Starting trees from seed can be one of the most rewarding gardening activities, but tree seeds often require a little more preparation than many common flower or vegetable seeds.
In most cases, there are two ways to start tree seeds: The natural way, which often includes sowing the seeds in the fall, or through forced or “assisted” germination, which is initially done indoors.
The Natural Way to Germinate Tree Seeds
Seeds have been sprouting and trees have been growing for an awfully long time without any help from humans. The “natural way” to germinate tree seeds, then, is to allow nature to take its course. Most seeds, when sown in the fall without any pre-treatment, will begin to germinate the following spring. Be sure to sow the seeds at the recommended depth. If the seeds are planted too deep, this could delay or inhibit the spring germination process. With some seed varieties you may see germination spread over two or three years with some seeds germinating in the first spring and others taking longer to break dormancy and germinate.
It is important to remember that many species originated in cooler climates where seeds drop to the ground and are covered by leaves in the fall. Over the winter, the seeds remain bedded in this cool moisture environment. As the warm spring weather arrives the seeds then begin the germination process. For many types of seeds, the embryo inside the seeds is immature and unable to germinate (this is called ‘dormancy’) until it matures in this manner. The delay in the germination process is vital to the survival of many tree species. In a natural forest, if seeds germinated immediately upon falling to the ground in late summer or fall, the tender seedlings would die off during the cold winter.
Forced or “Assisted” Germination
Although natural germination is an acceptable way to start most tree seeds, sometimes better and more consistent results can be achieved through forced or “assisted” germination. Basically, it means using various techniques to mimic the role nature plays in causing tree seeds to germinate.
There are several techniques that may be involved to force the germination of any given tree seed. Please carefully read the recommended steps listed on each individual seed package.
Many seeds require one or more treatment steps to stimulate the germination process. The three steps are: 1) Scarification, 2) Cold Stratification, and 3) Warm Stratification. Keep in mind that not all seeds require all of these steps. In fact some seeds do not require any pre-treatment whatsoever.
Scarification
Scarification is the process of reducing or breaking the seed coat so that moisture can penetrate and the embryo can begin the germination process. Scarification is commonly required on seeds with dense or hard seed shells. Many tree seeds do not require any scarification, and for those that do, the most common treatment is a simple water soak.
Hard seed coats can be broken down by a) a water soak, b) a physical or mechanical breaking of the seed coat, or c) a chemical or acid treatment (not commonly required).
a) Water soak: Pour water over the seeds and let them soak for the recommend time, often 6 to 24 hours. Most water treatments are done using room temperature water. It is best to use a glass container for soaking the seeds. Some seeds may require hot water as per instructions. Follow the above noted directions, using water at the recommended temperature.
b) Physical/Mechanical: Using a small file or sandpaper, rub the outside of the seed coat to reduce its density or to nick the seed coat so that moisture can more easily penetrate to the embryo. Take care to avoid damaging the seed embryo.
c) Chemical (Acid) Wash: The chemical wash method of scarification is generally used by commercial growers for select seed varieties and is often not required for home gardening purposes. If you are attempting it, you may want to consult a more detailed protocol and follow these basic guidelines:
- Wear goggles and protective clothing. Wash immediately if any is spilt on your skin
- Use a large glass jar or vessel
- Place seeds in the dry glass container
- Add the sulphuric acid concentrate at a volume about twice the volume of the seeds
- Stir the mixture with a glass rod
- Periodically check the seed for coat thickness by extracting a few seeds and cutting in half with pruners. Even in the same lot, the coat thickness may vary from seed to seed.
- After soaking the seeds, decant acid and seeds through a screening device and wash for 5 to 10 minutes under cold water
- Spread the seeds on a paper and allow to dry at room temperature. - be sure to spread the seeds out so that they do not clump
Cold Stratification
Stratification is the process of mimicking the natural over-wintering process by exposing the seeds to cool, moist conditions. The easiest way to undertake the stratification process is:
- Take a few handfuls of peat moss and soak it in water until it is saturated
- After soaking, use your hands to squeeze out as much water as possible
- Place a layer of the moist peat moss in the bottom of a zip-lock plastic sandwich bag
- Place the seeds on the layer and fill the rest of the bag with the peat moss
- Seal the bag closed
- Store the sealed bag in the bottom of the refrigerator for the appropriate stratification time.
During the cold stratification process, occasionally check the seeds for signs of early germination. If the seeds begin to germinate in the refrigerator, remove them and plant as normal.
After the prescribed stratification time in the refrigerator, remove the seeds and sow them in the normal manner.
Warm Stratification
The warm stratification step is designed to mimic the seed’s summer dormancy when it is often imbedded in warm damp soil or mud. For warm stratification, follow the same steps outlined in cold stratification, except place the zip-lock bag in a warm location at or slightly above room temperature for a target temperature range of about 72 to 86 degrees F. (Often placing the bag on top of the refrigerator achieves this.)
During the warm stratification process, occasionally check the seeds for signs of early germination. If the seeds begin to germinate, plant as normal.
Planting the Seeds
Seeds may be sown into individual containers or into seed trays. It is important to ensure that the seeds are planted at the recommend soil depth. Most tree seeds are planted much shallower than other annual seeds, but it typically depends on the size of the seed. Please follow the directions on each seed packet for appropriate planting depth. The seeds should be sown in a well-drained medium, such as a mixture of peat moss and vermiculite.
When sowing the seeds, fill the container or seed tray to about ½ inch form the top with the moist medium (soil). Level the medium by gently shaking or taping the container.
For larger seeds – those over a 1/3 of an inch tall, press half the seed into the medium. For smaller seeds, sprinkle them lightly over the surface of the soil. Cover the seeds with a fine layer of sand to a depth about the thickness of the seed.
After planting the seeds, gently water them and keep them moist but not wet. Maintaining high moisture and relative humidity is critical to germinating seeds. You can increase the humidity by enclosing the seed tray in a plastic tent. Be sure to poke some holes in the plastic cover to ensure adequate air circulation. Keep the trays in a warm but dimly lit location.
Germination can be as quick as a few days or as slow as several months, depending on the species and the environmental conditions. Once the seeds germinate, move the seedlings to a brighter location. You may need to nurse the seedlings indoors for a few months before planting outdoors. Try to give the young plants as much sun light as possible.