The seeds of many deciduous and evergreen trees and woody ornamentals (shrubs) along with some perennials and herbs will not germinate without a pre-treatment known as stratification. The following paragraphs will explain this simple process.
The pre treatment of seeds (warm, cold, or variable stratification) is a surprisingly simple measure that one uses to break a seed’s dormancy to enable the seed to germinate. By subjecting the seeds to the required pre-treatment, you are providing them with the natural effect that Mother Nature would have provided had the seeds been left to their natural course. By applying the required treatment yourself in a controlled environment such as a refrigerator or cold frame, you are better able to control the environment and reduce factors that would have been detrimental to a seed had it been left to make it on its own in nature. By controlled stratification of the seeds, you are also able to control the time frame under which the seeds will germinate. By not stratifying the seeds that require this treatment, you will have to be content to accept nature’s timeframe which could result in a year or more delay. Or, you may have no germination at all.
The term stratification, is derived from the age old practice of stimulating seed to germinate by placing alternate layers of a moist media and seed. This media may be sand, vermiculite, perlite, peat, composted bark, sawdust, or potting media. The actual stratification involves placing the seed in this moist medium to simulate the natural conditions it would normally receive from its native environment.
TYPES OF STRATIFICATION:
There are several types of stratification. Which type is proper for your seed would depend on what that seed would normally experience in nature. If you have purchased seeds from us, the proper type of stratification is listed on the seed package with brief details of that process. Reading through the following text should give you a better understanding of the required process.
Warm-moist. Seeds from trees and shrubs that ripen their seeds in the early fall, as a rule, require a warm moist treatment to induce germination.
Cool-moist. Seeds that ripen in the late fall or early winter, as a rule, require a cool moist treatment. This period of time is usually between 1- 4 months.
Warm-moist-cool-warm. Depending on the species, some need a combination of warm and cool treatments followed by a warm to germinate.
Perhaps you may have tried, without any success, to germinate seeds from trees or from woody ornamentals. The seeds of most trees, shrubs, and some perennials are incapable of germinating immediately after they ripen. Nature has provided this built-in system for various reasons, mostly to protect the species. For example, if a seed that dropped in late fall were to germinate at once, it would not be equipped to survive the up coming cold winter and die. Further, some seed embryos are incomplete and require a period to complete the development of these immature parts. Some seeds have a mechanical barrier to water, which is required for germination. These may require scarification. More on this later. Some cannot germinate because of a built in physiological barrier that inhibits germination. Stratification is used (among other methods) to remove mechanical moisture barriers and physiological blocks.
SCARIFICATION OF SEED
The seed coat of some seeds are tough and must be penetrated by special means. Hard seed coats must be scarified. Scarification involves breaking, scratching or softening the seed coat to allow moisture penetration. In nature, this occurs as the seed exposed to freezing temperatures or microbial activities that modify the seed coat during the winter. Scarification can also occur as seeds pass through the digestive tract of various birds and animals. Methods of scarification commonly used are mechanical, hot water, and acid. Best method is usually listed on the seed packet. If in doubt, please contact us for the best method.
Mechanical scarification involves breaking or weakening the seed coat with a file or sandpaper. Following the scarification, the seeds should be dull in appearance, but not deeply pitted or cracked as to damage the embryo. Once scarified, seeds will not store well and should be planted as soon as possible after treatment. This treatment works well for larger seeds. Smaller seeds may be rubbed between sheets of 120 grit sandpaper. If the seeds are too small to see the progress, a different method such as soaking should be used.
Hot water scarification involves placing seeds in water that is 170 to 210 degrees F. NOT BOILING! After the water cools, seeds should continue to soak for 12 to 24 hours. Use 10-20 times the volume of hot water as seed. Seeds with a waxy coating such as northern Bayberry should be washed several times in very hot water to remove the wax before the final soaking.
