Seeds may be as small as a grain of dust, or as big as a football, but all contain the same parts. The most important of these is the embryo, which is a living, breathing plant reduced to its most basic form. There is also a food source, which keeps the embryo alive while the seed is dormant and powers the germination process until the seedling starts to produce its own food. All of this is wrapped in a thick seed coat, which protects the embryo from adverse conditions until germination.<\/p>\n\n\n\n
There is something magical\nabout seeds (Jacks magic beans were nothing special, I have boxes and boxes of\nmagic seeds). I simply love to handle these neat little packages of genetic\ninformation and whenever I get seeds, which is often, I simply have to open the\npacket and handle them. In fact it has become something of a ritual for me. I\nlike to hold a handful of seeds in my hand and think of the previous\ngenerations of gardeners who have handled, cherished, planted and saved the\nvariety over the years (sometimes hundreds, or even thousands of years!).<\/p>\n\n\n\n
Requirements for seed germination<\/strong><\/p>\n\n\n\n A viable seed contains\neverything necessary to produce a healthy mature plant, but germination is a\nvery tenuous process and in the wild most seeds don\u2019t manage to achieve it\nsuccessfully. This is okay because plants often produce hundreds or thousands\nof seeds, so there wouldn\u2019t be room for them all to grow anyway. <\/p>\n\n\n\n For a seed to germinate\nsuccessfully the temperature must be high enough, and there must be sufficient\nmoisture and oxygen available (a few seeds also need light). Of course that is\nsimply the start of the growth process. For a seed to establish itself\nsuccessfully, it must also have a suitable soil to put down roots. This is\nwhere most wild seedlings fail, they simply don\u2019t end up in a hospitable enough\nplace.<\/p>\n\n\n\n Warmth<\/strong><\/p>\n\n\n\n Seeds require warmth to\ngerminate because chemical reactions occur more rapidly at higher temperatures.\nThe time needed for germination doubles for every 18\u00b0F drop in temperature from\n90\u00b0F. This means that a seed that germinates in 3 days at 80\u00b0F, may take a\nmonth at 50\u00b0F. <\/p>\n\n\n\n The optimal temperature for\ngermination varies according to species, but most crops germinate best at\ntemperatures of 75 – 85\u00b0F in the daytime and 5 – 10\u00b0F lower at night. Some warm\nweather crops may not germinate well below 55\u00b0F, while some cool weather crops\ncan germinate at temperatures of 40\u00b0F, or even lower (though it can take a long\ntime). There is also an upper temperature limit, above which a seed won\u2019t\ngerminate. This may be 95 or 100\u00b0F for a warm weather crop, or as low as 75\u00b0F\nfor a cool weather one.<\/p>\n\n\n\n All seeds take a long time\nto germinate at low temperatures and if a seed is planted in soil that is too\ncold, it will germinate so slowly that nothing seems to happen. If it sits for\ntoo long then carbohydrates will begin to leach out of the seed. These will be\nconsumed by bacteria and fungi, which may eventually move on to those in the\nseed itself and the seed will rot.<\/p>\n\n\n\n The optimal temperature for\nplant growth is usually about 10\u00b0F lower than that for germination (65 – 75\u00b0F).\nThis difference is significant because it makes it possible to germinate seeds\nin the warm greenhouse, when the soil is too cold for direct sowing. Once they\nhave reached a suitable size, the seedlings can be transplanted outside into\ncooler conditions, where they will still grow satisfactorily<\/p>\n\n\n\n Oxygen<\/strong><\/p>\n\n\n\n If a living seed has imbibed\nenough water and has sufficient oxygen, its respiration and other chemical\nprocesses speed up and it starts to germinate (ungerminated seeds need only\nminimal oxygen to stay alive). If the seed is close to the soil surface, this\noxygen is easily available from the air in the pore spaces (a rule of thumb\nstates that a seed should be buried at a depth of 3 – 5 times it diameter). If\na seed is buried too deeply, there is insufficient oxygen and it will just sit\nthere lying dormant. Weed seeds sometimes sit in the soil depths for years,\nuntil they either die, or a gardener disturbs the soil and brings them to the\nsurface, where they can germinate.<\/p>\n\n\n\n If the seed remains buried\nfor too long, if the surface of the soil is sealed (by crusting or algae), or\nif the soil is waterlogged, then the seed might never get enough oxygen and\nwill eventually die.<\/p>\n\n\n\n Light or dark<\/strong><\/p>\n\n\n\n A few kinds of seed won\u2019t\ngerminate unless they have been exposed to light, but most aren\u2019t particular.\nOf course as soon as they are able to photosynthesize they absolutely must have\nenough light for healthy growth, or they soon start to suffer and eventually\ndie.