Studying the Joshua Tree ( Yucca brevifolia ) at its northern most limits in central Nevada.
Geoscape Desert Nursery
Cold hardy cactus, succulents, and perennials for the rock gardening enthusiast.
1962 North Sparkling Place
Meridian, Idaho 83646
( 208 ) 884-1251
Captain Kirk, where are you?
"The devil's in the details" as the saying goes and on this page you will find detailed information on how to select the plants that ideally suit your local environment and how to plant them with the best combination of success factors, which include the following:
1. Sun Exposure
2. Cold Hardiness
3. Precipitation & Soil Composition
1. Our first success factor is Sun Exposure. The description "Full Sun", as declared by many plant sellers, 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. 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. A basic guideline here is 36 inches at plant maturity. In other words if the plant(s) your choosing are 36 inches or more in height at maturity, it's a good bet that it will need as much sun as possible to retain it's natural growth and shape. Then, conversely, if it's under 36 inches in height at maturity, it's a good bet the plant will have limitations on sun exposure, especially if conditions are similar to it's native habitat. 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. In the photo to the lower right, the Joshua Tree is a good example of a plant that demands full sun while smaller species of cacti, such as Mammillaria and Escobaria, grow next to or underneath the other perennial species to avoid being scorched in the afternoon sun. Species of Hedgehog ( Echinocereus ) cacti fall in between both extremes. Prickly pears ( Opuntia ) and Chollas ( Cylindropuntia ) fall mostly on the upper end of exposure.
On all sun exposure descriptions, I've now broken them down into 3 geographic Solar Regions:
1. Northeast & Pacific Northwest west of the Cascades = Areas less than 4.5 Kilowatts per hour, shaded as light yellowish green on the map. These areas are safe to provide as much sun as possible for all the plants listed on the Geoscape website. I would recommend a minimum of 6 summertime hours of direct sunlight for your plants if your in these areas.
2 . Southeast, Great Plains, and Intermountain West = Areas 4.5 to 6.0 Kilowatts per hour, including the yellow and both light and dark orange shaded areas of the map. These areas will have limits on 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 = 6.0 Kilowatts per hour and above, represented on the map by the brown and red shades. For these areas, shade is more critical to avoid turning your plants into "crispy critters" during the summer with full sun being limited to the tallest species with all others needing some degree of shade during the entirety of the day.
Utilize the map above in figure 1 to determine which Solar Region best fits your local conditions.
2. Our second success factor is 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 over-winter 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 including Cylindropuntia, Echinocereus, Escobaria, Opuntia, Pediocactus, and Sclerocactus can be locally abundant in areas of the Intermountain West and western Great Plains where winter conditions can become quite severe.
To help take out the guess work, all plant descriptions now indicate the plants native zone hardiness for each plant listing. In addition, I have a four tier rating system for cold hardiness that's based on the number of hours of freezing a plant can tolerate without experiencing severe damage or total loss:
Cold Tolerance Ratings:
1. Low tolerance = Plants can take light frosts above 20 degrees F ( -7 C ) less than 4-6 continuous hours.
2. Low to Medium tolerance = Plants can take heavy frosts down to 10 degrees F ( -13 C ) up to 12 continuous hours.
3. Medium to High tolerance = Plants can take heavy frosts down to zero degrees F ( -18 C ) up to 48 continuous hours.
4. High tolerance = Plants can stay frozen solid for weeks. Note: Plants that are in this tolerance category can be lost in the heat of summer ( and or from too much sun exposure ) if planted too far south or at a much lower elevation than the plants native habitat. Remember, it works both ways with regards to extreme heat or cold tolerance and your objective is to choose the best balance for your local environment.
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.
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: http://planthardiness.ars.usda.gov/phzmweb/Default.aspx
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 completely. Some areas of the high Andes in Argentina, and in southern Patagonia are probably closer to a zone 7 if not an outright zone 6 or lower at high altitude that this map doesn't define. Maihuenia and Pterocactus are two examples of cacti from Patagonia that can over-winter into a zone 6. Outside of these two species, a zone 7b or 8a will be the hardiest one could expect from Argentina.cacti. The same could be said of the species from northern Mexico.
Figure 3. World Temperature Map.
