The tomato is a warm-weather vegetable, it is very tender to frost and light freezes. Never plant near walnut family trees. The walnut trees excrete an acid that inhibits growth of nearby plants.
Plant basil nearby to repel flies and mosquitoes, and improve flavor and growth. Bee balm, chives, and mint will also improve health and flavor.
Corn and tomatoes are attacked by the same worm, and should not be planted next to each other because of this. Kohlrabi stunts tomato growth.
Keep potatoes away also, as they both can get early and late blight.
Also keep cabbage and cauliflower away from your tomato plants.
When to Plant
Tomatoes should not be planted outdoors until day and night temperatures are about 55 degrees. Low temperatures (below 55 degrees) prevent fruit set. Soil temperatures should be at least 55-60F to transplant. Otherwise plants may turn yellow, become stunted and slow to bear. Seed should be started indoors 6-8 weeks before plants are set out. The ideal plant size is 6-10 inches tall.
How to Plant
To start tomatoes indoors, sow seeds using expanding seed starting soil pods about 8 weeks before the last frost date for your area. Seedlings will be spindly with less than 12-14 hours of light per day, try to keep them in a warm sunny location.
When seedlings have 4 leaves, transfer to a deeper pot (3-4") and again when 8-10 inches tall. Each time, place the uppermost leaves just above the soil line and remove all lower leaves.
Transplant (see: guide to transplanting) into the garden when the stem above the soil has reached 8-10 inches tall. Be sure to harden them off before transplanting them outdoors. Allow up to 10 days for the tomato plants to harden off to the outside temperature fluctuations.
Set your seedlings out when the temperatures are fairly certain to be above 55 F throughout the night. Seedlings should be spaced about 2 feet apart for early tomatoes and 3 feet apart for main-season types.
Set tomato plants deeply, up to the first set of leaves; roots will form along the stem under ground and strengthen the support for the plant. A lanky seedling can be planted on its side, to the first leaves. It will right itself in a day or so. Use a starter solution (half-rate water-soluble fertilizer) when setting out tomatoes to give them a good, quick start. Listen for cold weather warnings; if late frost seems imminent, cover the plants at night with newspaper tents.
How to Grow
Some sources suggest that indeterminate and larger semi determinate varieties should be pruned of all suckers (tiny leaves and stems in the crotches of other stems) because they may steal nourishment from the fruits (see: Pruning Your Tomato Plant: A Suckers Guide to Removing Suckers).
Feed with a starter solution when the first plants are set out and again after the first flowers form. Continue to supplement with a weak fish emulsion or compost tea every 2-3 weeks. If you do not enrich the soil (see: Guide to Amending Soil) before planting, feed the tomatoes once a month with about 1/3 cup of 5-10-5 fertilizer scattered in a 2 foot wide band around each plant. Tomato plants need at least an inch of water per week; so water them well, especially during dry spells. If the plants are well mulched (see: Guide to Mulching), weeds should not be a problem. Try using a plastic mulch in either a red or black color. The mulch will help prevent weeds and keep soil borne pests from splashing up on the plants, in addition to helping control moisture. A generously moist growing season followed by a severe drought period, will often initiate blossom-end rot, which appears first as a water-soaked mark that develops to a flat, dark leathery spot. It can be discouraged with mulching and consistent water levels. Unlike most crops, you may solarize the soil as you grow tomatoes because they're very heat tolerant. Solarizing helps control disease, particularly verticillium wilt. Wet the soil and cover with clear plastic for the entire season for best results. Hand pollinate in greenhouses (see: How to Hand Pollinate Your Tomatoes).
To keep indeterminate plants from making too much leafy growth, prune them to a single main stem by breaking off side shoots as soon as they appear. You will notice these side "suckers" growing between the crotch formed between the main stem and the leaf stem. Cut them out while they are small (see: Pruning Your Tomato Plant: A Suckers Guide to Removing Suckers). The terminal shoot is pruned off when the plants reach the top of the 5-6 foot stake to stop their growth. These plants are also pruned of suckers, the side shoots that grow between the main stem and the leaf axils to moderate their vegetative growth. Most gardeners prefer to prune their tomato plants to one or two main stems (see: Spank Your Tomatoes! -Get More Fruit on Every Plant)
How to Harvest
Pick the fruit when it is red ripe, and check the plants every few days when the harvest starts coming. Store excess tomatoes in the refrigerator, but the flavor is best at room temperature. Tomato flavor starts to decline at temperatures below 55 degrees. When frost threatens, pick the remaining green tomatoes, wrap in newspaper, and keep in a moderately dark, warm place. They will ripen gradually well past the harvest season. Given warm weather and abundant rainfall, tomatoes ripen in 60-85 days from the time seedlings are set out. When the fruits begin to turn red, check the plants every day and pick those that are fully red and firm, but not hard. Overripe tomatoes will fall off the plant and rot quickly. To store an abundance of tomatoes at the end of the year, you can roast them, and store in a little olive oil in the refrigerator.
A very light frost will usually kill a few leaves, but the plant itself will continue to grow and produce. However, anything more severe than a touch of frost is likely to kill the entire plant. If frost is coming you can protect each plant by draping it in plastic sheeting or old bed sheets, or you can pull the plant up by its roots and hang in the basement until the fruit ripens. Neither method is guaranteed to work, and in cool areas an early frost almost always means the end of harvesting tomatoes.
How to Save Seeds
To save seeds from open-pollinated varieties, allow perfect fruits to ripen until they become soft. Cut them in half and squeeze the gel and seeds into a jar. Cover with 3 inches of water and shake well. Allow the mixture to sit at room temperature for 24 hours before pouring out the liquid. Discard the floating seeds, and rinse the larger seeds on the bottom in a strainer and then dry them at room temperature for approximately 2 weeks. If handled and stored properly in a cool dark place, tomato seeds can last up to 6 years.