Cold hardy cactus, succulents, and perennials for the rock gardening enthusiast.
1962 North Sparkling Place
Meridian, Idaho 83646
( 208 ) 884-1251
The following descriptions are a general guide as to how much sun exposure your plants should receive based on the average amount of sun exposure. Heat also plays a major role in these descriptions and is directly related to sun exposure and UV intensity.
1. Northeast, Pacific Northwest west of the Cascades, costal areas & Intermountain West ( above 5000 feet ) = For these areas, the challenge will be to provide enough sunlight to allow the plants to retain their natural form. I recommend an absolute minimum of 6 summertime hours of direct sunlight for your plants. Any less than this then shade plants should be considered.
2 . Southeast, Great Plains, Intermountain West ( below 5000 feet ), & Desert Southwest ( above 5000 feet )= These areas can have limits on some plants in regards to sun exposure with an emphasis on broken shade in the afternoon hours of the day when summer daytime highs are at their peak. For these areas I would recommend a minimum of 6 hours ( sunrise to early afternoon ) of direct sunlight followed by some degree of light shading in the afternoon hours.
3. Desert Southwest below 5000 feet= For these areas, partial to broken shade is more critical to prevent sunburn with full sun being limited to the tallest species and most others needing some degree of shade during most of the day. The exception is higher elevations above 5000 feet where cooler temperatures at night help offset the difference.
2.Cold Hardiness.The Department of Agriculture, over many decades, has developed a series of cold hardiness maps to aid farmers and gardeners alike to help understand which plants are hardy enough to overwinter in their respective areas. You'll notice on the map in figure 2 that most of the cactus species in the US are concentrated in the Desert Southwest where conditions are relatively warm and dry during the winter months, however a few species do overlap into much colder areas at the edge of their range or at high elevation, particularly in the Four Corners area of Arizona, Utah, Colorado, and New Mexico. Outside of the Desert Southwest, a handful of species can be locally abundant in areas of the Intermountain West and western Great Plains where winter conditions can become quite severe.
Utilize the map in figure 2 below to select plants that are within 1 1/2 zones of your local zone. Selecting plants that are two or more zones removed from your own will increase the odds of it being lost either in the cold of winter or the heat of summer without protective measures. For example, if your in zone 6a, you can safely grow plants from zones 4b to 7b. This is more critical with plants from milder climates to colder than colder to warmer.
If your not sure which zone your in, the USDA has developed a zip code program where you can enter your zip code and the program will display your zone. Use the following link ( this will open another window ) to enter your zip code:
Figure 1. Map provided by US Department of Energy - National Renewable Energy Laboratory ( NREL )
Figure 2 - USDA Zone Hardiness Map by PRISM Climate Group. Oregon State University.
Outside of the US, many semi-hardy cacti also come from northern Mexico and South America. In figure 3, a quick look at their native zones indicates that the semi-hardy species from these areas are a zone 9 or 8 at best, at least in general terms. The plants from these areas can experience hard frosts at night, but are typically offset by daytime temps well above freezing that moderates the overall median temperature and the plants are rarely frozen or completely frozen for very long.
Figure 3. World Temperature Map.
3.Soil Composition.This is the most challenging aspect of desert gardening where drainage is critical to keep your plants properly hydrated. The soil composition you make will depend directly on the amount of natural rainfall your area receives with a richer mix in desert areas then progressively leaner as annual precipitation increases. Areas with less than 15 inches of rain annually are considered "desert" areas and the challenge is to retain as much rainwater as possible while 15-30 inches per year is marginal and partial conservation of rainwater is only necessary in drought years to retain good moisture content. Above 30 inches then the challenge is to direct the excess rainwater away as quickly as possible. Humidity also factors in with high rainfall and the mix should be extra lean to encourage quick drying of the mix in the root zone.
Figure 4. US Annual Average Precipitation.
Determine your drainage requirements. Formulate your soil mixture based on the amount of natural rainfall you receive annually. In landscapes I recommend a minimum depth of 4 inches while 6 and above is ideal. If your growing in a container then fill the entire container. Containers should be a minimum of 12 inches wide and deep for the plants to survive outdoor conditions and provide enough space for roots to grow and should be made of a breathable material such as wood or concrete mixes like cinder block or hypertufa.
Basic Components: Pea gravel and sand. Pea gravel ( figures 5 & 6 ) is roughly defined as rock chips between 1/4 and 3/8 inch in diameter that are mechanically separated at the source and can come as coarse or rounded. Sand ( figure 7 ) is also mechanically separated for size uniformity and has many uses from sandblasting, concrete mixes ( often described as "Morters Sand " ), and "play sand". The quality of sand can vary depending on its source. Sources that use water as part of the separation process are superior than dry separated and retain little or no dirt which is critical for drainage. Dry separated sand can retain too much dirt and can harden like concrete when wetted down and dried. Avoid using dirt ( hardening ) or potting soil ( fungus, pests, pathogens ) in your mix that can contribute to problems in the root zone.
Figure 7. Washed sand should be nearly free of any fine dust or dirt and will not harden after getting wet.
Figure 8: Washed Play Sand - Home Depot
1. Sun Exposure:The description "Full Sun", as described on many plant tags, is somewhat misleading as we know that "Full Sun" in Boston is different than "Full Sun" in Phoenix. On the map below ( Figure 1 ) we can see that not all parts of the country are created equal with regards to hours of sun exposure and intensity. Most all cactus and succulent plants are generally assumed to need "full sun" when, in fact, it could be harmful or even fatal in some cases. A good percentage of cacti and succulent plants grow next to, or underneath, other plants to help shield them from the intense sun of their native environment. In fact, you will notice that most cactus species that occur within the US are mostly within the Desert Southwest where sun exposure is at its highest. Outside of their native habitats however they can be exposed to more sun and even full sun if the intensity is far enough lower than their native habitat.
