Psorothamnus spinosus

( soar - oh - THAM - nus     spin - OH - sus )

At-A-Glance Information

Commonly called Indigobush, Smoke Tree, Smokethorn, or Corona de Cristo and known alternatively under the botanical name Delea spinosa, this species of deciduous trees and shrubs needs full sun and little to no water, making them wonderful choices for water-wise gardens. This plant reaches 10-20 feet tall and 10-15 feet wide and has small, hairy, white leaves that drop off the plant in early spring, leaving behind spiny branchlets that make the tree look like it is smoldering. In late spring, clusters of violet-blue flowers bloom (and are great to cut for arrangements). These plants are native to Arizona, California and Baja California. It is common in Joshua Tree National Park. Smoke Tree grows best in zones 11-13 (to find your zone, click here: Zone Map). The genus Psorothamnus is a member of the plant family Fabaceae.

Caring for and pruning Psorothamnus spinosus shrubs

Plant Smoke Tree shrubs in full sun. These plants love hot, dry, desert-like conditions, so it should be getting plenty of direct sunlight. Ensure that soil is well-draining; sandy or gravelly soil is ideal, they do not do well in clay, heavy soils. Soil does not need to be nutrient-rich, Smoke Tree is well-adapted to poor soil conditions. These are highly drought-tolerant plants once established. Water only sparingly, allowing the soil to dry out completely between waterings. Overwatering these shrubs can quickly lead to problems like root rot. These shrubs do not need to be fertilized, and adding fertilizer to the surrounding soil can actually do more harm than good. Apply organic mulch around the base of the plant to increase soil moisture retention and suppress weeds. These shrubs are hardy down to 20-25 degrees Fahrenheit, and will need protection from lower temperatures. Prune your Smokethorn shrub as needed to remove dead, damaged, or diseased branches and foliage, and to shape the plant. Prunings are best completed in late winter to early springtime.

Propagation

Psorothamnus spinosus can be propagated either by using cuttings or from seed. To ensure that your propagated plant will have the exact same qualities and characteristics as the parent plant, use the cutting method. If you would like to experiment and see what plant you can grow from seed, just keep in mind that it will not necessarily be the same as the parent plant. To propagate your shrub, follow the instructions below:

Propagating Psorothamnus spinosus by Cutting
  1. Gather your materials. You'll need a clean, sanitized, sharp pair of garden shears, rooting hormone, a pencil, and a 3-inch pot filled with well-draining compost or other organic material. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) moss peat and horticultural-grade sand or grit sand.
  2. In spring or early summertime, choose the plant that you'd like to propagate. Ensure that this plant has had a good watering the night before, as this will improve the chances your cutting will take root. Cut one or more 6 inch portion(s) of the end of a non-blooming branch and strip the leaves off of the lower half. Make a hole in your planting medium by pushing your pencil into the soil. Dip the end of the stem in rooting hormone, and plant it in your pot. Fill in soil around the cutting to support it, but do not pack it down too much.
  3. Moisten with water, and ensure that the pot has good drainage. Your pot should be kept in warm, sunny conditions indoors (and not in direct sunlight). Water whenever your soil is dry under the top layer. After about four to six weeks have passed, your cutting should be rooted. When you notice new growth, you'll know you have roots.
  4. Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring, when you can transplant it to its new home. Ensure that the last frost of the year has occurred before doing this, as it can seriously damage your new shrub.

Propagating Psorothamnus spinosus by Seed
  1. Keep in mind that the plant you get from seed will not necessarily mimic all of the characteristics of the parent plant. To propagate by seed, you will need a paper bag, a razorblade or an abrasive surface (like sandpaper), a pot or tray filled with well-draining compost or other organic material, a spray bottle and a fork. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) moss peat and horticultural-grade sand or grit sand. You will also, after a few weeks, need individual 3-inch pots for each seedling that grows.
  2. Harvest your seeds. The seed pods are pretty easy to spot, they're large and dangle from the branches, left behind after flowers have faded. Once they have dried out and begun to split open, you can harvest the seeds inside. Cut them off of the plant, placing them in a paper bag. Harvest as many as you can, and place the paper bag in a dry, cool, dark area. They will begin to split open and release the seeds contained inside. Once they have been released, take them out of the bag and put them in a container so you can work with them.
  3. Scarify your seeds by rubbing them against an abrasive surface or cutting them with a razorblade. to prep them for germination. Water your prepared seed tray or pot filled with soil, and ensure that good drainage is occurring. Place your seeds in your prepared pot or seed tray about a quarter inch to a half-inch under the soil surface. Cover them with soil and water with your spray bottle. Place your tray of seeds in a warm area that receives bright, indirect light. A greenhouse is the most ideal climate, but a planting tray inside a window will do as well.
  4. Germination usually takes a few weeks to a couple of months, and the average germination rate falls around 50%. Once you begin to see seedlings sprout and they are able to be handled, transplant them to their own individual 3-inch pots, filled with the same potting medium as described in Step 1. Loosen the seedlings from the potting medium by digging down with the tines of your fork about an inch and a half, gently working the seedling up until it is out. Then, replant the seedling in its individual pot.
  5. Keep the seedling's potting medium moist, but not wet, by spraying the soil with your spray bottle. Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring (after the danger of frost has passed), when you can transplant it to its new home.

Problems and Pests

Psorothamnus spinosus plants are generally pest and disease-free.
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Gallery

These shrubs are called Smoke Tree because when they are out of leaf and flower, they appear to be plumes of smoke in the landscape.
In springtime, these shrubs and trees are covered in small dark blue flowers.
These trees do well in areas that are dry and desert-like.

Videos

All about Smoketree.
Learn about Corona de Cristo.

Resources

Psorothamnus spinosus (Smoketree)
Psorothamnus spinosus - Smoke Tree
Wikipedia: Psorothamnus spinosus
The New Sunset Western Garden Book: The Ultimate Gardening Guide (2012) - Page 538, Psorothamnus spinosus
If you have any questions, comments, concerns, or photos you’d like to add to the gallery, please let me know by sending me a message using the button below!
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