3. Our third and final success factor is Precipitation & Soil Composition. Cactus and succulents are well known for their ability to store water in their leaves and stems to conserve water, which helps them survive dry environments and drought. Their ability to survive these conditions also makes them sensitive to excessive rainfall and moisture, so when growing them under garden conditions the question becomes, how much precipitation or hand watering can these plants tolerate without losing them to root rot? The answer lies in studying the rainfall patterns that the plants are normally growing in. Figure 4 below shows the average annual precipitation in the continental US, which reveals that most native cactus and succulent species occur in areas that typically receive less than 15-20 inches per year, as represented by the light and dark orange, red, and reddish brown shades, however tropical cacti that occur in Central & South America can have average rainfall well over 100 inches, but they survive by growing on steep, rocky terrain, even cliff dwelling in some cases. Still others have adapted to growing in trees or on the ground where deep pockets of sand in combination with broken branches and leaves have accumulated, allowing the rainwater to pass through without pooling. Opuntia humifusa is an eastern US cactus that's a good example of this. In all cases, the rain drains away immediately, without puddling, which allows what little soil the cacti is growing in to dry out quickly. It's this example that you'll utilize in the garden by formulating a soil mix that's deep enough for roots to grab a hold of and sharp draining enough to allow any rainfall or supplementary water to pass through quickly.
Figure 4. US Annual Average Precipitation
Figures 18 and 19 below demonstrate not only the dry conditions most cacti and succulents grow in, but also the sandy or coarse, rocky soils that are themselves a only a thin cover over sloping bedrock. So even in dry environments the soil is fast draining and it's these kinds of soil drainage conditions you'll need to duplicate in the garden to grow them successfully.
Formulate your soil mixture based on the amount of natural rainfall you receive. Utilize the map in figure 4 above to determine which of the following formulas to use:
Note: On the Perennials Page, plants are classified as "Type 1 or 2" soil mix. "Type 1" refers to plants that occur in arid environments without a nearby water source and are as drought hardy as cactus can be. For these plants, use the same mix as you would for cactus and succulents. "Type 2" refers to plants that occur in arid environments, but have a nearby permanent water source, such as a spring or runoff areas next to creeks or rivers. In this case, the plants can endure the intense heat by drawing on the water source to stay hydrated. "Type 2" plants will need a richer soil mix to help retain moisture, so for these plants, add one more part fine sand to the mix. In addition, top dress the mix with a 1 inch layer of potting soil. Avoid mixing in with the soil mixture itself as this can cause fungus and rot in the root zone. You'll note that none of the cactus or succulent formulas below contain any organic ingredients for this reason.
Formula 1: For areas that receive less than 20 inches of annual rainfall, as represented by the orange through brown colors. Supplementary watering may be necessary to keep plants properly hydrated for flowering and growth with the emphasis on the month before and after the normal flowering cycle. Allow to dry completely to a minimum depth of 3 inches between watering when doing so.
1 part 3/8 inch red lava ( or black ) cinder fines to 1 part washed sand. This formula is ideally suited for desert or near desert like conditions where some moisture retention is beneficial to keep plants from overly dehydrating in the intense heat of summer. It contains a higher ratio of fines to coarse rock. Red lava cinder fines ( figures 5 & 6 ) are widely available in the western US at most garden centers. It typically contains 3/8 inch pieces down to dust. It usually prices in the $30-35 range per cubic yard. Washed fine sand ( figure 7 ) is the other key component that also gives good drainage while retaining some moisture for a short time before drying out completely. If you have a gravel pit nearby, this will be the most cost effective way to obtain larger quantities and prices in the $10-15 range per cubic yard. Washed fine sand ( also called play sand ) is also sold in 40 or 50 pound bags if you need a smaller amount and is commonly available at most garden centers or the big box stores like Home Depot or Lowes and prices in the $4-5 range per bag. The important thing when buying by the bag is make sure that it's washed. It will say this on the bag as in figure 8. If it doesn't, then consider it poor quality and not suitable for this use.
Formula 2: For areas that receive 20 to 40 inches of annual rainfall, as represented by the yellow, tan, and first two shades of green. Supplementary watering should only be necessary if dry conditions persist for more than 30 days in the critical month leading up to and after the flowering cycle. Beyond that, any supplementary watering should not be necessary. Soil mix should be completely dry to a depth of 4 or more inches between watering.
3 parts coarse pea gravel to 1 part washed fine sand. This formula contains a higher ratio of coarse rock to fine sand for sharper drainage and quicker evaporation. For this reason, this formula, and the next, contain no cinder or cinder like products ( such as pumice or perlite ) as they can retain moisture for too long in higher rainfall areas and can potentially cause rot. The coarse pea gravel ( Figure 9 ) can also be obtained at gravel pits if you have one nearby, at local nurseries, or even bagged ( figure 10 ) at many retail stores such as Home Depot or Lowes. Pond or aquarium supply outlets are another source, but are usually more expensive. A product called "Chicken or Poultry Grit" ( Figure 11 ) is another relatively low cost alternative that is essentially coarse pea gravel that is sold in 10 to 50 pound bags at local feed stores such as Agway or Southern States, just make sure that it's a 100% real stone formula as some can be derived from oyster shells or can have feed or other additives mixed in. It also comes in grit sizes ranging from 1 to 6 with 1 being the smallest size and 6 being the largest, so be aware of this if you buy and select grit size 3 or higher for the larger size pebbles.
