Growing Cold Climate Cacti
Echinocereus | Escobaria | Opuntia | Pediocactus |
The first and most important consideration to make with growing cold climate cacti is that they need to live in a region or zone, which will provide cold enough winter temperatures for them to flourish. Almost all cold climate cactus species need long periods of freezing winter temperatures in order to bloom in the following spring and/or summer. A good climate for growing cold tolerant cacti needs to at least have winters cold enough to grow tulips. If Calla Lilies flourish in outdoor areas, without winter protection, cactus varieties which will survive extended periods of 0 °F or lower are unlikely to do well. However, if you have to dig up plants like Calla Lilies and store them indoors over the winter, but leave tulips in the ground then cold climate cacti should have the right winter conditions.
A second consideration especially concerning globular varieties is a simple method of protecting them from fungus gnats. Small species of flies who lay their eggs in soils may often be attracted to globular cold hardy cacti. These kinds of flies apparently aren't a factor in the native habitats of cacti like Echinocereus, Escobaria and Pediocactus, but are very common in wetter climates. If soil flies, shore flies or fungus gnats lay eggs on the base of these plants the maggots will feed on the plants, and then the cactus plant will likely incur a deadly fungal infection where the larva were feeding. These insects can be stopped by putting a product containing Bacillus thuringiensis subsp. israelensis and is in a product called Mosquito Beater by Bonide. Sprinkle some of this product around the cacti once a month to stop this kind of insect attacks and give the globular cacti a greater opportunity for survival. This is a consideration which needs to be made for seedlings, but most adult plants will not be vulnerable to fungus gnat infestations.
Cold climate cactus plants can be planted either in rock gardens or in flower pots to be kept outdoors, but are unlikely to make good windowsill specimens. These species of cacti may seem like very tough plants because they are native to habitats that have very harsh weather with wide temperature swings, but they also have vulnerabilities that need to be understood. They survive in these harsh ecosystems by finding the right places to grow, and because they have some unique abilities warmer climate cacti lack. Even thought these plants seem tough it is important to carefully consider the advantages of any location for a rock garden, or where to keep their flower pots in the changing seasons.
Meaning of Terms
To specify the scope of meaning for terms on this web site the terms; 'cold hardy', 'cold climate' and 'cold tolerant' will mean cactus plants which will survive extended periods of 0 °F or lower.
The terms, 'Cold hardy cacti' are generally considered to apply to cactus varieties that will survive at temperatures of 45 °F or lower for prolonged periods of time. The information here may apply to these plants, but the primary purpose of these passages are to address issues for growing cacti which will survive extended periods of 0 °F or lower. The text below the photos are general guidelines for growing cold hardy cacti either in an Outdoor Potted Cactus format or as Rock Garden Cacti in colder four season climates.
Avoiding Terminal Conditions
Ice Kills Cacti
People who live in climates where weather may produce ice storms need to understand that these conditions are terminal to most cacti before buying and planting any cold hardy cactus outdoors. Ice storms or freezing rain are deadly conditions to cold hardy cacti, and when ice builds up on the skin of these plants they will usually die. Cacti grown outdoors will need to be covered before freezing rains begins, because there are only a few species of Opuntia which are able to survive a covering of ice. Please consider the weather conditions where you live, and whether you are prepared, as a grower, to take the necessary time to protect the cacti from ice before you purchase any cold hardy cactus plants. When weather conditions will produce these conditions these plants need to be covered with plastic, which can be removed after the threat of layering ice is over.
Ice, rather than snow, is the main feature that separates habitat where most cold climate cacti live from regions where fewer species will survive. Hundreds of species live in harsh habitats with deep snow and temperatures below zero °F, but freezing rain is more rare in their natural habitats than many places like the midwest. Even if a climate has similar winters to the native habitats of cold hardy cacti the additional feature of fall freezing rains will kill these plants. Ice formed by freezing rain in semiarid alpine habitats stays on the plants only for a few hours, whereas wet regions can have ice cover plants for several day or weeks. Cold hardy cactus natural habitats are in regions that rarely ever have these conditions, which produce a layer of ice that stay directly on the skin of these plants.
The Echinocereus plant in this photo was purposely subjected to ice to visually demonstrate the kind of ice conditions which kill cold hardy cacti. This plant had a root fungus which it would have failed to survive anyway, so it was chosen for the making of this picture. This photo shows the kind of ice that will kill nearly all species of cold hardy cacti, and these plants have to be protected from any weather that will cause this kind of ice.
