7 Factors that Affect Seed Germination

seedlings
I mentioned in my last entry that my vegetable seeds have started to germinate! I thought I’d take this opportunity to talk a little bit more about SEED GERMINATION and the factors that can affect germination rates. Some of these are your standard gardening book variety factors, some are a little less obvious and come from my experience working with native plant species (I’ve blogged in the past about my interest in using native plant species in homesteading).

Why Germination Conditions Matter

Basically, seeds are dormant when you purchase (or collect) them and they need certain environmental conditions to break that dormancy. Germinating most common varieties of vegetable seeds is fairly straight forward. However, many herb, flower, and woody species are more difficult to germinate.

Why Start Plants from Seed?

I’ve mentioned before that I’m interested in integrating as many permaculture principles into my lifestyle as possible. Agriculture based on permaculture principles should mimic the natural world as much as possible – and natural ecosystems typically include diversity of both species and genetics. Starting plants from seed is not only more economical than purchasing young plants, but it also allows for more genetic diversity (at least with open pollinated species). Knowing what factors could be an issue is the first step to troubleshooting difficult to germinate species.

1) WATER
This one seems obvious but there is more to it than you might think. Seedlings generally need to be kept moist but not wet. This water should also (ideally) be chlorine-free. Urban water supplies are heavily chlorinated, so I let my water stand on the counter for at least 24 hours before I use it to water my seedlings. Also, if seeds are allowed to dry out after they have initially been wet some species will enter a second dormancy that is much harder to break!

2) OXYGEN
This often-neglected factor is actually very important! Most seeds will not germinate in saturated (waterlogged) soil. Seeds and the seedlings they produce need to breath just like we do, so drainage in seed trays is very important!

3) SUBSTRATE
You might have expected me to say “soil” here, but seedlings actually don’t need soil per-say. Soil, by definition of the NRCS, is “The unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants.” Soil is a limited resource, and it is not the only option.

Seedlings need a substrate to germinate in but that substrate can be partially-decomposed plant matter (such as peat moss or compost), minerals (such as vermiculite or perlite), a combination of these (found in many soilless growing mixes), or just about anything that gives seedlings the structure they need. Generally things to consider when choosing a germination substrate are:

  • pH: Without going into too much chemistry, pH is a measure of the acidity or alkalinity of a substance. Soil is acidic at pH levels under 7 and alkaline at pH levels greater than 7. Soil pH also affects the availability of nutrients and minerals needed for plant growth. Most seedlings will grow in slightly acidic soil (pH around 6), but some species (blueberries, parsley, potatoes) prefer much more acidic conditions. A few species (asparagus, leeks, marjoram) prefer alkaline soils.
  • Texture: The size of the substrate particles affects how uniformly moist the seeds are kept. Finer particle sizes will keep seeds evenly moist. A coarse substrate will not retain moisture well enough to be a good germination substrate for most seeds.
  • Compaction: Substrates that are more compact will generally keep seeds more evenly moist. Soils that are overly compacted (such as soils that are frequently driven over by a tractor) will be too compact. Some pore space in the substrate is necessary to make sure the seeds do not become waterlogged and to make sure seedling roots can penetrate the substrate. However, too much pore space means that the substrate will dry out too quickly.
4) LIGHT
Some seeds germinate just fine in the dark while other seeds need light to break dormancy. I put all of my seeds under lights just in case. Even though some species don’t require light to germinate, all young seedlings do need light so light can’t hurt in either case! A sunny windowsill is good enough for some seedlings, but if you don’t have enough light coming in your windows then fluorescent lights can be a good supplement/substitute. Some seeds respond to the length of the day to germinate, so adding lights can be a good way to force germination of species that typically wouldn’t germinate early in the year when the days are still short.
5) TEMPERATURE
Some seeds require heat for germination, others simply benefit from it. There is also the occasional species that germinates best on cool soil. In my experience, most vegetable seeds benefit from added heat (with a few exceptions – spinach and parsnips have their optimum germination at room temperature). However, some herbs and ornamental species germinate best at cooler temperatures. There are also species that need alternating warm and cool periods for germination. For some species, day-night warm-cool cycles are needed.
6) OTHER DORMANCY-BREAKING FACTORS
Some seeds need additional help in breaking dormancy. Abrasion of the seed coat can help in some cases. This can be accomplished by rubbing the surface of the seeds with sandpaper or other abrasive surface. This is not necessary with most common vegetable varieties, but is fairly common for tree and shrub seeds.  For other seeds, an extended period of cold – known as a stratification period – is needed to break the seed’s dormancy. Additionally, the seeds of some species will benefit from soaking prior to germination. More about seed pretreatments can be found here.
7) NUTRIENTS and MINERALS
Seeds contain stored nutrients, so generally seeds don’t technically need additional nutrients to germinate, however young seedlings do. Most substrate mixes are formulated to contain the necessary nutrients, so this isn’t something that is much of an issue unless you are formulating your own custom mix.
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