Formula 3: For areas that receive more than 40 inches of annual rainfall. For these high rainfall areas, plants should be restricted to areas with overhead protection, such as a roof overhang or patio covering, but still open enough to allow full sun exposure for a minimum of 6 summertime hours. Container gardens are ideal in these situations for maximized evaporation. In addition, you should apply a lime fertilizer ( pelletized form is best ) to offset the acidity of your soil due to the high rainfall.
5 parts coarse pea gravel to 1 part washed fine sand. This formula has the highest ratio of coarse gravel to sand for the sharpest drainage and maximized air flow through the entire soil bed to prevent excessive moisture retention from not only rainfall, but to also offset the higher humidity as well. If using a container, select one that allows the air to flow through the sides and bottom, such as a wood cedar planter ( figure 12 ) or hypertufa container or trough ( figure 13 ) which are popular in alpine gardening. I recommend doing an Internet search on "hypertufa pots" for many online recipes for making your own. Some gardeners have also reported great success using hollow cinder blocks ( figure 14 ) filling in the cavity with the soil mixture. A great idea! In a larger setting, the blocks can be set in rows of successive height for a "great wall" effect. The skies the limit for creativity here, so have fun!
Figure 5: Red Lava Cinder Fines at a typical garden center
Figure 6 : Red Lava Fines close up, containing coarse pieces down to dust.
Figure 7. Washed Fine Sand. Should be nearly free of any fine dust or dirt and will not harden after getting wet and drying out.
Figure 8: Washed Play Sand - Home Depot
Figure 9: Coarse Pea Gravel
Figure 10: Bagged Pea Gravel - Lowes
Figure 11: Chicken Grit - Southern States
Figure 12: Wood Cedar Planters - Home Depot
Figure 13: Hypertufa Planter
Figure 14: Cinder Block Wall Garden
Figure 15 below is an example of how to set up your planting bed for Formula 1, which is a semi-arid to arid climate with an emphasis on trapping rainwater. The rainwater quickly passes through the upper layer where it's then directed into water traps by the substrate layer, where it's more slowly absorbed in very much the same manor as terracing. The substrate layer is a 50% mixture of formula 1 and 50% native soil and should be raised above the surrounding ground a minimum of 24 inches at it's highest point to avoid any long term pooling of water. If your native soil is a heavier clay type, you'll have to increase the ratio of the formula 1 to native soil mix. A good test is mix up a small batch and completely douse it with water to where it pools up about half inch deep. If it takes more than 30 seconds for the standing water to disappear, then increase the ratio. Place a minimum of 12 inches of formula 1 mixture over the substrate layer. Place anchor rocks or boulders evenly around to help stabilize the planting bed.
Figure 15: The formula 1 planting bed is set up like a mini terrace with an emphasis on rainwater retention. Plants on the surface will send roots down towards the rainwater traps to maximize their uptake.
Example of a formula 1 planting bed with terraced layers inside and retaining wall on the outside.
For Formulas 2&3, where the rainfall is far above the average they would normally receive, the drainage has to be sharp and evaporation quick. The formula mix should be a minimum of 12 inches in depth to allow the roots to spread out evenly. Two other key factors are the substrate layer and the type of retaining wall. The substrate layer should be raised a minimum of 24 inches above the surrounding ground and be smooth and sloping down to the foot of the retaining wall to direct the rainwater away from the center to the edge. Its composition is a 50% formula 2 or 3 mix to 50% native soil, which should result in a far more dense layer than the one above. The retaining wall is also key in that it should be made of a breathable or porous material, such as untreated wood timbers or cinder block. This encourages any water accumulation at its base to pass through quickly and allow the wind to dry its exposed surfaces, which then draws more moisture from the layers inside. To stabilize the bed, the anchor rocks should be more deeply placed into the substrate layer. This design mimics the natural setting as shown in the photo to the right in figure 17 where the Opuntia aurea is growing in a pocket of sand underlined by sloping bedrock. This setting eliminates any possibility of standing water and the soil dry's out quickly.
Figure 16: Diagram for a formula 2 & 3 planting bed that mimics the natural drainage as in the photo to the right.
Figure 17: Opuntia aurea growing in pure sand above a solid layer of sloping bedrock. Washington County, Utah.
Figure 18: Grizzly Bear cactus ( Opuntia erinacea var. ursina ) growing on a thin layer of rocky soil over sloping limestone bedrock. Clarke County, Nevada.
Figure 19: Large padded Opuntia dulcis and woodsii growing on a thin layer of rocky soil over sloping bedrock below. Washington County, Utah.