The following photos are only a few examples of cactus plants that grow in extrememy cold climates.
"Claret Cup Cactus"
This is the Echinocereus species with the broadest habitat covering most of the Rocky Mountain region. They can be found from Southwestern Canada to Northern Mexico at moderate to very high altitudes. These plants vary with stem size and spine coverings, but their flowers are highly consistent in appearance. They are divided into numerous subspecies, and the way these devisions are made seems to depend on the whims of people with scientific credibility (please excuse my snark). The plants produce offsets and grow into large colonies, which often rise into impressive mounds. They grow in sparsely wooded areas along with grass, and their brilliant flowers are true humming bird attractors. Their flower colors are in the reds with only slight variations of bright red ranging from either slightly orange to deep crimson. If a plant looks like these, but produces a dusty red sort of colored flower it is surely is a hybrid. Spine color is usually grey and their overall visual texture varies making each plant a unique individual. Some variations of this plant are nearly spineless and the spines they grow are shed before the end of summer. At least one version of this cactus constantly grows and remains spineless. These plants survive in harsh climates where water availability is often sparse. They are able to grow in rocky soils where day and night time temperatures widely vary. Their cold tolerance abilities vary because the climates of their locations differ through out widely spread habitats. ↑
Echinocereus viridiflorus var. montanus
"Green Flowered Rainbow" or "Green Flowered Torch Cactus"
The range of their native habitat covers most of the Rocky Mountain western states at high altitudes. These plants and their flowers vary in appearance, and are divided into numerous subspecies. They tend to grow as small singular stemmed plants rarely more than two inches thick, but older plants may produce offsets. Their flower colors range from rusty oranges, browns, greens and chartreuse. A few versions of this cactus are noted for having lemon scented flowers. They often grow in sparsely wooded areas visually blending in with grass, which makes them very hard to see at first glance. Short new spines often grow in rusty red colors and their spiny visual texture varies for each plant as well as for colonies in habitat locations. These plants survive some of the coldest extreme high altitude climates the rocky mountains have to offer. They are able to grow in rocky soils where day and night time temperatures widely vary. They are a very impressive example of a cacti which survives extreme conditions. ↑
"Missouri Pincushion Cactus"
These cacti should be recognized as the globular species most likely to be found growing in cold wet habitats. They grow in a wide range of western state habitats that include wet locations where no other globular cacti naturally grow. They are sometimes found growing at high altitudes, but mostly they grow in plains habitats where they often hide in grasslands. They have a generally similar appearance with only a few subspecies. Some populations will only grow as singular stemmed plants while others will make offsets by the hundreds. The long narrow shape of their flower peddles and flower colors give the the appearance of grasses with green, yellow and bronze tones on every peddle. Their over all visual texture with long pointed tubercles and skinny straw colored spines produces the appearance of the whole plant seeming like a tuft of grass. Stem diameter can vary from one to six inches depending on the colony and the height of the plants depends on the season. The extend their stems to approximately the diameter of the plant in the spring and summer, but part of their winter survival strategy is unique to only a few species of cacti. These plants dramatically shrink their stems into the ground just before winter, and sometimes they shrink enough to disappear from view. These cacti are survivors in extreme environments, and often live in habitats where rain is plentiful and winters are harshly cold. ↑
"Spiny Star", "Ball Cactus" or "Beehive Cactus"
These cacti range from low to high altitudes in a vast habitat from Southwestern Canada to Northern Mexico, and west of the Mississippi River. Throughout their habitat these cacti have a generally similar appearance, but are divided into several subspecies. Some populations will only grow as singular stemmed plants while others will make offsets by the hundreds. Their flower colors are very florescent in intensity ranging from pink to magenta and violet tones. Their spines help them blend into the appearance of dry grasses and rocky ground making these cacti most easily seen when they show the intense colors of their inflorescence. They survive best in very dry conditions and often tend to rot with exposed to too much rainfall. A few populations may survive climates with heavy rainfall, and nearly all tolerate very cold winters. These plants are survivors in extreme environments, but any individual plant's abilities will vary depending on plant habitat because they are widespread. ↑
Their habitat ranges from the south western states to Canada, and farther north than any other cacti in British Columbia. They have colonies into the eastern states as far as Michigan, and north all the way to the upper peninsula of Michigan. Their flowers are usually yellow, but maybe there are some rare magenta or white flowering plants too. The pads are small from half an inch to a few inches long, and depending on the colony visual spine texture varies slightly too. Plants form mounds from two to fifteen feet wide, and they remain close to the ground mounding few inches high. These cacti rarely produce seeds, and their main method of distributing themselves over a vast range of habitat is by stem propagation. Their pads are only slightly attached to one another and it's possible for pads to catch on passing wildlife then drop off in a new place. Pads will send down roots and generate a new plant allowing this plant to easily establish itself in new places without seeds. All cacti have the ability to grow new roots from their stems, and this species specializes in propagating itself this way. Some colonies thrive in areas with heavy rainfall and others require arid conditions. This is often considered to be the most cold tolerant cactus by any standard for survivors of extreme conditions. ↑
