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Deno at long last establishes a system for understanding the germination of seeds. His theory is that fruit and seeds contain chemicals in their pulp, seed coat or embryo that inhibit germination. Deno is a physical chemist not a botanist so he understands this stuff. These chemicals are barriers to germination and must be removed, often in order for seed to germinate. Forget terms like stratification, they are now archaic. It originally referred to the practice of layering seed in beds that were kept cold, to layer or to stratify. I prefer to use the terms that Deno uses, because they more accurately describe what is going on. These terms are 'germination inhibitors' for the chemicals that must be removed, and 'pre-treatment' for the processes involved. I think it is the least that we can do to honor the man's work.
The chemical reactions that take place to break down some of the germination inhibitors are quite unusual in that they require moisture and temperature within a specific range, not colder or hotter. As a physical chemist Deno has identified this type of reaction and explains how it can happen, although we needn't be concerned about it here. Another inhibitor is broken down at warm temperature, another by a fungus, one by lack of light, and one in the presence of light, although the light triggered ones seem to work in conjunction with the fungus. One type of inhibitor is present in most fruit pulp and must be physically removed. In addition there are physical barriers that must be broken down such as impermeable seed coats. Deno lists all of these in what he calls the nine principles. I am not going to give you all nine, but I will summarize the most common problems in seed germination.
Some inhibitors are broken down under moist conditions at 70 degrees (Fahrenheit) and others at 40 degrees. Seeds may, and often do contain more than one inhibitor. There is a curve of germination for each species that describes the rate of germination over time, but for our purposes neither of these cycles takes longer than ninety days for the vast majority of plants. If the cycle is shorter than this you can easily determine it by checking the paper towels each week to see if the seed has started to sprout.
Most temperate woody seeds will contain an inhibitor that will require a forty degree cycle followed by a seventy degree cycle. This only makes sense, since it is natures way of protecting seed from germinating while it is still too cold. Sometimes the inhibitors must be broken down in order, first forty then seventy, or vice versa. The seed will not germinate until this happens. Some species require multiple cycles. That is, there may be more than one forty or seventy degree inhibitor present, and when this happens they are broken down one at a time. Thus you must alternate three months of forty followed by three months of seventy until germination occurs. I have written that Acer japonicum requires just such a multiple cycle.
The beauty of using moist paper towels and plastic bags to pretreat seed is that you can do all this work without trying to stick a flat of dirt in the refrigerator. When the seed finally germinates, remove it from the bag and sow, one at a time if necessary, and this is often the case. I prefer to wait until the radicle just emerges and then plant it in a plug tray or cell. This eliminates much of the waste involved in planting seed. You know that the seed you planted is going to come up because it has already germinated! This is particularly nice for rare and valuable seed when you only have a few.
This has worked very well for me with Dogwood, (Cornus), Ginkgo, and Prunus mume.
If you get nothing at the end of two weeks put it in the fridge for three months, checking it each week for signs of germination. Often fresh woody seed will begin germinating after one month. At the end of three months and no germination takes place, then a seventy degree inhibitor is most likely present. Keep the seed at about 70. For most woody seed the inhibitor is broken down quickly and it will begin to sprout in a week or two. If not, hold it at 70 for three months. If nothing happens, a second 40 degree inhibitor is present (assuming of course that the seed is viable). Back to the fridge, repeat the cycles until germination occurs, you have a fit, or the seed rots.
You can of course, run multiple experiments if you have no information at all on your seed. One bag in fridge, one at 70, etc.
A few notes on care. The seed must stay moist INTERNALLY throughout this process. Deno doesn't talk about this much, but my experience with woody seed is that it can stay a lot drier on the outside than most people would believe. Keeping it this dry eliminates a lot of the fungal problems involved with long storage times. This is why I am so particular about getting fresh seed that has not been dried, it already has internal moisture and if an impermeable seed coat is present it won't make any difference.
Some tips on determining the proper moisture: Know the difference between moist and wet. If a film of moisture is on the seed or the plastic bag it is wet not moist. The paper towel should feel almost dry. If it starts to get stiff during the process, it is dry, and a very few drops of water should be added or a single spritz from a spray bottle.
For seed that does not take long to pretreat such as Cedrus I don't even use paper towels or other media, I soak the seed, dry it in the sun for about fifteen minutes until the outer husk feels dry to the touch and put it in a baggie and into the fridge. The seed is very fleshy and retains adequate water for the month that it must stay in there. Cedrus is VERY sensitive to excess water and will rot in an instant (see my Cedrus article for more info).
If your seed does get very moldy but has not yet cracked the seed coat you can wash it with a ten percent bleach solution. Let dry, then return to storage in fresh bag and towels. Deno points out that sound seed has natural antibodies for most fungi, and this is true. But keeping seed too wet is just too risky. Once you get a pathogen it seems like you have it for life and precautions are in order.
I have spent hundreds of dollars on Acer palmatum seed without a single seed germinating until I finally got smart and collected my own in Oct and Nov. The best seed will still have some color to the wings and the fleshy part will still be a little moist. If the seed is collected at this stage and refrigerated without further drying it will keep for some time, but most companies don't bother doing this. Other problem seed like this is Carpinus, and Fagus. I use to think that over drying the seed put it some state of deep dormancy, but now based on Deno's work, I think it either kills, as in the case of Acer rubrum, or allows the seed and seed coat to dry and harden and become semi-impermeable so that you cannot get water to the embryo no matter how long you soak.
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