Understanding how plants grow in their natural habitat goes a long way towards growing them successfully in the garden and possibly even better than they would in nature. To do this, there are three main areas I'm going to focus on here for you to consider when selecting your plants and preparing your desert garden, they are:
1. Sun Exposure.
2. Cold Hardiness.
3. Soil Composition.
Figure 5: Pea gravel.
Figure 6: Bagged pea gravel - Lowes
Sourcing your materials:For landscapes requiring larger quantities, local quarries or gravel pits will be your most cost effective approach. Again, be sure they are utilizing water to screen their materials for the highest quality. For small gardens or container gardens you can buy by the bag and both materials are readily available at local retail stores like Home Depot or Lowes. Figures 6 & 8.
Less than 15 inches per year = 1-1 ratio + amendments*: 1 part pea gravel to 1 part sand. For western gardens with less than 15 inches per year you can substitute volcanic cinder fines in place of the standard pea gravel for better moisture retention.
15 - 30 inches per year = 3-1 ratio + amendments*: 3 parts pea gravel to 1 part sand.
30 inches and above per year = 5-1 ratio + amendments*: 5 parts pea gravel to 1 part sand.
*Amendments: Once you've determined your drainage requirements then the next step is to add essential trace minerals and fertilizer to the mix for growing vigor, disease and cold resistance.
N= Nitrogen, essential in all plant functions. Cacti and succulents, with their slow metabolisms, require low amounts of nitrogen and adequate amounts can be obtained from rainwater alone in higher ( approximately 20 inches and above ) rainfall areas. Earthworm castings, applied topically, then watered in, are another good natural source for low rainfall areas.
P= Phosphorus, essential for photosynthesis, blooming vigor and root development. Sources include superphosphate derived from natural dolomitic limestone or bone meal. Both can be purchased separately at some retail outlets or online ( see figure 9 ) and should be used sparingly at 1 tablespoon per 5 gallon container or 1 cup per wheelbarrow load. If you decide to use animal byproducts like bone or blood meal etc., then I recommend they only be applied topically to avoid fungal, disease formation in the root zone from bacterial breakdown. Applying them topically will allow the sun to sterilize and break them down for more ideal uptake.
K= Potassium, for growing vigor, disease & cold resistance. Sources are somewhat similar to phosphorus including dolomitic limestone and gypsum, which are most likely to be found only through online sources like Amazon ( see figures 10 &11 ). Application is higher as the concentrations are less at 1 cup per 5 gallon container or 4 cups per wheelbarrow load. Both products listed also contain high natural concentrations of calcium which cacti need lots of for formation of their spines
Roses are distantly related to cactus and have similar requirements so if you'd like to keep it simple then you can use rose fertilizer to fulfill the NPK recommendations.
Trace Minerals: For photosynthesis and structural formation. This is also highly recommended in addition to the NPK descriptions above. I recommend a product called Azomite ( see figure 12 ) which contains trace minerals like magnesium, sulphur, iron and others. Application is 1/2 cup per 5 gallon container or 2 cups per wheelbarrow load. Actually there's no limit to how many trace minerals to add into your mix unless it compromises the drainage.
Figure 9. Triple superphosphate - Amazon.
Figure 10. Dolomite limestone - Amazon.
Figure 11. Gypsum - Amazon
Figure 12. Azomite - Amazon
Additional Fertilizers: For summer / fall blooming desert perennials, I do recommend using liquid fertilizers derived from seaweed or compost tea. They can be lightly applied topically during the late evening hours just before sunset or more concentrated anytime around the base of the plant and watered in approximately every two weeks. This should be discontinued in late summer to early fall to harden the plant for winter. Succulents, with their slower metabolisms, can also be fertilized safely but should be done only once per month. With cacti it's even less with most species normally growing in soils nearly devoid of organic compost and active microbes. A safe application would be once per year in late winter to early spring just before or as the plants become active.
Landscaping basics for desert gardens: If you have flat ground you'll have to create a raised bed in order to avoid standing water from the spring thaw or excessive rainfall. Precautions should be taken to direct the flow of water away from any raised beds. Beds should be raised a minimum of three feet above the existing ground and should slope down from a higher central point. This is less critical if your property is on an existing slope. For western gardens ( figure 13 ) a substrate layer consisting of 1/2 cactus mix and 1/2 native soil is put down in order to absorb and hold moisture but allow excessive moisture to still drain away. In addition, terracing of the surface will enhance the effect. This is especially helpful in drought stricken areas with 15 inches or less of annual rainfall. For higher rainfall areas above 30 inches per year ( Figure 14 ) the substrate layer needs to be less porous, draining the rainwater away more quickly with minimal absorption into the substrate layer.
Figure 13. Diagram for landscapes with low annual rainfall below 30 inches per year. The goal is to capture and hold as much rainwater as possible without allowing it pool up and stagnate. The uppermost layer of cactus mix allows the rainwater to pass through quickly where its absorbed slowly by the substrate layer.
Figure 14. Diagram for areas with higher rainfall above 30 inches per year. The goal is to drain away rainwater quickly with the substrate layer consisting up to 100% native soil to minimize absorbption. The exeption is if you have a porous, sandy native soil already and any rainwater is absorbed quickly into the ground without standing. The anchor rocks should be more deeply placed into the substrate layer to help stabilize the upper layer and, if you have a retaining wall, it should be made from a breatheable or porrus material such as wood, layered stone or concrete that will allow the wind to help dry out the material behind it.