Q: I have a bag of open potting soil in the garage. Why must I use soilless seed starting mix?
A: Soilless seed starting mix contains all the right stuff and none of the wrong stuff. Seed starting media has excellent drainage, just the right amount of nutrition to get the seedlings off to a good start and none of soil borne pathogens that can attack and kill those young seedlings. That open bag of potting soil likely contains a mix that is too "heavy" for seed starting and since it has been open, likely now contains disease pathogens from contaminated gardening tools. Save that bag for the outdoor garden.
Q: I kept unused seed from 2018. Will I be able to use it this spring?
A: That is one of those yes, no and maybe questions. In general, the larger the seed, the longer it will last. Think snap beans versus lettuce seed or sunflower versus marigold. Those tiny seeds with limited food reserves may not make it even a year later. In addition, seed stored in a cool, dry environment will last longer than seed stored in hot, dry conditions. Seed stored in tight-sealing containers will last longer than leftover seed in the open seed packet. In general, the germination rate goes down about 10 percent each year. Best bet is to buy and use fresh seed packaged for 2019. If old 2018 seed is used, increase the sowing rate by at least 10 percent.
Q: Once I have sown my seed, how often do I need to water it?
A: Seeds do need to take up soil moisture to start the germination process, but do not require saturated soils. Soils that are damp are good enough. As they begin to show up in the seedling tray, they still do not need a lot of water, as root systems are very limited. Moisten the soil and consider covering the flat with a plastic wrap to keep moisture in. Once many of the seedlings have emerged, the covering comes off and you are off to the races. Then pay close attention to their watering needs and keep the water off the foliage when you do water.
Q: Do I need to sow all the seeds at once?
A: Absolutely not. In fact, sowing the seeds in stages allows you to "hedge your bet" on the actual date you will be able to get out in your garden to do the transplanting, since spring weather is so difficult to predict. Different sowing dates also may be driven by what you are growing and how many weeks before planting you need to start. Cold or cool season crops are some of the earliest ones to start, while warming loving crops are near the end of your 2019 seed sowing efforts.
For questions, contact your local Master Gardener Help Desk or learn more about vegetable gardening at https://extension.illinois.edu/veggies