Acid treatment requires SAFETY measures that should not be overlooked. There are two different acid treatments. One uses concentrated sulfuric acid and the other uses common household vinegar. Sulfuric acid can be very dangerous for an inexperienced individual and should be used with extreme caution! Vinegar is safer, of course, but may not be strong enough for all species and requires a longer soaking time. The technique is the basically the same as with sulfuric acid: Seeds are placed in a glass (Do not use any other type) container and covered with the proper concentration of sulfuric acid. NOTE: NEVER, EVER add water to acid! ALWAYS add the acid to the water!!! The seeds are gently stirred with a glass rod and allowed to soak for 10 minutes to several hours, depending on the species. If this method is recommended by us, then the appropriate time is listed on the seed packet along with the proper acid concentration. Otherwise, there are reference books available which list proper soaking times. When the seed coat has been soaked for the proper amount of time, the seeds are removed, washed, and stratified. NOTE: We accept no responsibility for any accidents or injury resulting from the use of acid or chemicals.
Stratifying Your Seeds
Gather up all the necessary items before beginning. You will need:
Sterile sand, vermiculite, perlite, or peat moss
Plastic labels and waterproof pen (such as a Sharpie)
Sterile Plastic Zip Lock freezer bags or plastic containers with tight fitting lids. The semi-disposable plastic containers by Glad or Rubbermaid work well and are durable, uniform, and inexpensive
Step 1: Label.
Label both the containers and plastic stakes with a waterproof pen. Include the stratification date started and when they are due to be removed from stratification.
Step 2a: Scarification.
If the seed requires mechanical scarification, follow the directions either on the seed packet or above.
Step 2b: Soaking.
A seed needs to begin absorbing moisture before it will sprout. Soak in warm to hot (not boiling) water overnight before placing them into stratification unless other wise instructed on the seed packet.
Step 3: Mixing your stratification medium.
We use medium grade vermiculite.
Use 10-20 times the volume of medium vs. seed volume. Your stratification medium should be moist to the touch, but not soaking wet. A ratio of 1 part water to one part medium, mixed well, and allowed to drain excess water through a strainer is a good mix. What to use: We use medium grade vermiculite for all of our stratification. You can also use a vermiculite-perlite mixture, sterile sand, or sterile milled sphagnum peat moss. If sawdust is used, it should be aged and not fresh. Never use walnut sawdust as it contains chemicals that will prevent your seeds from germinating. Moisture is a major factor in the stratification process. In theory it is possible to increase the amount of moisture in the medium to speed up the stratification process. As a rule: you should not be able to squeeze any dripping water out of a handful of medium after thoroughly and uniformly moistening it. Do not squeeze vermiculite excessively hard as it will ruin the structure. High moisture levels in the sealed containers can cause fungus growth that can harm the seed. The use of a fungicide when mixing the medium won’t hurt. It is easier to prevent than to cure! You can expect some of the seeds to germinate in the sealed containers during stratification. If this happens, remove them and sow. These seeds will grow normally, if they are planted carefully.
Step 4: Stratify!
Make note on your calendar when you place your seeds into stratification and when to remove them. Place a label inside the container as well as one on the outside. Place the sealed containers into the proper cold or warm stratification environment.
Cold: 32-40 degrees F. The vegetable compartment in your refrigerator is a good spot.
Warm: 62-75 degrees F. On top of your refrigerator is a good spot. Keep out of the sun!
Check the next day and pour out any standing water.
Check every 2 weeks. If the mix starts to dry, add enough water to moisten, squeeze out any unused moisture, and re-seal the container. Re-check again in one day for standing water.
IMPORTANT: Once stratification is started, a time clock begins ticking in the seed and the shelf life begins to decrease. Seed should not stay in stratification many months past its recommended stratification time as it will use up its energy reserves and die.
Step 5: Remove and Sow
Don’t be overly concerned about exact lengths of pre-treatment time. If it is recommended that a particular species of seed will benefit from 2-3 months cold stratification, this means that past experience finds that this seed’s dormancy is usually overcome by approximately this length of cold stratification and the seeds are more susceptible to germination and will generally sprout at a more even rate. Within reason, the longer the pre-treatment… the more uniformity in germination rates after sowing. After the required time period (or after 20% germination is noticed), remove the seed and sow in flats or outside in the spring.
