Winter has barely started, but it is already time to begin the planning process for starting vegetable transplants. Very hardy vegetables like broccoli and cabbage can be planted outdoors without protection 4-6 weeks before the last average frost-free date, which means transplants need to be seeded well in advance of this date. The Illinois State Water Survey http://www.isws.illinois.edu/ and the National Oceanic and Atmospheric Administration (NOAA) https://www.ncdc.noaa.gov/ both provide resources to aid in determining the last average freeze date for any given area in Illinois or the US, respectively. For my location in the St Louis Metro East, historical climate data has determined that planting on April 2nd carries a 50% probability of a later freeze occurring, compared to April 16 which historically has a 10% probability of a later freeze occurring. I usually use the earlier date for determining when to plant cool-season vegetables and the later date for warm-season vegetables. My logic for this is cool-season vegetables can take some frost so I am willing to gamble a bit. Whereas warm-season vegetables don't tolerate frost and I am not a reliable frost protection provider, so I am a bit more conservative.
Generally, a minimum of 4-6 weeks is needed to grow a transplant with a well-developed root system, sufficient to pull transplants from plug trays without the root ball falling apart or the roots pulling free. To allow for a full six weeks, very hardy vegetable transplants need to be seeded by February 19th in my location. That gives me about six weeks to order seed, set up an area to grow the transplants and prepare the growing media.
I don't use garden soil in my transplant trays. Not only is garden soil not sterile (damping off pathogens), it is usually too heavy and poorly drained for transplant production. A better choice is a sterile media that usually contains an equal amount (by volume) of fine particle pinebark, sphagnum peat moss, and perlite. Unless I am using a soil blocker which requires a coarser textured media in order to hold its shape, I usually purchase germination mix for seeding transplants because it provides seeds with a fine, uniform seedbed that is well aerated and loose. It is also free of insects, disease organisms and weed seeds.
Unless you are growing lettuce transplants, which requires light for germination, most vegetable seeds are happy to germinate in either light or dark. Seeds that require light to germinate should be left on the media surface and not covered with soil. All other seeds should be planted shallowly under a thin layer of media. In addition to a south-facing window, supplemental light can be provided by fluorescent fixtures suspended 6 to 12 inches above the medium for 16 hours a day for all but those seeds that require dark to germinate. Pansy (Viola) is an example of an edible flower whose seeds require dark to germinate. For seeds like these, cover the tray to block light (aluminum foil works) and check regularly. Immediately upon germination, remove the cover and expose to light. Regardless of seed type, all seedlings require light and will become leggy and fragile and will not produce to their potential if they do not have sufficient light.
In general, seeds need warmer soil temperatures during germination, making a seed germination mat a handy tool. Once germinated, most transplants prefer night air temperatures of 60-65⁰F and 10⁰F higher during the day. Cool-season plants prefer 10⁰F cooler, both day and night. Maintain uniformly moist media; never soggy or so dry seedlings wilt. Some species, like the cole crops, are especially sensitive to interruptions in growth which can result in forever stunting. Young, tender seedlings are also easily damaged by too much nitrogen fertilizer, so dilute fertilizer rates by 50-75% until true leaves develop.
It is also good to know which types of vegetables transplant well. In general, plants that take well to transplanting are generally improved by the process (including transplanting into a larger container). Vegetables and herbs that don't take to transplanting include beans, Chinese cabbage, vine crops, many root crops (not including beets, turnips and celeriac), borage, chervil and coriander. A way to get around this problem and still get a jump on the season with transplants is to seed into biodegradable trays or pots. This allows for transplanting of the entire container with minimal impact on the sensitive root system.