Germination of Tree Seed
Growing trees from seed can be fun. However, the seed of most tree species won't germinate immediately when planted because they are in a dormant state. Dormancy must be broken before the seed can germinate.
In some tree species, dormancy is the result of a thick, hard seed coat. The seed coat may be broken in a variety of ways and the process is referred to as scarification. Mechanical means, such as a metal file or coarse sandpaper, can be used to break the seed coat. Treatment with boiling water has also been successful for a number of tree species. In nature, the seed coat may be broken by microbial action, passage of the seed through the digestive tract of a bird or other animal, exposure to alternate freezing and thawing, or fire.
The seed of many tree species will not germinate until they have been exposed to cool temperatures and moist conditions for several weeks or months. Winter weather in Iowa provides the necessary conditions to break dormancy. Gardeners can accomplish the same results by a process called stratification. Tree seed can be stratified by placing the seed in a moist 50:50 mixture of sand and peat moss. Suitable containers include coffee cans, plastic jars, and cottage cheese containers. (Punch holes in the lid of the container to provide air.) Seed can also be stratified in plastic bags. Stratify the seed in the refrigerator.
The seed of some trees, such as redbud, have hard impermeable seed coats and dormant embryos. They require both scarification and stratification for germination.
Specific information on collecting and planting seed from several tree species follows.
Maples (Acer species)
When mature, maple fruit (samaras) turn from green to yellow or brown and fall to the ground. Collect mature fruit from the lawn, driveway, or gutters. There is no need to remove the seed from the fruit.
The fruit of red (Acer rubrum) and silver maple (Acer saccharinum) mature in late spring or early summer. Neither requires a pregermination treatment and should be planted immediately. The fruit of most maple species mature in the fall. Sow seed directly outdoors in the fall or plant stratified seed in the spring. Seed of the sugar maple (Acer saccharum) should be stratified for 40 to 90 days at 33 to 41 F, while seed of the Norway maple (Acer platanoides) require 90 to 120 days at 41 F. Plant the seed (fruit) 1/4 to 1 inch deep.
Horsechestnuts and Buckeyes (Aesculus species)
Gather the fruit (capsules) of the horsechestnuts and buckeyes as soon as they fall to the ground. Dry the fruit at room temperature until the capsules split open, then remove the shiny, dark brown seeds.
Plant the seed in the fall or stratify the seed and plant in the spring. The Ohio buckeye (Aesculus glabra) and common horsechestnut (Aesculus hippocastanum) should be stratified for 120 days at 41 F. Seed should be planted 1 to 2 inches deep.
Hickories (Carya species)
Collect the fruit as they fall to the ground. Remove the husks by hand. Sow the seed in the fall or plant stratified seed in the spring. Prior to stratifying, soak the nuts in water at room temperature for 2 to 4 days (change the water once or twice a day). Then stratify the nuts of the shagbark (Carya ovata) and bitternut (Carya cordiformis) hickories at 33 to 40 F for 90 to 120 days. Pecans (Carya illinoiensis) require only 30 to 90 days of cold stratification. Plant the seeds 3/4 to 1 1/2 inches deep.
Redbud (Cercis canadensis)
Collect the pods of redbud when they turn brown in the fall. Air dry the pods, then remove the seed. Seed of the redbud have hard, impermeable seed coats in addition to dormant embryos. Seed require both scarification and cold stratification before they will germinate. Redbud seed can be scarified by soaking seed in concentrated sulfuric acid for 30 minutes or by submerging seed in boiling water for one minute. Once scarified, the seed should be stratified at 35 to 41 F for 5 to 8 weeks. Seed should be planted promptly at a depth of 1/4 inch.
Kentucky Coffeetree (Gymnocladus dioicus)
Collect the fruit (pods) when they drop to the ground in the fall. Remove the seed by hand. The seed of Kentucky coffeetree have hard, thick seed coats. The seed coat can be broken by filing through it with a hand file. Plant the seed in the spring at a depth of 1 inch.
Black Walnuts (Juglans nigra)
Collect walnuts after they fall to the ground. Remove the husks, then place the nuts in water. Those nuts that float on the water are not viable and can be discarded. The good, viable nuts will sink to the bottom. Sow walnuts in the fall or stratify the nuts at 34 to 41 F for 90 to 120 days and plant in the spring. Walnuts should be planted 1 to 2 inches deep.
Apples and Crabapples (Malus species)
Most apples and crabapples will not reproduce true from seed. They are usually propagated by grafting and budding. However, the seed of most apples and crabapples are viable and can be germinated.
Collect fruit from trees as they ripen. Remove the seed. Sow the seed in the fall or stratify the seed for 60 to 120 days at 37 to 41 F and plant in the spring. Sow the seed 1/2 to 1 inch deep.
Cherry, Peach, and Plum (Prunus species)
Harvest fruit when full mature. Remove the seed. Seed may be sown in the fall or stratified seed may be planted in the spring. Stratify the seeds at 33 to 41 F. The sour cherry (Prunus cerasus) requires 90 to 150 days, the peach (Prunus persica) 98 to 105 days, and European plum (Prunus domestica) 90 days. Seed of plums and peaches should be planted 2 inches deep. Sow the seed of sour cherry at a depth of 1/2 inch. (Like the apples and crabapples, the seed of most cultivated cherries, plums, and peaches will not reproduce true from seed.)
Oaks (Quercus species)
Ripe acorns should be collected as soon as they fall to the ground. Sound, viable seed can be separated from damaged or unfilled acorns by placing them in water. Sound acorns will sink. Most of the floating acorns are not viable and can be discarded.
The acorns of white oak (Quercus alba) and swamp white oak (Quercus bicolor) should be planted in the fall. They will germinate immediately after sowing.
Acorns of bur (Quercus macrocarpa), pin (Quercus palustris), and red (Quercus rubra) oaks can be planted in the fall or stratified seed can be sown in spring. Stratify the seed at a temperature of 32 to 41 F. Acorns of the bur oak require 30 to 60 days, while red and pin oaks require 30 to 45 days.
Excellent references on the propagation of trees and shrubs include
- The Reference Manual of Woody Plant Propagation by Michael Dirr and Charles Heuser Jr.,
- Plant Propagation: Principles and Practices by Hudson Hartmann, Dale Kester, Fred Davies Jr., and Robert Geneve, and
- Seeds of Woody Plants in North America by James Young and Cheryl Young.
This article originally appeared in the August 11, 2000 issue, pp. 102-103.