There are four environmental factors that affect seed germination: Water, Light, Oxygen, and Heat. Check out these four tips to learn how much of each and when.
Water
This diagram of a barley seed, by Ross Koning, is a good example for showing how germination takes place.
The first step of germination is a process called inbibition (absorption of water), which is usually what tells the seed to start germinating.
It’s important to supply adequate and consistent water to seeds. It can cause an issue if you don’t because if the seed starts to germinate, but then it doesn’t have enough moisture available, it can die before it’s able to set its roots down into the ground and get established. So, we need to provide the water the seed needs to initiate the germination process, and to continue its growth.
To give your seedlings the right amount of water, take the observation approach. For the first week your soil should stay wet to allow the inbibition process to signal germination. After that first week, maintain an even moisture – a good rule of thumb here is to never let your soil get any drier that a wrung-out sponge. Humidity and temperature will affect how often you need to water. You should also lift up the Seed Germination tray to see if there’s any water in the tray underneath. If the bottom tray is empty and the soil looks a little dry, it’s a good time to water.
After about a day after watering, soil will reach “Field Capacity,” a fancy term for that perfect zone where water and oxygen are balanced just right. If your soil is glistening, you shouldn’t add any more water. The soil needs to be allowed to drain. It’s kinda like that perfect moment on your day off when you don’t have any more work to do, you’ve gotten enough sleep, and you’re lounging on the couch watching Netflix and eating a snack. Boom. Field Capacity Reached. You should give yourself and your plants this luxury to go from surviving, to thriving.
Light
Light is also, in some instances, a stimulant for germination.
This is an example of a lettuce seed’s process and you can see here at the bottom it mentions “photo activation”, so there’s this phytochrome cell that absorbs light, and that lets the seed know that it’s time to germinate.
Let’s say you have some wild lettuces that just started to put their seeds out, and then they were covered in snow. When it starts warming up and the snow melts, the sun hits the seed and the photochrome cells absorb this light which tells the seed, “now it’s time to start germinating”.
They have these functions built in to keep them from germinating too quickly, which can potentially lead to them failing. So, it’s common with some of the cool season crops, like lettuces, to plant them on the surface and barely cover them up so they can get the light they need to start germinating.
We don’t plant tomato or pepper seeds on the surface like lettuces because they are stimulated by water first, rather than light. (This makes sense for a crop that you can start germinating during the cold winter months so that it will already have its “True Leaves” when that spring sunshine is available. I talk more about True Leaves here in my article, "How to Choose Soil for Spring Transplants".
A supplemental light fixture with bulbs somewhere between 6000 and 6500 degrees Kelvin is just as effective as planting seeds outdoors in the sunshine; this makes it possible to create an environment indoors that is ideal for seedlings to germinate. It may vary a little, but generally speaking, your transplants need a maximum of 12-14 hours of light every day.
Oxygen
Respiration is one of the basic needs of any cell, whether it’s in the animal or plant kingdom. For seeds, respiration begins once imbibition has occurred, so once that spring transplant has started to germinate, there needs to be some oxygen available.
Where a problem can occur is if you keep your soil too wet – the plant won’t be able to get the oxygen it needs. Unfortunately, this can cause your seeds to rot. If your seedlings don’t germinate properly, you can dig down in the soil and find the seed to check if it has, indeed, began to rot – this can happen both when the conditions are too wet, or the temperature isn’t quite right. Which brings me to the last tip:
Heat
Heat is very important because we’re trying to establish the best germination temperature for each type of seed to thrive. A function of these plants and their nature is that they’re activated at specific temperatures.
There needs to be a range of 65-75 degrees for most plants to begin germination. You can use heating pads, lights, and ambient temperature to keep the seeds in the perfect zone.
Check the temperature of the space you're growing your plants. Is the surface cold? Even though the ambient temperature may be warm enough, a cold surface will wick heat away from your seedlings quickly. That’s when a heating mat can help establish the temperature you need.
During one of my Spring Transplants Workshops, Richard Job asked, “Do you recommend testing the temperature of the soil?”
This is an excellent question. The answer is yes! I do. A Seed Sprouting Thermostat can regulate the temperature for you. This specialized thermostat connects to the heating mat, and has a probe you put into the soil that feeds back the soil temperature data. This is a good way to give your seedlings the constant temperature they need without sacrificing any sleep, or having them crawl into bed with you during those really cold nights.
Here is our resource on Ideal Germination Temperatures. You don’t have to memorize this stuff – you just need to know that it’s available! You can always find this and other planting tools here on our Resources page.
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