Growing Native Trees from Seed - October 3, 2001
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
Being a forester by training, I naturally gravitate toward trees and we have some truly spectacular native trees in the Verde Valley. There is something about a fine tree specimen that commands your respect: its age, size, appearance, feel, and an innate ability to cope with our highly variable environmental conditions. These species are also familiar to local wildlife and birds. Being adapted to our climate also means they will be somewhat drought tolerant (with the exception of riparian species). One warning: it takes some patience because most of these trees are not speedy growers and require close attention to environmental conditions for successful germination.
Late summer and fall are good times of year to collect seed from many of our native trees for propagation. Be sure you have permission to collect seed wherever you go. Trees have many adaptations for survival over their native range. Some seeds need some after ripening to germinate other need cold moist treatment to break out of dormancy. Think about it…without these adaptations, they may germinate at the wrong time of year and be killed by frost, heat, drought, hungry predators, or pathogens. Mortality is to be expected. If every seed produced grew into a mature tree, we would be climbing through a dense thicket just to get from our house to the road.
Almost all tree seeds require a treatment to break dormancy. Stratification is the most common. Stratification is a period of exposure to moist, cold (33 to 41 degrees F) conditions for a specified period of time (1 to 6 months). In species that require stratification, the treatment causes gradual and progressive internal physiological changes that lead to more successful germination. Simply put, stratification mimics a cold, damp winter. Commercial producers use fancy equipment, but at home you can usually stratify seed by wrapping it in a moist (not dripping wet) paper towel inside a closed ziplock bag and placing it in a corner of your refrigerator.
Other plants have hard seed coats that physically prevent germination by simply not allowing moisture to enter the seed. For these species, a nick in the seed coat with a file or sandpaper will increase germination. Sometimes heat is also used, especially in fire-adapted species. These techniques are called scarification. Commercial producers often used sulfuric acid to scarify large quantities of seed.
Once germinated, many tree species can be grown in pots until transplanting. However, some species will do best when grown directly in native soil, which allows them to put down deep roots following germination. Oaks, walnuts, and other large seeded plants often fall into this category. Seeds grown in native soil will also require less frequent watering. They may also require some protection from herbivores (rabbits, deer, etc).
Given this small amount of background information, I will provide some guidelines for germinating some of my favorite tree species. You may also just try a few different techniques to experiment on your own.
Arizona ash (Fraxinus velutina). Stratify for 90 days and sow covering with ¼ to ¾ inch of soil. Shading germinated seedling for a week or two may be desirable.
Arizona cypress (Cupressus arizonica). Cut mature, two year-old cones from tree and allow to air dry until scales separate. Seed will fall out of cone. Stratify for 21 days and sow covering with less than ¼ inch of soil.
Arizona sycamore (Platanus wrightii). Stratify for 60 to 90 days and sow covering with ¼ inch of soil or mulch. Protect from birds and small mammals. Arizona sycamore requires moderate to frequent irrigation.
Desert willow (Chilopsis linearis). Harvest mature seedpods just before pods open. Lay pods out to dry and collect seed when pods open. No stratification is necessary, but storage in wet sand for several days speeds germination. Sow ¼ inch deep.
Netleaf hackberry (Celtis reticulata). Harvest mature fruits and remove flesh by soaking in water and rubbing against a screen (this process is called wet maceration). Stratify for 60 to 90 days and sow covering with ½ inch of firmed soil. Protect from birds and small mammals.
Velvet mesquite (Prosopis velutina). Harvest mature seedpods just before pods open. Allow to dry then crush to extract seed. Many of the seeds will be likely be killed by weevils (small holes in seeds). Scarify seeds by nicking with a single edge razor or knife. Sow covering with ¼ inch soil.
Western soapberry (Sapindus drummondii). Harvest mature fruits in fall and remove flesh by soaking in water and screening. Stratify seed for 90 days and sow covering with ¾ inch of firmed soil. Germination may also be enhanced by scarification.
I do not encourage planting of the four following tree species: tree of heaven (Ailanthus altissima), Siberia elm (Ulmus pumila), salt cedar (Tamarix ramosissima), and Russian olive (Elaeganus angustifolia). In my opinion, these trees are invasive and undesirable. There are so many better choices, that I strongly discourage planting these species.
The University of Arizona Cooperative Extension has publications and information on planting and culture of landscape trees. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: March 15, 2001
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