"New Mexico Prickly Pear", "Brown Spined Prickly Pear" or "Engelmann's Prickly Pear"
This species grows from arid plains to high elevations throughout the south west states in extremely hot summer conditions and many colonies survive where winters are harsh. They grow low to the ground producing pads in long chain like formations, and patches spread out about three to ten feet across. These plants keeps moving as they grow putting down new roots with each new pad. Most colonies of this cactus stay low to the ground, but some may form mounds as high as three feet. The spines are usually rusty in color making a beautiful contrast to the bluish green skin of their pads. The pad size can vary from four to eight inches across and the overall visual texture may vary between populations. They mostly grow in dry rocky locations where the grass stays brown for eleven months out of the year. Many habitats have very cold winters, and the cold tolerance for phaeacantha will vary according to plant location. Their flowers are slightly muted yellows usually with deep rusty red centers. Seed pods are a deep rusty red to maroon making the color pallet for this cactus a spectrum in ferric shades. Some prickly pears are edible, however this is one to avoid eating and should be grown as an ornamental non food source plant. This cactus thrives in extremely hot and dry climates, but is much more difficult to grow in cold wet climates. ↑
"Starvation Cactus" or "Plains Prickly Pear"
This species lives in wide spread locations of the western states, and has habitat as far north as Canada. They grow from lower plains to medium high elevations in sparsely treed habitat with some grasses and rocky ground. They tend to grow in dryer climates and it is unlikely to find colonies outside of arid and semi arid regions. Their general visual texture is highly variable making them a plant suitable for study by anyone who wants to specialize in one species of Opuntia. They sometimes have tuberous roots, but their most noted habit is the lack of fleshy mass inside of the fruit to accompany the seed at maturity. They grow low to the ground and pile up their pads to form clumps a few feet across, but sometimes colonies may spread out more than five feet across. The spines of these cacti vary in color, and appearance may vary in density from thick to showing no spines . The pad size may vary, but usually pads are small from less than two to five inches across. They have a wide variety of flower colors like white, pink, magenta, red, chartreuse, and colors in the yellows sometimes with red centers. Some flowers of this species have different colored edges of their peddles. Many colonies survive in extremely hot and dry conditions while others grow where winters are long, cold and the snow piles high. They are true survivors of extreme climates and will tolerate different climate conditions depending on the specifics of their habitat. ↑
Pediocactus simpsonii var. minor
These cacti specialize in high altitude conditions and have some of the most interesting survival strategies of any cold hardy plant. They are small globular cacti with plenty of spines. They are visually distinct in the way tubercles line up around the stem creating spiral channels. When seeds drop from the top of the stem they can fall down the sides of the plants beneath the spines falling in a spiral path. Many seeds collect at the base of these plants as they grow where they may stay without germinating for the life of the plant. They seem to have plain looking flowers in photos, but if you ever see their pink to white flowers in person they are very intense. Perhaps they are some of the most beautiful and mysterious plants in the world. They are similar to other cacti which survive in extremely cold conditions in that they shrink before winter, and some of them can become almost flat to the ground. They also have some unique and interesting chemistry that happens inside the stems of these plants which sets them apart from other extreme cold survivors. Beneath the winter snow they create their own snow caves by using some sort of chemical method to melt the snow near the stems. From all of the cacti who are survivors of extreme habitats this one is very unique. ↑
Keeping Cold Climate Cacti in Flower Pots
The easiest way to meet the specialized needs of cold tolerant cactus plants is to keep them in a flower pots, and move them to ideal locations for every season. In the summer cactus plants in flower pots can be kept in sunny locations during fair weather, but then moved to cooler locations on severely hot days. Cactus plants in this format can be placed in the sunniest locations in the spring and fall where the availability of sun changes with the onset and falling of leaves on deciduous tree. The greatest advantage to keeping these kinds of cacti in pots is that, in the winter, they can more easily be kept in a very cold location that is protected from winter winds and ice. Great locations can always be found as long as the plants can be moved, but how much of an advantage flower pots have over planting a cold climate cacti in a rock garden will also depend on the cactus variety. Small globular cold hardy cacti can thrive in flower pots, but Opuntia may do better with more room for growing roots.
