For those of you new to starting seeds indoors, or those who have never used a seed tray, I would really suggest giving one a try. Almost every seed I start indoors begins in a seed tray.
The main reason is when starting seeds you almost always sow a number of seeds, which don’t all germinate, then you choose one or more from what’s left. A seed tray works a lot better, because instead of putting a few seeds in a number of different pots, you start with all the seeds in one place.
For example, if you want three of the same tomato plants, you might plant 10 seeds in a tray. Out of these 10 seeds, a few probably won’t germinate and one or two others won’t be healthy for some obvious reason. You can then choose the five best seedlings, and transplant them into their own containers. Out of these five seedlings, you can plant the best three and give the others away or discard them. In this way, you end up with the three best plants available from the 10 seeds you started with.
If you have six or eight different kinds of plants you want to grow at the same time, you can just make little rows in the same tray, and use plastic markers so you know what is what (like in the picture). Since trays take up a lot less space than pots, you will need fewer lights and less room to start so many different plants.
Another possibility is you have some old seeds, and you don’t know if they will germinate anymore. For example, I grow celeriac (celery root) each year, and the seeds come in packets containing hundreds of tiny seeds. If you take suspect seed like this and simply broadcast it over an entire seed tray, as thickly as you think is appropriate, and even just a small percentage of it germinates, you can just transplant out the seedlings that do emerge. Even if you get a much higher rate of germination than you expect, it’s not difficult to deal with a lot of seedlings in a tray as long as you don’t wait too long and let them become too established.
Some plants, most notably tomatoes, become stronger if they are transplanted. Tomatoes have naturally weak root systems, and also have the ability to form roots on any part of the plant that becomes buried. What I do is transplant them from the seed tray, as deeply as possible into a plastic pot. Only the top few leaves should remain above the soil. Then when I transplant again out into the garden, I again bury the plant all the way up to the top few leaves. In this way, the plant is ‘shocked’ into developing stronger roots.
Mostly I have very close to a 100% transplant success rate with seedlings from trays, only a few kinds of plants will not tolerate being transplanted out of a tray into a pot.
While you can use a number of things as a homemade tray, I suggest buying a proper tray from a garden center. It will have the proper drainage holes on the bottom, and be the proper depth for working. Trays come in many different sizes and they all work equally well. My trays are about 15x20cm. If you use a heating pad when starting seeds indoors, think about buying trays that will fit nicely on it.
A heating pad can be very useful. Some plants, like peppers will not germinate unless you keep them above 22C/70F day and night, and since many people’s homes are not that warm at night a heating pad is the perfect solution. Keep in mind a heating pad will cause your plants to dry out more quickly, so be sure to water it often. Once your seeds have germinated, the heating pad is no longer useful and should be turned off.
It can help seeds germinate if you cover the tray with a piece of kitchen plastic with a few holes poked in it with a fork for air. This helps keep the moisture in. Be sure to remove this plastic after 24-48 hours or mold can form. I use a purchased ‘propagator’, which is a heated tray with a plastic lid that works in a similar way.
Normally you want to transplant the seedlings out of the tray as soon as the first set of ‘real’ leaves form, this is the set of leaves that come after the initial cotelydons. To transplant, just loosen the planting medium with a table knife or something similar and pluck the seedlings out gently with your fingers. This will be easier if the seed tray is not too crowded. Except for tomatoes which can be planted much deeper, most seedlings should be transplanted to about the same depth they were in the tray. If you have trouble at this stage with the seedlings breaking or being too delicate, it’s sign they didn’t get enough light.
Before starting any indoor planting project, especially if you are reusing containers from the previous year, higiene is very important to avoid plant diseases. I suggest first cleaning everything with a little soap and water, then sterilizing it by soaking for a few minutes in water with a little bleach added.
You can make your own seed starting mix from homemade compost, but it has to be sterilized by cooking it in a warm oven for about 30 minutes. Otherwise, commercial seed starting mix can be purchased. When making your own seed starting mixes, be sure everything is sterilized first, and remember seedlings are very delicate and even the most gentle or natural of fertilizers can be too strong and kill them. It’s commonly suggested that you let the seed starting mix sit around for a month or two after adding your own fertilizer to it in order to let it ‘cool off’.
One of the best overall planting guides I’ve seen, as well as charts for determining when to start different plants indoors, can be found here.