Essentials for Tree Seed Propagation

Trees use seeds as a principal means of establishing their next generation in the natural world. Seeds serve as a delivery system for the transfer of genetic material from one generation to the next. This fascinating chain of events (the formation of seed to dispersal to germination) is very complex and still poorly understood. Some trees can easily be grown from seed but, for some trees, it may be much quicker and easier to propagate them from cuttings. Seed propagation can be a tricky process for a number of tree species. A small seedling can be very tiny and delicate when first germinated and often require much more care than a cutting. Seeds collected off tree hybrids or grafted stock can be sterile or the tree may  be off-character from the parent. For example, seeds collected from a pink dogwood will most likely flower white. What Stops Seeds From Germinating There are a number of important reasons a seed refuses to germinate under artificial conditions. Two major causes for unsuccessful tree seed germination are hard seed coats and dormant seed embryos. Both conditions are species-specific and every tree species has to subject the seeds to unique conditions to assure germination. Treating the seed properly is necessary before germination occurs and a seedling can be assured. Seed scarification and stratification are the most common methods of seed treatment and they will increase the chances of seed or nut germination. Scarification and Stratification The hard protective coating on some tree seeds is natures way of protecting the seed. But hard coats on some hard seed species actually inhibit the germination of the seed, because water and air cannot penetrate the hard coating. Interestingly, many tree seeds require two dormant periods (two winters) before the protective coating breaks down enough to germinate. The seeds must lay on the ground completely dormant for one full growing season, and then germinate the following growing season. Scarification is an artificial way to prepare hard seed coats for germination. There are three methods or treatments that will usually make seed-coats permeable to water: soaking in a solution of sulfuric acid, soaking in hot water or immersing the seed for a short period in boiling water, or mechanical scarification. Many dormant tree seeds need to be after-ripened before they can germinate. This is the most common cause of seeds failing to germinate. If the seed embryo produced by a tree is dormant, it must be stored at the proper temperature and in the presence of abundant supplies of moisture and air. Stratification is the process of mixing the seed in a moist (not wet) medium like peat moss, sand or sawdust, then placed in a storage container and stored in an area where the temperature is controlled at a low enough level to ripen the seed. This storage is usually over a definite period of time at a specific temperature (around 40 degrees F). Methods of Tree Seed Treatment by Species Hickory: This tree nut is generally considered to exhibit embryo dormancy. The common treatment is to stratify the nuts in a moist medium at 33 to 50 degrees F for 30 to 150 days. If cold storage facilities are not available, stratification in a pit with a covering of about 0.5 m (1.5 feet) of compost, leaves, or soil to prevent freezing will suffice. Prior to any cold stratification, nuts should be soaked in water at room temperature for two to four days with one or two water changes each day.Black Walnut: A walnut is generally considered to exhibit embryo dormancy. The common treatment is to stratify the nuts in a moist medium at 33 to 50 degrees F for two or three months. Although the seed coat is extremely hard it usually cracks, becomes water permeable, and does not need scarification.Pecan: A pecan does not fall into dormancy like other hickories and can be planted at any time with the expectation that the embryo will germinate. Still, the  pecan nut  is often collected and cold-stored for planting the next spring.Oak: Acorns of the white oak group generally have little or no dormancy and will germinate almost immediately after falling. These species should usually be planted in the fall. Acorns of the black oak group that exhibit variable dormancy and stratification are usually recommended before spring sowing. For best results, moist acorns should be held for four to 12 weeks at temperatures of 40 to 50 degrees F and can be placed in plastic bags without a medium, if turned frequently.Persimmon: Natural germination of common persimmon usually occurs in April or May, but two- to three-year delays have been observed. The main cause of the delay is a seed covering that causes a major decrease in water absorption. Seed dormancy also needs to be broken by stratification in sand or peat for 60 to 90 days at 37 to 50 degrees F. Persimmon is hard to artificially germinate.Sycamore:  American sycamore  needs no dormancy, and pre-germination treatments ar e usually not required for prompt germination.Pine: Seeds of most pines in temperate climates are shed in the autumn and germinate promptly the next spring. Seeds of most pines germinate without treatment, but germination rates and amounts are greatly increased by pretreating the seeds. This means storing seeds using moist, cold stratification.Elm: Under natural conditions, elm seeds that ripen in the spring usually germinate in the same growing season. Seeds that ripen in the fall germinate in the following spring. Although seeds of most elm species require no planting treatment, American elm will remain dormant until the second season.Beech: Seeds from beech trees need to overcome dormancy and require cold stratification for prompt germination. The seeds may take a combination of stratification and storage. Seed moisture level is the key to successful stratification in beech seeds. Beech is difficult to artificially germinate in significant amounts.