Flower Pot Opuntia
Prickly pear, or Opuntia species would gain the least benefit from being in flower pots, because they can tolerate extremely high temperatures in the summer and thrive with lots of room for roots. The only reason to keep cold hardy Opuntia in flower pots is if the specific species is unable to tolerate the coldest parts of winter in the climate where you live. As long as an Opuntia variety will take the coldest days of winter it will almost always be healthier growing in a rock garden than in a flower pot. If Opuntia are going to be planted in flower pots the best way to keep Prickly Pear Cacti growing is to plant them in the largest and deepest flower pots possible. In huge flower pots they will grow faster and bloom sooner than they would in confined containers.
Flower Pot Echinocereus & Escobaria
Globular varieties like Echinocereus, and Escobaria may do equally well in flower pots or in rock gardens, but keeping them in flower pots will provide them with good winter survival advantages. They are very tough plants when it comes to taking wide temperature swings and they flower more when placed in full sun through out the entire growing season. In the winter, some of these species will tolerate extreme cold, and remain under the dark cover of snow for more than four months out of the year. The greatest danger to them in the winter is being directly exposed to very cold winds or ice if it builds up as a layer on their skin. If an outdoor location like a rock garden canÕt be protected from harsh winter winds or dangerous ice, then keeping these species of cacti in flower pots will be very advantageous to their survival. Keeping these kinds of cacti in pots allows them to be moved to ideal locations in the winter like a cold frame, unheated storage building, or an unheated entry room of a home. It is far more important to protect these kinds of cacti from winter winds and ice than to attempt to provide sun light using an unprotected location. These varieties of cacti have unique abilities which makes cold dark still air far better for their winter survival than sun with wind or ice exposure. In their native habitats these varieties often grow in spaces protected from winter winds by rock formations, grass, sage brush and/or deep snow.
Flower Pot Pediocactus simpsonii var. minor
Species of Pediocactus are best kept in outdoor flower pots unless they are being grown in their native Rocky Mountain habitat, and may do very well as rock garden plants. Growing highly challenging cacti like Pedios in flower pots allows them to be moved to the best possible locations for every season and greatly increases their ability to live healthy cactus lives. Like most globular cacti cold hardy Pediocactus plants need as much sun as possible in the growing seasons, but on the hottest summer days they may occasionally need to be moved to cooler locations.
In the summer Pedios in pots can be allowed as much sun as possible as long as they are kept out of high heat. On the hottest days they need to be moved to locations with partial shade to prevent them from over heating. High concentrations of moisture in the air will make the summer heat even more dangerous. In humid climates day time temperatures above 90 °F can cause Pedios to suffer heat damage, and this high heat exposure can be fatal to these plants. Pedios will require cooling methods on the hottest days of summer such as moving the potted cacti to partially shaded locations or giving the plants a light spray of water, in the evening, to help cool them down for the night.
In the fall, the most important feature for their location will be an area which dries quickly after rain. Many species of fungi are active in the fall and keeping Pedios out of areas that stay damp for extended periods of time will help protect them from this seasonal hazard of fungal infections.
In the winter potted Pedios can be placed in very cold locations as long as they are protected from the wind. Pediocactus plants need a well defined cold dry and dark season of rest in the winter for two to three months. If they are given this long cold dark dry winter sleep they will thrive in outdoor flower pots through out the growing seasons. For this dry winter period they need little or no light of any kind, and can be kept in a snow covered cold frame, storage building or any very cold unheated room. Try to dry out their soil as much as possible in late fall and keep the soil completely dry all winter, and then they should remain dry until early spring. Around the end of February Pedios can be set out in the open to receive plenty of spring snow and rain.
