How To Grow And Care For Tomatoes Indoors

Keep your indoor tomato plants thriving with these top tips.

From planting and tending heirloom varieties to pruning and avoiding mistakes, we've covered lots of ground for growing tomatoes (Solanum lycopersicum). Potential pitfalls and tasty triumphs when growing tomatoes make them an edible garden favorite. Turn the warm-weather fruit-producing season into a year-long adventure by exchanging tomatoes grown staked outdoors for indoor gardens. While you can grow tasty, fresh tomatoes indoors throughout the winter, several care conditions remain to meet to produce the best harvest.

However, we've been learning how to grow tomatoes best indoors. If you currently lack garden space or prefer to grow veggies indoors where you can more easily control environmental factors like temperature and pest control, read on for some tips on starting tomatoes indoors. As a member of the nightshade family, tomato plant leaves, stems, and roots are toxic to people and pets, so be mindful when growing indoors.

Plant Attributes

Tomato Care

Meeting specific environmental factors is necessary to keep your indoor tomato plants thriving. First, they need good lighting, at least six, and up to eight or ten hours of direct sunlight daily. 

Use seed trays or even an empty egg carton filled with planting mix to start the plants. A seed-starting potting mix geared toward vegetable planting is more likely to help you avoid the fungal and bacterial issues often associated with topsoils. A hydroponic system can also be an excellent strategy for growing tomatoes indoors. Self-watering systems and built-in LED lamps can ease responsibilities for new gardeners.

If you're wondering when to start tomato seeds indoors (and if you intend eventually to transplant them into gardens or containers outdoors), then sow them about four to six weeks before the anticipated date of your area's last spring frost.

Light

Tomatoes thrive in full sun—at least six hours daily. When growing tomatoes indoors, you'll want to place your tomato plant in a sunny window for warmth or invest in an LED grow lamp for a more reliable light source.

Soil

Use a pre-made soilless potting mix when growing tomatoes indoors. You can use a combination of vermiculite and peat moss and incorporate a 5-10-5 fertilizer to add nutrients to the soil. Keep the soil moist but well-draining and slightly acidic with a pH between 5.8 and 6.8.

Water

Tomato plants growing indoors require more frequent watering than those grown outdoors. Use your hands to feel the soil to know when it's time to water again. Wait until the first few inches are dry to water again. Keep the soil consistently moist but well-drained. Provide plenty of air circulation and drainage holes—Using a saucer until the tomato plant can help with drainage and prevent the roots from getting waterlogged.

Temperature and Humidity

Tomatoes love warm weather, so keep your plants in toasty temperatures—70°F or higher. Mimic sunny summer environments to help them thrive. Avoid placing plants near air vents. Shield it from cold drafts and excessive heat from furnace vents. Keep tomatoes in an environment with slightly less humidity at night than during the day, which should be about 80 to 90 percent.

Fertilizer

Once the plants begin to grow and emerge, you'll need fertilizer. Regular fertilization helps tomato plants thrive since frequent watering depletes container plants of nutrients—Fertilization includes adding some during planting time. Use a weekly water-soluble or organic fertilizer for edible plants. Add a granular fertilizer every four to six weeks.

Types of Tomatoes

Your first instinct may be to find the tastiest tomatoes imaginable, but there are a few more things to consider when choosing which tomatoes to grow. For an indoor plant, your best bet will be to choose a compact variety that produces small fruits, like a cherry tomato or grape tomato. Hanging cultivars can also be suitable for small spaces. You can buy a starter kit, which will set you up for success, or you can find seeds at your local garden store. Here are some popular smaller varieties:

• 'Better Bush': This variety bears medium-sized fruits on a compact plant. These tomatoes grow well upright and staked for support.

• 'Candyland Red': Produces tiny, tasty, currant-type tomatoes, ideal for various foods.

• 'Yellow Pear': A heirloom tomato that bears large, bright, yellow, bite-sized, tangy fruits.

Pruning

Depending on the variety, pruning tomato plants is relatively simple. Determinate varieties do not need pruning. You must remove small stems that grow off the main branch in indeterminate varieties. The small stems do not produce fruit, so removing the excess stems will encourage the fruit-bearing branches to develop more than the foliage. 

