Cool-weather crops like lettuce, kale, and broccoli can be sown outdoors once the soil is 50 degrees. (Watch the trees. Once trees are leafing out, you're good to go.) Or you can start them indoors for an earlier, faster start.
If they start indoors at room temperature, they should stay indoors until they have at least 3 pairs of leaves, then be moved outside gradually before transplanting. Even cold-hardy veggies will be shocked if they go from warm to cold too fast. See the section on getting ready to transplant, below.
Warm-weather crops can be planted indoors and transplanted out after the soil is warm. To know when to start them, look up your last frost date HERE. Then you can start them 2-8 weeks before that, depending on variety.
Tomatoes, peppers, eggplant, and onions need to be started 4-8 weeks before you want to put them outside—March is good in many places. You want seedlings that are 6"-12" tall and growing actively when it's time to transplant. If you don't have heat mats, err on the side of more time. If you have heat, lights, and rich potting mix, they will need less time. Don't start really, really early unless you have the space for big pots indoors, because rootbound plants don't grow well. Most professional growers prefer smaller transplants.
Melons, cucumbers, squashes, basil, okra (as well as chia in some places) can be started a month before planting out. Melons need very warm soil and should go out later than the others.
Beans and corn should be sown direct in the garden or about 2 weeks before putting outside.
A good habit is to sow only part of your seed at first, saving the rest to plant a week or two later. That way, if the seeds come to harm in some way, you still have some. And when harvest time comes, you won’t have all your produce at once, but can pick over a long period. This is referred to in books and catalogs as “succession-sowing”
Supplies you'll need:
Garden crops need much brighter light than houseplants. Most gardeners will need to use artificial light. Don't pay extra money for “grow lights” or special “full-spectrum” lights. A plug-in florescent fixture (often called a "shop light") with cool white bulbs is perfect. Cool White is very similar to spring sunlight. Incandescent or “warm white” light will produce slower growth—it is more like sunset or fall light.
The light needs to be close to the plants--light loses strength quickly with distance. Hang your light on adjustable chain or cord so that it can start 4-6” from the soil, and raise it as the plants grow (or start by putting the plants on blocks and lower them.) Plants that are tall and thin, with lots of stem between leaves, need more light. If a plant gets bleached leaves, the light is too close. If they are so stretched out and thin that they flop over, start over. They will not survive outdoors.
A sunny window will work, if all the seedlings can get light—But bear in mind that the light is from the side. You will have to turn the flat often. If the window “almost works” but is not quite enough, consider hanging a light over the plants, or to the side away from the window. A compact florescent (larger size) in a cheap “drop light” from the auto supply store could be just the boost they need.
What pots to use?
When you first start the seeds, they can be in anything that will hold soil and drain well. The top should be wider than the bottom to minimize transplant damage. A yogurt container with holes in the bottom would work. But beware--if you use something shallow, you will have to transplant promptly into a deeper container before the roots get cramped. And if you are not home, or forget to water, a shallow container will dry out very quickly.
I recommend 3" or 4" pots like flowers come in at the nursery. They fit well in a flat, have enough root room, and are easy to handle. If you invest in standard flats and square pots sized for the flat, you will be able to fit more in a small space, and they will fit on a standard heating mat. But if you need to improvise, just make sure that your pots are at least 3" deep.
I do NOT recommend the small six-packs you buy annual flowers in, or egg crates, or anything else with a very small pocket of soil. The small soil volume makes your plants very vulnerable. They could easily dry out and die while you are at work, or gone for any reason, or just forget them for a few hours when its very sunny. You would also have to fertilize sooner, and transplant sooner, than if you use a bigger pot. Professional growers can use small pots because they often have automatic watering, and are scheduling their day around taking care of seedlings--it is their job. If you don't want to be checking them several times a day, use bigger containers.
Germinate seeds at 65 to 75 degrees F. Tomatoes, peppers, melons, eggplant, and basil will germinate best with additional heat, such as the garden heat mats you can get at the garden store. The top of a refrigerator, gas stove with a pilot light, or any electric appliance that is on for long period will often add just enough extra. You don't need anything above room temperature once leaves appear.
Use a good-quality soil mix or make one up from garden soil sand and compost in roughly equal parts. Price is a good indicator of quality in potting soil. Earthworm castings in the ingredients list is another sign of good quality. Soil should be moist but not wet. We recommend using a fungal inoculant in your soil mix to prevent damping off, help the seedling grow faster, and extend the roots’ ability to find nutrients and water.
Once they Sprout:
Growth should be continuous--if it slows, feed with compost tea or other mild liquid fertilizer. Check to see that roots have not filled the pot. Once seedlings are as tall as the pot is deep and have several pairs of leaves, they should be transplanted to a larger container or into the garden.
If seedlings appear withered at soil level and fall over, they’ve been attacked by fungi and will die. This is a called damping off. Here’s how to prevent it:
-Use a loose soil mix that drains well and doesn’t compact.
-Use mycorrhizal inoculant to prevent disease (the good fungi fight the bad fungi)
-Maintain good air circulation
-Keep temperature around 65 degrees F
-Avoid overwatering - VERY IMPORTANT! Once the seedlings are up, wait til the soil surface looks dry before watering.
Getting Ready to Transplant:
For about a week before transplanting to the garden, the seedlings need to be “hardened off.” Set them outside during the day and bring them in at night. Make sure they don’t dry out and protect them from wind and critters. Or put them in a cold frame and open the lid longer each day.
Plants that turn purple or yellow are stressed and need more warmth or nutrients.
If your seedlings are in a standard nursery flat, you can get clear plastic covers that make it easy to give them shelter at night. This gives them a step in between coming inside at night and being fully outdoors. You could use those covers over plants in the ground as well, after transplant.
Once your seedlings are in the ground, monitor the weather forecast. If temperatures are predicted to be 36 degrees or less, cover your plants with bedsheets, fleece (Reemay, Agribon, etc.), or make a rudimentary frame (ie, stick branches in the ground) and drape it with clear plastic. If it will only be cold during the night and you can take it off in the morning, a cardboard box is good insulation. However, you MUST take it off in the daytime. I haunt the thrift stores for old white sheets, a good cover in case of frost, and a good bit of light shade in hot summer weather as well.