Potting the Cacti
Many varieties of cacti, including cold tolerant ones, can be kept outdoors for the summer in flower pots, and on hot sunny days these plants can be damaged if planted below the rim of their flower pots. Heat can be reflected back to the stems of cactus plants from the edge of the flower pots and cause heat burns on the cacti. The flower pots need to be filled with rocks to the brim with the base of the cactus stems planted level with the edges of the pots. An air space around the cactus plants is necessary to prevent skin burns, by keeping them cooler on the hottest days. ↑
Growing Cacti in Four Season Rock Gardens
Flower pots have many advantages over planting cold hardy cacti directly in outdoor rock gardens especially for globular species. However, if a rock garden has great conditions many species of cacti will grow and thrive far better than they ever could growing in flower pots.
Understanding the needs of cold hardy cactus plants and finding species to match the climate are important considerations, and the following information may be helpful if you plan to grow cold tolerant cactus plants in a four season rock garden. If the right plants are planted in suitable conditions they should live and grow free from insect and fungal attacks. Most of the insects and fungi, which may destroy cacti, are lovers of dark and damp conditions, so choosing a sunny dry location is the first step for growing cold hardy cacti. Finding the right plants may take some trial and error as well as some research, and the three best generics to consider growing in regions where winter temperatures remain below freezing for extended periods of time are Opuntia, Echinocereus and Escobaria.
Rock Garden Opuntia
Opuntia are sometimes called 'Prickly Pear Cacti' and they are the best variety to grow in cold climate outdoor rock gardens. They grow quickly and when they suffer setbacks they are more able to recover from injuries than globular varieties. There are hundreds of species with lots of flower colors, and unfortunately many of these variations arenÕt even available in cultivation. Hopefully more of them will become available as horticulture and landscaping plants, and people will realize how easy and wonderful these cold climate cacti are to grow. Some varieties of cold tolerant Opuntia can occasionally be found in nurseries, and they may also be available from internet sellers.
There are far too many cold hardy species of Opuntia to list and sometimes there needs to be more information than a scientific name, because many species vary in their tolerance for cold weather. For instance, Opuntia phaeacantha can be found growing in both warmer and extremely cold climates. When the species name for an Opuntia variety ranges through several different growing zones itÕs a good idea to find a cold tolerant species whose parent plants were native to a cold climate similar to the zone where the rock garden is going to be located. These are cactus plants well worth looking for, but the key to finding cold hardy Opuntia suitable for any specific zone is to know if the parent plants came from a similar cold climate.
Rock Garden Echinocereus and Escobaria
Imitating Habitat Conditions
Rock Protects Cactus from Cold Winter Wind
This photo of an Echinocereus in habitat is a good example of how to think about planting cold hardy cacti in rock garden locations. The rock and clump of grass are on the north and west side of the cactus and protect it from the cold winter winds. Think about the direction cold fronts come from in the rock garden climate and this will be the direction of origin for dangerous winter winds. Placing rocks in locations near the cacti that will protect the plants from cold winds will improve their winter survival rate.
There are many species of globular cacti that will thrive in four season rock gardens if care and consideration is taken in choosing a location which provides for their needs in all four seasons. Here are some species names to look for in the generic Escobaria; missouriensis, organensis, sneedii, & vivipara, and under the generic Echinocereus; chloranthus, coccineus, fendleri, kuenzleri, reichenbachii, triglochidiatus & viridiflorus. There are many other generics with cold tolerant species to consider, but these are among the most versatile hardy species in regions where winter temperatures remain below freezing for extended periods of time. The warmer the zone where the rock garden is to be established the more types of globular cactus varieties will likely grow, and it may be worth trying different ones to see what will grow in the climate where the rock garden will be located.
A rock garden containing cactus plants should be in an elevated location where the ground is above the surrounding landscape allowing water to drain away from the cactus garden.
Most garden soils will cause cacti to rot if it remains in contact with the stems, so there needs to be plenty of gravel covering the soil to prevent mud from splashing on the cactus plants when it rains.
Competition from Native Plants
It may be helpful to include weed preventing soil cloth in the design of the rock garden to prevent competition from native plants, and it's no fun to remove weeds from around the base of the spiny cactus plants.