When pruning plants, cut dry plants early in the morning every few weeks. Continue to remove suckers as they emerge, using your fingers to pinch off suckers under two inches long. After staking the tomato plant, remove any excess foliage that touches the soil, as this makes it susceptible to infections.

Propagating Tomatoes

Tomatoes can easily propagate using cuttings taken from seedlings. Here's how:

1. Keep your tomatoes in a warm environment when making cuttings. Choose shoots with suckers on them that do not have buds.

2. Use sterile pruning shears to select a six-to-eight-inch selection.

3. Remove all leaves but the top set.

4. Place cuttings directly in clean water or a container filled with damp soil, such as potting soil or vermiculite. Use your fingers or a pencil to create a hole a few inches deep in the soil.

5. Cuttings growing in water should root in about a week but won't be as strong as those growing in soil.

How to Grow Tomatoes From Seed

Tomatoes grow quickly from seeds, making them a perfect garden plant. Here's how:

1. Using a seed-starting tray, fill the container with damp potting soil and add two or three seeds in each section. Water deeply when sowing seeds.

2. Cover the entire container to preserve moisture—place in a dark location, preferably around 70°F to 80°F.

3. Seeds will sprout within one to two weeks. After sprouting, move the container to a sunny location or use a garden light for artificial light.

4. Transplant seedlings after leaves appear. Ensure the new container has plenty of drainage holes.

5. Begin gradually exposing tomato plants to outdoor elements for a few hours daily. Prevent tomato plants from temperatures below 50°F.

6. If preferred, transplant tomatoes to outdoor containers or a garden after two weeks of cold exposure. Add a layer of mulch around the roots if necessary to retain moisture.

7. Continue to water and fertilize tomato plants consistently. Prune branches that touch the ground to prevent diseases and remove fruit attacked by pests.

Potting and Repotting Tomatoes

Tomato plants prefer to keep their roots intact once established, so try to select containers large enough to accommodate the plant's growth. If necessary, to repot plants, carefully dig around the root system and place the plant in the new container so a portion of the stem is below the soil line. Gently pack the soil around the base of the stem and water deeply when transferring. Fertilizing the plant at the time of planting can help add nutrients to the plant it may have lost during repotting.

Overwintering

Tomatoes growing indoors still need plenty of light during the winter season. While it is possible to grow these plants throughout the colder season, pay close attention to the water and fertilizer schedule so the depleted nutrients do not impact the plant's growth. Using a garden lamp can help provide artificial light for the tomato plants growing indoors, but avoid placing the container near a furnace or heat vent.

Common Pests & Plant Diseases

Growing tomato plants indoors does not mean pests and diseases will not impact them. Aphids, whiteflies, spider mites, and common pests can infest the plant but are treatable if caught early. Spraying plants with water is often the most effective method of removing pests, but if the issue persists, there are ways to trap or handpick pests. If these methods are not solving the problem, consider using neem oil.

Some diseases that impact tomato plants growing indoors include rot, wilt, and blight from waterlogged roots. Maintaining proper care will help prevent these diseases from developing, including providing plenty of direct sunlight, drainage holes, and the correct amount of fertilizer.

Common Problems With Tomato Plants

While tomatoes need specific environmental ideals to thrive indoors, there's usually an apparent fix for what ails them. Keep an eye on things to ensure they get the care they need.

Curling Leaves

If your tomatoes aren't thriving indoors, check on the light and water they're receiving. Adding fertilizer or amending the soil can also help. If flowers appear on your tomato plants, but no fruits are developing, this could result from nighttime temperatures that are too low or too little water. If your leaves start to wither and curl, this could be due to high temperatures, overwatering, and aggressive pruning.

Leaves Turning Yellow

Tomato plants need many nutrients to thrive, especially container plants growing indoors. As indoor plants require more frequent watering, the soil nutrients often deplete faster than plants grown in an outdoor garden. To ensure healthy tomato plants and avoid yellowing leaves, use a fertilizer with iron, magnesium, and nitrogen. Yellowing leaves are also a sign of older plants, but if you are beginning your tomato plant's growth, the lack of nutrients is probably the reason for the foliage discoloration.

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