When to Plant Vegetables by Seed or Seedling
The last frost is the date when you're free to plant vegetable seedlings or direct-sow vegetable seeds into the garden. The last frost date is in late spring, but the date varies from year to year and from location to location. You can find the correct date at the local garden center or from the nearest office of the Cooperative Extension Service.
Transplanting seedlings into the garden
If you have seedlings ready to go into the garden after the soil is ready and sufficiently warmed up, your garden can get off to an earlier start. This timing puts fresh food on the table sooner. Vegetables that you can start early indoors include cabbage, tomatoes, peppers, and eggplant.
Remember, if you start too early, your seedlings may be too big too early, making them a little hard to accommodate and care for — you may even have to start over. Here's a general list to get you started; you can tinker as you get more experience raising various sorts of seeds. Yep, get out your calendar — some counting backwards is in order:
Onions: 12 to 14 weeks before the safe planting-out day (which in the case of onions is 4 to 6 weeks before the last frost)
Broccoli, collards, and cabbage: 5 to 6 weeks before the safe planting-out date (which is after the danger of snow and ice is past but while nights are still chilly)
Lettuce: 5 to 6 weeks before the safe planting-out day (which is 4 to 5 weeks before the last frost)
Peppers: 8 to 12 weeks before the last frost
Tomatoes and eggplant: 6 to 8 weeks before the last frost
Cucumbers and melons: 2 to 4 weeks before the last frost
Sowing seeds directly into the garden
Gardeners generally sow seeds directly in the garden after the last frost, after the soil has warmed up and the weather seems to have settled into an early-summer groove. Direct-sowing in cold and/or soggy soil is a bad idea — it's muddy work for you, and the seeds usually sprout poorly or rot; then you have to start over. Best to wait for the right time.
Though you can buy tools and gadgets to help you with this work, there remains something so primal and satisfying about going out with a seed packet and digging in the dirt with your hands.
Vegetables that you can direct-sow include lettuce, onions, peas, radishes, turnips, beets, cabbage, carrots, beans, corn, parsnips, cucumber, lettuce, and tomatoes (in warm climates).

Gardening Glossary
annuals
Plants that complete their entire life cycle within one growing season. The plant germinates from seed, grows and blooms, and then produces seed and dies.

Gardening Glossary
biennials
A plant that take two growing seasons to complete its life cycle. It germinates and grows leaves and stems in the first year; produces flowers and fruit (seed) in the second, and then dies.

Gardening Glossary
bolt
When a plant flowers or produces seed prematurely.

Gardening Glossary
cold frame
A wooden or concrete block box in which you can grow plants or hold dormant during the cold winter months.

Gardening Glossary
cole crops
A family of vegetables, including broccoli, cabbage, cauliflower, kale, and Brussels sprouts. They thrive in cooler weather.

Gardening Glossary
complete fertilizer
Any fertilizer that contains all three of the primary nutrients, N-P-K (nitrogen, phosphorus, potassium). Phrase is based on regulations governing the fertilizer industry. Does not mean that the fertilizer literally contains everything a plant needs to thrive.

Gardening Glossary
deadheading
The practice of pinching or cutting off spent flowers

Gardening Glossary
evaporative-pad humidifier
A humidifier in which fans blow across a moisture-laden pad that sits in a reservoir of water.

Gardening Glossary
harden off
The process of acclimating plants grown indoors gradually to the brighter light and cooler temperatures of the outside world.

Gardening Glossary
hardiness
The ability of a plant to survive is called its hardiness.

Gardening Glossary
humus
A stable end product of organic-matter decomposition that's believed to increase microbial activity in soil, improve soil structure, and enhance the root development of plants.

Gardening Glossary
Bacillus thuringiensis Bt
An effective bacteria that attacks only the larvae of caterpillar family insects. It is safe to other insects, animals, and humans.

Gardening Glossary
macronutrients
Mineral nutrients that plants need in the largest quantities: nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.

Gardening Glossary
mulch
Organic or inorganic material placed over the surface of soil, usually directly over the root zone of growing plants. Used to conserve moisture, kill weed seedlings, modify soil temperature, provide attractive covering to garden beds.

Gardening Glossary
organic matter
Once-living stuff like compost, sawdust, animal manure, ground bark, grass clippings, and leaf mold (composted tree leaves). Used to enrich soil and improve soil texture.

Gardening Glossary
perennials
Any plant with a life cycle of three or more years. Herbaceous (non-woody) perennials include flowering plants and herbs, mainly. Woody perennials include trees and shrubs. Longevity depends on the plant and growing conditions.

Gardening Glossary
pH
The measure of soil's acidity. Soil with low pH means it's too acidic; soil with high pH means it's alkaline. Most plants grow best in soil with a pH value between 6.5 and 7.2. Neutral soils measure 7.

Gardening Glossary
photosynthesis
The process through which plants take nutrients from the air and from the water in the soil to produce sugars that fuels the plant's growth.

Gardening Glossary
primary nutrients
Nitrogen, phosphorous, and potassium are the three nutrients plants need in the largest quantities.

Gardening Glossary
root crops
Plants with edible underground roots such as onions, carrots, beets, potatoes, turnips. Most root crops are cold-weather crops.

Gardening Glossary
self-blanching
A type of cauliflower with leaves that naturally curl over the head and exclude light. Requires cool temperatures for leaves to curl effectively.

Gardening Glossary
sets
Small onion bulbs, about 1/2-inch wide, that were started from seed the previous year. Grow onion sets with the pointy end up.

Gardening Glossary
side-dressing
The act of adding a small amount of fertilizer around or "on the side" of plants after they're growing.

Gardening Glossary
succession planting
Planting small, 2-to-4-foot patches of plants every two weeks throughout the growing season so that you can harvest a crop over an extended period of time.

Gardening Glossary
thinning
The act of cutting the least robust seedlings in your garden to give the healthier plants more room to grow.

Gardening Glossary
vining crops
Crops that grow on vines, such as cucumbers, melons, pumpkins, and winter squash. They usually require support (staking, trellising, etc.) to keep them off the ground.