Debris like small tree branches, leaves and other wind blown objects may be captured by cactus spines especially in regions where deciduous trees are common. The natural habitats of many species of cold hardy cacti are often more along the lines of rocky soils with sparse grasses, conifer trees and sage type plants. If leaves or other debris accumulate around cactus plants they need to be removed to prevent the cacti from becoming shaded from the sun.
The rock garden needs to be in a location that will dry out quickly after rain to avoid promoting the kinds of molds and fungi that attack plant life in this season.
The rock garden location should accumulate snow in the winter, as opposed to being in a wind blown location which losses snow cover to winter winds.
The rock garden needs to be located in an area which has as much sun exposure as possible, and for cold hardy cacti sun is more important in this season than summer. Spring is a wonderful time to watch cold climate cacti, because this is their main growing season and they are very likely to bloom. These cacti have the unique ability to remain in the dark for many months beneath snow, but when the snow melts they will need plenty of sun and will tolerate lots of rain.
The rock garden location needs to be within reach of a garden hose in order to spray the cacti and cool them off in the evenings following the hottest days of summer. In the high altitude habitats, where most cold climate cacti grow, night time temperatures drop dramatically, allowing these plants the ability to cool down. In contrast, climates like the humid midwest will have hot nights following hot days, and this condition is dangerous to many of these species of cacti. When cold climate cacti are unable to cool down in the evening they will be forced into a state of rest, and this prevents them from carrying out the chemical activities necessary for plant life like photosynthesis. Cacti are awake at night and sleep during the day, which is the opposite way most other plants live, so night time temperatures are very important to cacti. In the hottest humid days of summer when temperatures are above 90 °F cold climate cacti may need some help cooling off in the evening. The cacti wonÕt need to be soaked with the hose unless there hasnÕt been any rain for a long time, and enough water to wet them will help these plants cool off at night and stay healthy.
Rock gardens are typically located in sunny fast drying locations, which is ideal for most cactus plants including cold climate varieties. However, cold hardy cacti can get heat burns under certain conditions which can cause narcotic tissue. The development of necrotic tissue is dangerous, because fungi can establish themselves as a deadly plant infection originating in dead plant tissue. This kind of condition where dead plant tissue can cause the rest of the plant to die will also happen if cacti are exposed to cold winter winds, so both cold and heat burns need to be avoided.
It is important to avoid locating the cactus plants next to a vertical surface that is less than an inch away from the stem of the plants. A surface like a rock or the edge of a flower pot may reflect and/or radiate heat back towards the stems on very hot sunny days. To some extent, cacti both radiate and reflect heat to keep themselves cooler in hot weather, and they need some open space around the stem of the plants to unload their excess heat into and stay cool. Strangely, plants like grasses or other succulents may actually help cacti stay cool, and many cacti grow with grasses and sedums in their natural habitat. Except for plants like low growing grasses it is very important to leave space around the base of cold climate cacti at ground level, and this open space will help prevent them from getting heat damage.
Many pests with names like 'Sow Bugs', 'Millipede' or 'Slugs' are lovers of dark damp places. One of their favorite shadow world places to live is under rocks and there are usually rocks in rock gardens.
A good way to help prevent these kinds of pest from living under rocks is to help keep these areas dry by covering the ground with plenty of gravel before laying down the large rocks. The wet conditions these animals love will more easily occur if the rocks are directly on hummus type soils, and the rocks will help rich garden soil stay wet enough to keep these pests comfortable. Gravel will dry out faster than garden soil and dryer conditions will help prevent these kinds of pests from establishing colonies in the rock garden.
Porcellio scaber (Sow Bugs )
Sow Bugs may feed on the roots of cacti and may eventually cause the plants to die from a fungal infestation. They are also called "Lawn Shrimp" and "Wood-louse" which seem like innocent names for some kind of fun animal. Outdoor areas which are frequently wet and have plenty of shade are favorite hiding places for these small crustaceans. If any of these sow bugs are found around cactus plants they will need to be destroyed.
Schizophyllum sabulosum (Millipedes)
Another lover of damp dark conditions are millipedes and they may feed on some types of globular cacti if they are given an opportunity. These crustacean are rarely seen in sunny areas, unless they are suffering from parasites, and millipedes live where soil has decaying organic matter. They are unlikely to find their way into sunny locations where a rock garden would be located, but in some weather conditions they may become a problem. If there are cloudy rainy conditions for many days millipedes will begin to look for food over a wider area, and may find their way into a rock garden. They may be interested in eating cactus plants, and prolonged wet periods with little sun may allow them an opportunity to dine on cactus stems. In these kinds of conditions it is a good idea to watch for these small animals and remove or destroy them.