<\/p>\n\n\n\n Moisture<\/strong><\/p>\n\n\n\n A dry seed may contain as\nlittle as 10% water. When it gets wet it soaks up water like a sponge and its\nwater content may increase to as much as 70%. This process is known as\nimbibition and is a purely mechanical process that occurs even in dead seeds.\nThe resulting change in appearance can sometimes fool you into thinking a dead\nseed is germinating. <\/p>\n\n\n\n Germination<\/strong><\/p>\n\n\n\n The process of germination\nstarts when the moisture absorbed during imbibition causes various enzymes to\nbe activated and the seed wakes up. These enzymes start to convert the store of\nstarch in the seed into glucose which provides the energy necessary for the\ngrowth of the embryo. Once the germination process has begun there is no\nturning back; the seed must have everything it needs to keep going, otherwise\nit will die.<\/p>\n\n\n\n The embryo uses the energy\nin the glucose for cell division and \ngrowth, slowly breaking out of the seed coat and growing down to form\nthe radicle (or root) and upward to form the epicotyle (or shoot). The roots\nform first, as they are needed to supply water and nutrients to the growing\nplant. In fact a plant won\u2019t (can\u2019t) expend much energy on leaf growth until it\nhas established a good root system.<\/p>\n\n\n\n The germinating plant first\nsends down roots and puts out its cotyledons or seed leaves (unless it is a\ndicot, but I\u2019m not even going there). These are followed by the first true\nleaves, which are tiny versions of the leaves of a mature plant. If a plant\ngets to this stage, it will start to grow rapidly as it puts on more and more\nleaves. The bigger it gets the better its chances of survival. <\/p>\n\n\n\n The seed of some annual\ncrops have special characteristics that affect germination. In some members of\nthe Apiaceae<\/em> (carrot, parsnip, parsley) the seed embryo isn\u2019t fully\ndeveloped and only matures after the seed has imbibed moisture (this is why\nthose seeds are some of the slowest to germinate). Pepper and beet seed (actually a capsules containing\nseveral seed) contain a germination inhibitor that must be leached out before\nthey can germinate (their germination can be hastened by soaking the seed\novernight before planting). <\/p>\n\n\n\n Germination is a complex process, but you don\u2019t have to worry too much about the mechanics of it; the plant has that covered. You just have to make sure it gets all of the things it needs and it will do the rest.<\/p>\n\n\n\n Minimum germination percentage<\/strong><\/p>\n\n\n\n New seed legally has to have\na minimum germination percentage and most reputable companies are reliable. I\nhave had my doubts about a few packets of seed, but usually sowing technique is\nthe problem rather than the seed. Seed packets usually have the date when they\nwere packed written on them, so be suspicious of those that don\u2019t.<\/p>\n\n\n\n Seed may eventually become\nso weak it has a hard time getting out of its own seed coat. I once acquired a\nlarge collection of unusual old seeds, averaging 6 or 7 years old which had\nbeen stored in a garden shed (some packets had even been wet at some point). A\nsurprising number of these seeds germinated satisfactorily, but quite a few had\nlittle vigor (some oregano seedlings sat around in their cotyledons for two\nmonths before I lost patience with them!)<\/p>\n\n\n\n Seed size<\/strong><\/p>\n\n\n\n The size of a seed also affects its vigor. Large seeds have greater reserves of food than small ones, so live longer, are more vigorous and produce larger seedlings (which commonly mature faster). You may have noticed that some newly germinated seedlings quickly grow much bigger than the rest, well this is usually because they came from the larger seeds. If you have a lot of seed and only need a few plants, I think it makes sense to separate out the biggest ones (I do this with a sieve) and only plant them.<\/p>\n\n\n\n Germination testing<\/strong><\/p>\n\n\n\n Seed that has been stored for any length of time should be tested for germination capacity before you plant it. In this way you don\u2019t waste time and effort on non-viable seed. You can do this yourself quite easily. Select a small quantity of seeds, a minimum of 10, preferably 20, and lay them on one side of a sheet of absorbent paper (blotting paper, a coffee filter, or a strong paper towel – the kind that don\u2019t fall apart when wet). Moisten the paper (don\u2019t saturate) and then fold one side over the other to cover it. Roll up this little package, put it in a plastic bag or sandwich bag and put it in a warm place (75 – 95\u00b0F). Check the seeds every day or two, to keep them moist and look for germination. Note when the first seeds germinate (fast germination indicates high vigor) and how long it takes for the rest. Then count up how many germinate altogether to find the germination percentage. You will then have an idea of how many seeds to sow to obtain the number of plants you need.<\/p>\n\n\n
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