How to Plant Tomatoes Indoors

Tomatoes (Solanum lycopersicum) are often planted indoors to get an early start on garden production. You may also grow them entirely indoors to protect them from unfavorable outdoor growing conditions. New gardeners can successfully start or grow tomatoes indoors as long as the plants' basic needs are met.

Tomato varieties come in two types: determinate and indeterminate. The determinate varieties, also called "bush tomatoes," produce a lot of fruit all at once and then their productive lives are over. This type often does not need trellising. Indeterminate varieties, or "vine tomatoes," produce a few fruits at any given time, but will continue to produce until cold weather kills them. Indeterminate vines can easily grow to 10 feet long or more in a single season, and are often trellised to keep the fruit off the ground. If you are planting tomatoes for indoor production, ensure you have enough room for indeterminate varieties, or use a bush variety for more compact plants.

Fill one or more containers with starting soil or growing media. Plant each tomato seed about 1/4-inch deep, one seed per container or tray cell, then cover lightly with more soil. Keep the soil uniformly moist and warm throughout the germination period. Provide bright light for at least 12 hours per day during germination. The seedlings will start to emerge after five to seven days. After the seedlings develop their first true leaves, they can be transplanted into larger containers.

Paper or foam cups, peat pots, plastic or clay pots, and seedling trays are all fine. If you want to plant just a few tomatoes, small pots work well. If you want to plant a lot of tomatoes, a seedling tray will keep them all organized and make watering easier. Ensure that your containers allow drainage at the bottom and set them on some kind of tray to collect water that drains out. This tray only needs to be about 1 inch deep.

Tomato seeds germinate between 50 to 95 degrees Fahrenheit, but their optimum temperature is 85 F. You can heat the root zone with a heating mat or soil heating cable under the seedling tray or pot. If you don't heat your containers, the seeds will eventually sprout, but it may take longer.

Provide at least 12 hours of strong light -- enough to cast a sharp shadow -- each day while tomato seedlings are becoming established. You can also use a windowsill or naturally sunny spot. If you do not have sufficient natural light, you can use standard fluorescent or grow lights as supplements. If your light source is localized, turn the plants every few days for even coverage.

Your soil mix should be moist, but not soggy. Roll some soil into a ball in your hand, then squeeze. If you can squeeze just a few drops of water from the ball, that's perfect. If the soil won't form a ball, it's too dry, and if it's drippy or muddy, it's too wet. Once you've planted your seeds, maintain this level of moisture in the soil. Start by watering once a day and see how long the soil remains moist. If needed, add a second watering per day, but be careful not to over-water.

One helpful practice is to water from the bottom, pouring water into the trays that hold the containers. This allows water to be drawn up into the soil or growing media without disturbing the seed. Start by adding enough water to half-fill the tray. If it is drawn up by the soil within only a few minutes, add more. If it is drawn up by the soil slowly over the next hour, that's just about perfect. Once the water has been drawn up by the soil, let the tray stay dry for at least eight hours before watering again.

Purchase starter mix or make your own with four parts compost, one part perlite, one part vermiculite and one part peat moss. Avoid using straight garden soil, which has a wide range of fungal spores that can overwhelm seedlings before they even get started. Once the seedlings have at least one set of true leaves, transplant them into a larger container using standard potting soil, which will provide nutrition for the rest of the year. If you use hydroponic growing media instead, you will need to start adding either liquid or powder nutrient formulations -- available through indoor gardening and hydroponic suppliers -- to feed your young tomato plants.

Transplant your tomatoes outdoors after all danger of frost has passed, or to an indoor pot that is several gallons in size. In either location, if you continue to offer the plants bright light for at least 12 hours a day, consistent watering and temperatures above 55 F, you will be rewarded with beautiful plants and plentiful tomatoes.