Deroceras reticulatum (Slugs)
Slugs and snails are common garden pests who love cool damp dark conditions, and can be especially destructive to cacti. If wet overcast conditions last for many days slugs seem to multiply quickly and spread their progeny everywhere. Slugs are attracted to cacti as a food source, and will slip themselves between the spaces in the spines to feed on the cactus stems. If wet weather lasts long enough for slugs to find their way into a rock garden area they will need to be baited and destroyed. Many species of Opuntia and Echinocereus are capable of surviving slug attacks, and the only remaining damage will be scares on the stems where the slugs were feeding. For other cold hardy species like Escobaria, Pediocactus, Glandulicactus and Sclerocactus the damage caused by the slugs will surely lead to fungal infections and death.
Family Ephydridae (Shore Flies)
Many varieties of cacti may tolerate cold winters, but in more humid climates some varieties end up as food for flies if planted in rock gardens. Glandulicactus and Sclerocactus are very prone to being eaten by a variety small fly maggots, which may go unnoticed for weeks as they inhabit and devour these cacti. Climates with plenty of rivers, lakes and rain will support a wide variety of small flies who are very opportunistic about where they lay their eggs, and Glands and Scleros will be seen by these kinds of flies as nurseries for their larva. Some varieties of cold hardy cacti may simply be impossible to grow in rock gardens where the weather is wetter then their natural habitats because of flies. Glandulicactus and Sclerocactus plants may be kept in outdoor flower pots in regions where these kinds of flies are present in the habitat, but they will still require special care to prevent infestations.
Anasa tristis (Squash Bugs)
There are common garden pests like 'Squash Bugs' and who may choose to lay their eggs on cacti, and 'Prickly Pear Cactus', Opuntia are especially tempting for these kinds of insects. The eggs will hatch inside of the cactus flesh and the larva will likely feed there unnoticed until the plants are nearly destroyed. Adult insects like this may also suck sap from the stems of plants and this will potentially cause a fungal infection which also may be terminal to cacti. Either by feeding and/or laying eggs on cacti these insects are pests you will want to catch and destroy as soon as possible. They usually can be found as adults, picked off of the plants and destroyed, hopefully, before they've had an opportunity to lay any eggs.
There are many species of mites that can feed on cacti and some of them can be very difficult to destroy. Most kinds of mites that live in colder climates can be treated the same way they would if they were on roses. There are a wide variety of insecticides and herbal alternatives that can put an end to most mite problems. Mites are most likely to feed on 'Prickly Pear Cactus', Opuntia, and because mites are common in almost any climate plants should be frequently examined for infestations.
Dactylopius coccus (Cochineal Insects)
Cochineal is a Carmen colored dye derived from the insect named Dactylopius coccus, but they are undesirable pests for rock garden cacti. 'Cochineal Insects' are a kind of scale that spins pinkish webbing making fuzzy structures that sort of look like large mealy bug webs and they can be very difficult to destroy. The best time of year to go after these types of pests is in the early spring as they are hatching out and beginning to spread. Whether insecticide or other herbal remedies are being used the plants should be treated week after week for at least a month and this will kill off the scales as they are hatching and crawling on the stems of their host plants.
They will feed on 'Prickly Pear Cactus', Opuntia and even though they originate in warmer regions their eggs can survive prolonged periods of below freezing temperatures. 'Cochineal Insects' are a serious enough problem that if they can't be destroyed the plants themselves may need to be destroyed, otherwise the scale will continue to spread to other 'Prickly Pear Cacti'.
Opinions in a Nutshell
Success in growing cold hardy cacti in rock gardens or outdoor flower pots depends on understanding the needs of these cacti. Cold hardy cacti have very specialized skills for survival, and even though they are tough plants they depend on specific favorable conditions in every season in order to grow and thrive.
Remember that many cold hardy cacti will actually live better if they are surrounded by low growing grasses like buffalo grass. Grasses often provide protection for cacti in their natural habitat and cold hardy cacti frequently grow with the protection of grasses, sage brush and pine trees. These cacti benefit from other low growing plants by gaining protection in both the coldest and the hottest days of the seasons. ↑