This wonderful FAQ provided by Botanist extraordinaire Kay Lancaster just west of Portland, OR; USDA zone 8:
Problem #l: Seeds have to be moist enough to germinate, dry enough that the fungi don't move in, in soil damp enough to keep them going, but with enough air spaces that oxygen can get to the seeds and the roots.
General solution: a well-drained potting soil and plastic pots with good drainage holes. Keep soil about as moist as a sponge you'd use to wipe off the kitchen counter.
Problem #2: Drought and drown. Most of us don't have the time to watch seedlings really, really carefully and water them every time they need it. Plus some of us (ahem! who me?) have minds like sieves and just plain forget to water now and then. A seed that's started to germinate and then dries out is dead.
General solution: Keep the humidity up so the soil doesn't dry out so rapidly. If you've got seeds in soil, cover with plastic wrap so the tray doesn't dry out. Once you see the first signs of seedlings, though, you need to remove the cover and start paying closer attention.
Problem #3: The seedlings come up, they look wonderful, and then start flopping over, with a pinched stem at the soil line just like a tiny lumberjack has been clearcutting. It's the dreaded "damping off' group of fungal diseases.
General solution: Instead of covering the seeds with soil, cover them with a thin layer of sand (yep, like you give the kids for the sandbox) or milled sphagnum moss. Keep the air circulating, not stagnant, once the seedlings are up (which, of course, causes some water loss—try a combination of a humidifier or steamer and a small fan). If you see damping off, use chamomile tea for watering (haven't tried this, but some very good gardeners in this group swear by it) or one of the commercial fungicides made for this purpose. Personally, I haven't needed to resort to either remedy...l find sand or sphagnum plus good air circulation is plenty for me.
Problem #4: watering. The soil is so dry when you put it in the pots, and then you plant the seeds, and try to water, and the seeds float right out. Later, the trays are either floating in water or the top is damp and the bottom soil is dry.
Solutions: Fill the trays with soil, then set them in a shallow pan of water for several hours, till the soil is moistened all the way through by capillary action. Let the excess water drain from the trays for several hours before planting. Plant the seeds directly on top of the soil, then cover with a bit of sand or milled sphagnum. Water by misting or by bottom watering until the seedlings are up.
Problem #5: The packet says they ought to be up in 14 days, and it's been 3 weeks, etc.
Solution: germination is also cued by temperature. At the optimum germination temperature, the time for germination is minimized. For most of our common garden plants 70-75F *soil* temperature during the day and 65° or so at night is about ideal. Some plants must have cooler temperatures; others must have warmer temps (the packets usually have this information). Folks who keep their house at 65° will generally have soil temps about 62° if they leave the trays uncovered, the seedlings will be slow to germinate and often more prone to damping off. While the seeds are germinating, many people find the top of the refrigerator, the top of the water heater, or the spare bathroom with the register open and the door closed gets the soil temp up to the proper levels. Other folks resort to heating mats of various designs. Be SURE to use mats (and other electrical equipment) designed for use in wet places and connected to GFI receptacles. Fried gardener is not a tasty dish.
Problem #6: How much light? When? How? What if the packet says germinate in the dark? Aren't you supposed to start seeds in the dark and put them in the light later?
Solution: think about it. If seeds had to germinate in the darkness for several days, they sure wouldn't grow outside! (There are a few weird plants with this dark/light germination trigger, like a certain sub strain of Grand Rapids lettuce. But they're rare.) Even seeds in the soil get a significant amount of light. The red wavelengths that trigger germination can penetrate 18" of good soil. Seedlings need light... lots more than most people think...pretty much from the start. South or west-facing window are the old standby, but the windowsill is often too chilly. Most folks opt for fluorescent lights, usually a 4 ft shop light fixture placed about 3" above the tops of the seedlings (yes, you need to keep raising the light fixtures as the plants grow.) Lots of light means nice stocky, dark green plants that grow quickly. No, you don't need expensive "grolight" tubes. But you do need to replace the tubes after they've been used for 3-6 months, because the light output drops greatly. The $0.99 tubes from the discount stores work fine. 18 hours a day for light is plenty. 24 hours a day screws up the flowering times for some plants.
Problem #7: They're growing! They're growing! Now I've got to transplant them, don't I? They're too small! I'll kill them! Can't I just give them more fertilizer instead?
Solution: I sow seeds in rows in my seed trays, trying to get a single file line of plants, (well, I try. I don't always get). When the seedlings have two true leaves (the first pair are cotyledons, seed leaves and then you wait for two more leaves to appear), they need to be transplanted to give them room to grow. I use the "pony packs" nurseries use... I like the size that's about 6 plants per pack. You can buy whole sheets of these and the trays they fit in. I take a plant label and scoop down beside a line of seedlings, lifting a little bunch of them at a time...maybe 6-10. Carefully separate one from the bunch by holding on to a cotyledon (not the stem!) and shaking a bit, pulling the bunch apart. Put the seedling down on top of a compartment of the pony pack (already filled with soil) and poke the roots down into a pre-made hole in the soil gently with a finger or a blunt pencil. Just one poke does it once you get the rhythm. As soon as possible (within 30 min or so), water the tray gently from the top with a very fine spray of tepid water. Baby bath water temperature is fine. Let the tray drain, then water again. And again, until you're sure the soil has settled around the roots and the soil is wetted thoroughly. Return the tray to the same temperature and light the plants were in before. Be really careful watering for the next few days until the roots have had a chance to start growing into the soil, lest you wash out the plants.Problem #8: How about some fertilizer now? They're getting bigger!
Problem #9: Everything looks great, and I want to plant them out this weekend.
Solution: Greenhouse growers can use fertilizer profitably on their plants, because their goal is producing blooming sized plants for the consumer. They can afford the soil tests that tell them what and how much they need to add. As a home gardener. I'm trying to produce plants that will grow well in my garden. I've found that, for most plants. I'm better off NOT trying to grow them like the pros do. Instead, if the plants get to the stage where the roots are pretty well filling their compartment, I don't fertilize... I transplant. Either to the great outdoors, or to a little bigger pot. If you do transplant to a larger pot, choose one that the current roots will take up at least 1/3 of the volume of the pot. Over potting is often a death sentence unless you're really persnickety about watering.
Problem #10: It all grew! I don't have room for it all! I'm going to be awash in zucchini!
Solution: Whoops—probably you forgot to start "conditioning" the plants to the great outdoors a couple of weeks ago. Outdoors generally has lower humidity, less frequent rainfall and higher light levels, and if you plant your babies out directly, they may wilt. Instead, start taking them outside in the shade for a few hours a day—then letting them have some direct sunlight for a couple hours a day, shade the rest. Let them wilt just a tiny, tiny bit while they're outside. Give them at least 3-4 days of this sort of "abuse" before you plant them out. This is called "hardening off' and it generally results in plants that just keep right on growing without breaking stride.
Solution: Don't plant it all. Palm the extra seedlings off on friends, relatives and neighbors. Put them on the curb with a "free to good home" sign. But don't crowd up your plantings just because those seedlings look so small now, and you've got 153 tomato seedlings that you've grown. You'll get better yields per plant by giving them the room they need to grow, and besides, who wants to spend all of August canning?