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Stratification (botany)


In horticulture, stratification is the process of pretreating seeds to simulate natural conditions that a seed must endure before germination. Many seed species have what is called an embryonic dormancy and generally speaking will not sprout until this dormancy is broken. 

For seeds of trees and shrubs from temperate climates, stratification involves soaking and chilling seeds prior to sowing. This simulates natural conditions where the seeds would remain through a winter on cold, wet ground. Seeds will usually germinate promptly and uniformly after stratification. Unstratified seeds may take up to two years to germinate, if they do so at all. 

In the wild, "seed dormancy" is usually overcome by the seed spending time in the ground through a winter period and having their hard seed coat soften up a bit. By doing so the seed is undergoing a natural form of "cold stratification" or pretreatment. This cold moist period triggers the seed's embryo, its growth and subsequent expansion eventually break through the softened seed coat in its search for sun and nutrients. In its most basic form, when we control the cold stratification process, the pretreatment amounts to nothing more than subjecting the seeds to storage in a cool (not freezing) and moist environment for a period found to be sufficient for the species in question. This period of time is often and usually found to be somewhere between 1 through 3 months. 

To accomplish this you merely place your tree seeds in a sealed plastic baggie with moistened vermiculite (or peat) and place in a refrigerator. Use three times the amount of vermiculite as seeds. It is important to only slightly dampen the vermiculite. Excessive moisture can cause the seeds to mildew and grow mouldy in the baggie. As such, err on the side of drier rather than wetter. To give you an idea-you should not be able to squeeze any dripping water out of a handful of vermiculite. 

The bottom vegetable crisper in the refrigerator is an ideal place to cold stratify seeds. After undergoing the recommended period of cold stratification, the seeds are ready to be removed and sown in the nursery bed for germination. 

Sanitary measures

Many sources recommend using peat when cold stratifying seeds in the belief that peat is highly sterile and pathogen free. This has not been my experience. Using it often leads to fungus growing on the seeds and this is especially the case if too much moisture and/or no fungicide has been applied. This fungus can either cause the seeds so much injury as to prevent germination and can attack those parts that emerge from the seeds. My advice is to use vermiculite which is sterile, inexpensive, and found in just about any garden center. If you do use peat keep a closer watch on your seeds while they are undergoing the pretreatment and do be sure to use a fungicide. 

Preparing Your Stratifying Medium

Many sources do not mention that using a fungicide to moisten your stratifying vermiculite is a great help. I always use a liquid fungicidelike "DAMP OFF" or a similar product which is inexpensive, comes in a little bottle and again, usually found in any garden center. 

One moistens the stratifying medium by applying the fungicide to the vermiculite (or peat) by using a spray/spritzer bottle according to the diluted strength indicated on the "DAMP OFF" bottle, however exact mixing ratios are not critical here. If you are pretreating many seeds you should spread your seeds/stratification mixture into a few different baggies rather than putting them all into one baggie. That way if you have a fungus outbreak it will be restricted to only some seeds. If you forego the use of fungicide and just use water to moisten your vermiculite-keep a closer check on them. 

If you do have an outbreak of fungus all is by no means lost. Remove the tree seeds and respray them with your fungicide mix, then place them back in a new baggie with new slightly moistened vermiculite. Always keep the baggie sealed. It is a good idea to check your stratifying seeds on a regular basis for either fungus or germination. If any tree seeds germinate while in the refrigerator you simply remove them and plant. 

Any seeds that are indicated as needing a period of "warm stratification" followed by cold stratification should simply be subjected to the same measures but this time the seeds should be stratified in a warm area first, followed by the cold period in your refrigerator thereafter. I use the top of my refrigerator (outside on top) where it is usually about 62-70 degrees Fahrenheit for any warm stratification periods. In many instances, warm stratification followed by cold stratification requirements can also be met by simply planting the seeds in summer (around July/August) in a nicely mulched bed for expected germination the following spring. (Some seeds may not germinate until the 2nd spring) 

It helps to soak your seeds in cold water overnight immediately before placing them in cold stratification. This can cut down somewhat on the amount of time needed for stratification as the seed naturally needs to absorb moisture while stratifying. I also soak them overnight prior to planting if the seeds have the appearance of not having swelled/split much during stratification. 

Although the amount of time it takes to stratify tree seeds depends on species and conditions, in a great many cases two months is usually sufficient to break the tree seeds dormancy. This means that if your spring starts around May/June you should start stratifying your seeds 1 or 2 months earlier so that they will be ready to plant at the beginning of warm weather. 
After undergoing the cool moist treatment the seeds are ready to plant and will usually sprout in a few weeks. Before planting I give the actual seeds another spray of fungicide/water mixture. I also give the hole I am planting the seeds into a good spray as well. 

Sowing and Seedlings

All seedlings, whether grown in pots or beds, benefit from good air circulation which wards off fungus growth and promotes sturdy stems. Potting and germinating medium/soil is not critical as long as the "soil" is light as well as lightly tamped down but not heavily compacted. Be sure and use a potting soil which is described as being sterile, otherwise you may have problems with stem rot and "damping off". These problems are much more likely to occur if air circulation is poor and this is why letting your seedlings grow outdoors is better. 

Most tree seeds need only be planted 1/4 to a 1/2 inch deep in order to germinate. If you plan on planting your seeds outdoors, then plant them a little deeper to about 3/4 inch because the disturbance caused by heavy rainfall has a tendency to turn the seeds up. Keep a check on the seeds and push them back in if they do come up. Lightly mulching your seed bed helps to provide protection against heavy rain in the winter but you should lighten or remove completely all the mulch cover in the spring-once any seedlings break through the surface remove all mulch entirely if any has been left on. Make sure to keep the nursery bed lightly damp but never soaking wet and never let it dry out completely. 

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