I've done tomatoes in a few different ways. The list of steps below seems long and involved, but it's really pretty easy.
- Start 6-8 weeks before the last frost date for your area, or 6-8 weeks before you plan to transplant to the garden. That's about 1 week to germinate, 1 week for growing before potting up, 3-5 weeks of growing in the second container, and 1 week to harden off.
- Get your supplies: seeds, containers, seed starting mix, light source. The seeds are obvious.
For containers, you have tons of choices:
- Plastic "six packs" -- these are ok, but you'll have to transplant from these to a larger pot before transplanting into the garden, and getting the plant out of the six pack does a fair bit of root disturbance which sets the plant back a bit. You also don't get much density if you want to start a bunch of plants; for a beginner this isn't much of an issue.
- Plastic 11x20 trays. Last year I started about 50 tomatoes in one of these by putting the seeds on a 1" x 1" grid in the tray. Transplanting disturbs the roots a lot (see previous bullet) because you have to prick out the seedlings to put them in pots.
- Half gallon milk cartons are, in my mind, possibly the best container. (That's a link to my blog, with a picture of parsley seedlings from last year in milk cartons.) They're free, easy to make (cut off at 4-5" high and drill holes in the bottom), they reuse something I've got tons of, easy to store, they fit nicely in the 11x20 trays, and they're a good size for keeping the plants until transplanting to the garden. The only drawback is that they can be a hassle to remove the plant when transplanting -- a sharp knife to slit the container open along the sides does the trick though. If you use milk cartons, put two seeds per container and thin to the strongest one.
- 32oz yogurt tubs. These are almost as good as milk cartons, but they don't fit as nicely into trays because of their shape.
- 6"x24" (or whatever size) window box planters. Last year, after starting in trays, I planted up to window boxes because I didn't have enough milk cartons. These are nice because they're quite deep (and I had several that weren't in use in the spring). The drawback here is that there's quite a bit of root disturbance when transplanting into the garden because you have to separate / tear apart the plants since they're not in separate containers.
- Lastly, you can go containerless and use soil blocks. Since you haven't mentioned soil blocks and this is your first time, I won't go into any more detail except to say that this bypasses the need for containers and minimizes transplant shock.
For seed starting mix, you again have tons of choices, but at a basic level, you can buy seed starting mix from the store, or you can mix your own using a simple recipe:
- 5 gal sifted garden soil
- 5 gal sifted mature horse manure compost (we use a wood-based bedding product, and the compost has some of the desirable properties of peat)
- 5 gal coarse sand
- "sprinkling" of equal parts lime, greensand, and rock phosphate -- strength depending on what I'm potting [fairly weak for starting seeds]
See other answers on that question for different recipes. Keep in mind that if you use the sterile "seed starting mix" from the store, it's probably just something like vermiculite and peat, it has no nutrients and it is only appropriate for germinating seeds that you will transplant to a larger container with soil that has nutrients. My recipe above is suitable for growing from seeds to planting out in the garden. Don't use any nitrogen fertilizer in a seedling starting mix; it's not necessary and can stimulate too much growth, especially in tomatoes.
Lighting is important! I see a lot of advice to "put the seeds in a sunny window", but I don't think this is ideal. You're better off with a $10 fluorescent fixture from the big box store; a single fixture can accommodate two trays, or 40 milk cartons set into trays. The fixtures with chains are best -- you can raise/lower the light so that it stays between ½-1" away from the growing plants. If the light is further away, the plants will shoot up quickly and get leggy and weak (they won't survive in the garden, if they even make it that long). I buy one "warm" bulb and one "cool" bulb to try to get a full spectrum; I don't know how much this really affects plant growth. You can also get "full spectrum" bulbs made specifically for plants but they're a specialty item and really expensive.
Now that you've got all your stuff, fill up your selected container with your selected soil mix. Get the soil moist, but not soggy. Plant the seeds according to the directions on the packet (a general rule of thumb is to plant seeds 2x the depth of the size of the seed; for tomatoes it's about ¼"). Cover lightly with your soil mix. Don't tamp them down; seeds need to breathe, and you don't want to make them fight to break out. (I don't even cover the seeds that I plant in soil blocks.)
Many people recommend covering the trays with plastic wrap or those domed covers you can get, but this is an invitation to fungal disease. Air flow is very important at this stage. Leave them uncovered, but keep the soil moist while the seeds are germinating. If you do cover them, remove the cover as soon as the plants poke through the soil so they can get light and don't get a fungus (eg. damping off).
Make sure the lights stay about ½-1" above the plants. It doesn't hurt them if they touch the lights a little bit.
Make sure the soil stays moist but not wet. I use a moisture probe to see how wet the bottom of the container is; it is very hard to judge by looking at the surface, which can be completely dry while the bottom of the container is quite wet. Make sure your containers have good drainage holes. (Though when I used trays last year, I didn't have drainage holes, I was just very careful about not overwatering.)
If you planted into a tiny container (eg. six pack) or shallow tray, you'll need to "pot up" / "prick out" when the seedlings have four "true leaves" (the first leaves that come up are cotyledons, "seed leaves", and don't count). Don't put this off -- the roots grow really fast and they will quickly become root bound, and you'll shock the plant more than you have to if you wait too long. Potting up from a six pack isn't hard: just prepare your next container (with good soil, see above), squeeze the container a little bit to free up the plant, turn it upside down and the whole thing should fall into your hand, then drop it into a hole you've made in the soil in the larger container. Pricking out from a tray isn't much harder: use a small tool (I use a tiny ¾" wide planting trowel) gently dig/tease apart the roots from the tray, always holding the plant by a leaf not the stem, and drop it into the larger container, gently firming around the roots. For either technique, bury the plant up to the first set of true leaves. This will help it grow extra roots.
As long as you used good soil in your second container (or first container if you're sticking to milk cartons for the entire time), you shouldn't need to fertilize. But if the plants become yellowish and aren't growing well, you can apply a dilute liquid fertilizer. Again: not too much nitrogen!
A week before you're going to transplant, start hardening off. (I'll give a more complete answer to hardening on your other question.)
Transplant into the garden ideally on an overcast, calm day. If you can't manage an overcast day, at least avoid doing it in the middle of the heat of the day. Early morning or (better, in my mind) late afternoon would be best. Remove the plant from the container, disturbing the roots as little as possible. Plant into prepared holes in the garden. I sometimes put a little scoop (maybe 1-2 tablespoons? -- read directions on the bag) of tomato fertilizer in the planting hole. If I know I'm planting into fertile soil, I'll skip the fertilizer. I've also added just pulverized eggshells -- calcium is important to tomato growth. Set the plant deeper than it was into the container -- bury the first set of leaves. If your plants are leggy, you can lay them a bit sideways in sort of a trough, again burying some of the stem. The above-ground part of the stem will turn upright, and the hairs on the below-ground part will turn into roots. Water well. If it is a sunny day, and you have a way to provide a little shade, do so.