Starting indoor seeds is a lot easier than it sounds. Even inexperienced gardeners can start seeds with just a bare minimum of equipment. There are as many ways to start seeds as there are gardeners, but the following tips have always worked for me.
Plastic flats -- I prefer the cheap ones with the separate cells. If you have a choice, buy flats with white trays rather than black. The black heats up in the sun, which is nice for germination, but when the flats are placed outside later in the spring, it can overheat the seedlings. Watering more frequently can help to counteract this problem. You can reuse the same trays and pony packs for 2-3 years before they fall apart. Just wash them in the bathtub with soapy water before reusing them.
Sterilized seed starting mix -- This works much better than potting soil, even though it's a little more expensive. Never use garden soil or previously used potting soil. I don't recommend using peat pellets because they tend to stay soggy, which can cause damping-off disease.
A spray bottle -- To remoisten the surface of the mix, if necessary, while the seeds are germinating.
Seaweed fertilizer -- Optional, but very helpful. A little goes a long way! I use half the strength recommended on the bottle to avoid burning the tender young roots. I like Maxicrop kelp extract, available from Peaceful Valley Farm Supply.
A rack to put the flats on -- Rubbermaid makes a nice 4 tier storage shelf. It has enough spacing between the shelves for the flats to get adequate sunlight. Or you can attach grow lights, if necessary.
A bright window, preferably south or east-facing -- I set my rack in front of the sliding glass door in my east-facing kitchen. South-facing would be even better. If you don't have a south- or east-facing window, you could try west-facing, as long as you also supplement the plants with flourescent lighting. Hang the grow lights (or cheap flourescent lights) from chains 4" above the tops of the seedlings as they grow. Leave them on for no more than 12-16 hours a day (plants need to rest at night just like we do).
Now, it's time to start those seeds. Don't be scared off by all of the instructions -- it's really easier than it sounds.
Seed Starting Tips
Fill a flat with seed starting mix. Press it firmly into every cell, since the mix will shrink when the water is added and the air pockets are gone.
Moisten the mix with warm water. Don't use cold water, because peat moss (the main ingredient) repels cold water. My flats usually require between 1 and 1.5 quarts of water. I pour it slowly over the flat, trying to put roughly the same amount into each cell without washing out the soil. Don't use too much water -- you don't want to waterlog the soil. When the mix is thoroughly moistened, lightly press it down to remove air pockets.
Wait for a few minutes for the mix to become evenly moist before you start planting the seeds. If there is excess water in the bottom of the tray after 15 minutes, pour it off.
Most seeds are planted about 2 times as deep as they are wide. This means that tiny seeds like campanula are best sown on the surface or covered with just the thinnest amount of mix. Press the seeds lightly onto the surface to ensure good soil contact. A little spray of water with a mister will also help the seeds to settle in. Before sowing larger seeds, poke a shallow hole in the center of each cell. Sow about 3 seeds per cell. For very tiny seeds, sow 5-8 seeds. Extra seedlings can be thinned later. But try to avoid the temptation to sow hundreds of tiny seeds into one cell. The seedlings won't do well, and they become a thinning nightmare. It's also a good idea to save some seeds for next year or for disasters (like your toddler pulling up the seedlings).
Whenever I try a new type of seed, I start some indoors and some outdoors to see what works best. For seeds with a high germination rate, just pull back the mulch, scratch up the dirt with a hand-held cultivator, scatter a few seeds over the dirt, then scratch them in and pat them down. Don't replace the mulch, or they won't germinate (mulch keeps weeds and flower seedlings down). This method is less reliable for difficult seeds. When in doubt, it's best to prepare a special seed bed in a place where the soil is good (like the vegetable garden), and turn in some weed-free compost or well-composted manure before planting the seeds. Most perennials, except those with taproots, can be moved later to permanent spots in the flower garden. Leave annuals where they are sown -- collect the seeds of hardy annuals in the fall and scatter them in flower beds.
Back indoors: You must keep accurate records of what you planted. Believe me, you won't remember in what order you planted the seeds by the time you finish planting them. I sow one type of seed in each pony pack, writing down what I planted as I go. You may also want to mark the side of the flat where you started with masking tape, and write in your notes something like, "Starting from top left side, going down"...(followed by your list of seeds and how many pony packs of each that you sowed).
Place the clear plastic lid over the flat. This retains the humidity for surface-sown seeds. Remove the lid if the flat gets too warm sitting in the sunshine. As soon as several seedlings appear, remove the lid permanently, and do not use it again or you risk getting damping off disease, a fungus that kills young seedlings if they are kept too wet or don't get enough air circulation. I've never had this problem because I'm very careful to avoid overwatering (and there's a heat vent in the floor nearby, which probably helps). Some people run a fan to ensure good air circulation.
If you start seeds that require pre-chilling, such as columbine or penstemon, you should place the flat in your refrigerator for 3-4 weeks after sowing. When the time is up, place the flat in a sunny window, and the seedlings should begin to appear within 2-3 weeks. I often start my columbine in part of a flat, then add the rest of the cells and seeds after the columbine is finished pre-chilling.
Seedlings must be observed every day from now until transplanting time. When about half of the cells are looking dry, and the flat feels lighter than normal, then it's time to give it a good watering. Pour about 1 quart of water in the bottom of the tray. Watering from the bottom up is better for seedlings. Just be sure to drain off any excess water after giving it time to soak in. Unfortunately, some cells tend to dry out before the others (especially the cells in the corners), and they need to be watered more often. Do this carefully to avoid damaging tiny seedlings. Before the seedlings germinate, I use a spray bottle to moisten the soil surface if it dries out too much. Once the seedlings have emerged, bottom watering is best to reduce the risk of damping-off disease.
After the seedlings have developed their second set of leaves (the "true" leaves), it's time to use some seaweed fertilizer. Add the seaweed at half the strength recommended on the bottle. Fertilize about once a week, adding the fertilizer to the quart of water that you put in the bottom of the tray.
When the seedlings are sturdy enough, use a small pair of manicure scissors to thin out the smallest or most gangly ones. Don't attempt to pull up the seedlings, which could damage the roots of their neighbors. There are some things that don't require thinning, such as sweet alyssum. Avoid thinning weak seedlings like snapdragons until they are well established, since the fragile stems frequently break, leading to a sort of self-thinning.
About 3-4 weeks before the last frost date, start introducing the seedlings to the great outdoors. Just set the rack outside on the east side of your house or in the dappled shade of a tree. Only do this for half an hour or less on the first day. Watch the seedlings closely for signs of wilting, and don't put them out on cold or windy days. Slowly increase the amount of time that they spend outside each day until they are staying outside all day and only coming in at night. During the hardening-off phase, plants will need to be watered as often as twice a day to keep them from drying out.
It would be wise to transplant the seedlings on different days, just in case of disaster. And, of course, water them frequently until they are well established. Adding seaweed to the water for the first one or two waterings will encourage stronger root development. Don't give in to the temptation to transplant them into the garden before the last frost date, no matter how nice the weather seems. One year, I planted out 100 seedlings a few days before the last frost date (May 15 here in northern Utah), and two days later the temperature suddenly dropped to freezing. I placed pots over as many seedlings as I could, but I still lost about half of them.
Don't be shocked and dismayed if up to 10% or more of your seedlings die before they grow up. This happens to even the most experienced gardeners. Just enjoy the plants that make it.
Once you have successfully started your first plants from seed, you'll be hooked for life!
Check out Diane's outstanding website at: dianeseeds.com lots of great gardening articles and information to help YOUR gardening efforts.