SOWING AND GROWING
When the time has come to sow after treatment, you have a choice of planting direct into nursery beds in the garden or into flats. Nursery beds should be shaded in the afternoon and be weed seed free. The soil should also be friable with lots of organic matter and not prone to crusting over after a rain. We use standard 11 x 21″ flats filled with a seed starting mix such as Jiffy Mix or Pro Mix. Media should be evenly moist, but not soaking wet. Do not sow the seeds too thickly as this can lead to poor air circulation and damp off problems. We do not separate the stratification medium from the seeds. The mixture is spread over the surface of the media in the flat and covered with 1/4 – 1/2″ of fine perlite or sand. After planting, immediately water in the seed to facilitate good seed to soil contact, and keep the seed properly moistened. These flats are then placed over bottom heat of 75 degrees F and kept moist, but not wet. After germination, provide good air circulation and do not let the seedlings dry out. Watering from the bottom will help to prevent fungal diseases from forming on the surface. Use of a fungicide drench is recommended. Once the seedlings have developed their first set of true leaves, transplant to 3 x 3 x 3 pots or nursery beds. Trees should be transplanted to Root-Trainer pots or directly to a nursery bed. Do not allow to become pot bound. Shift to larger containers or nursery beds as required.
FIRST YEAR CARE AND WINTER OVER
Special precautions are necessary on most species the first winter. In nature, these fledging seedlings are naturally protected by fallen leaves and grass. Of course, those then are not, are eaten by critters or succumb to winter’s cold. A mulch of shredded leaves or straw will protect them from nature over winter. Apply this after the ground is frozen and remove in very early spring. Most can be transplanted to their final location the second spring after sowing.
If you don’t want to stratify in the ‘fridge, then you can always rely on nature’s way and sow in the Fall (Autumn). The Fall planting of seeds directly into a nursery bed for germination the following Spring usually satisfies a seed’s requirement for cold stratification. This means you can skip all the above outlined pre-treatment in your refrigerator as the over-wintering of the seeds in the earth accomplishes the same thing. As a rule, the results are as good (and some instances better) as those resulting from seeds which have undergone artificial cold stratification and is a widely practiced means of germination by commercial nurseries. Depending on the species and variable depth of dormancy some seeds will sprout in the second and third Spring after sowing. Don’t give up on any seeds that you planted but did not germinate immediately. This is a frequent outcome and they will more often then not come up in the next spring..
Some Examples of Seed Treatments
Cold stratify 3-4 months See individual seed packets for exact times.
1 to 4 months of cold-moist stratification. Seeds will germinate in 30 to 60 days. See individual seed packets for exact times.
Artificial stratification is recommended. Germinating holly seeds can be very difficult. Germination is extremely slow under outdoor conditions; it may take 2 to 3 years because of the holly’s hard seed coat and an immature (rudimentary) embryo. See individual seed packets for exact times.
Germination is inhibited by an impermeable seed coat and embryo dormancy. Soak for 30 minutes in vinegar followed by 3 months cold stratification. Mechanical scarification will yield satisfactory results. See individual seed packets for exact times.
Spring-maturing seeds of such species as red and silver maple should be collected immediately when mature, not permitted to dry, and sown immediately. For seeds of other species which mature in the fall, cold-moist stratification for 90 to 120 days is necessary. See individual seed packets for exact times.
Acorns of white oak do not become dormant. When planted in the fall, roots will emerge during winter; shoots will emerge in the spring. Acorns of red oaks should be planted in the fall or stratified for 1 to 3 months. See individual seed packets for exact times.
The seeds have a waxy coating which must be removed and can be accomplished by rubbing against a screen. Follow this with several soakings in very warm water with a small amount of Dawn dishwashing soap. When all wax is removed, rinse, soak and stratify. Cold stratify for 90 days Seed may start to germinate in cold stratification. Sow seed 1/8″ deep
Willows. (salix) No stratification necessary! Collect and/or sow at once on the surface of media kept moist at all times. Seed life of most types is one to two weeks.