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Grow Guide: Tomatoes

Starting seeds may seem intimidating to a beginner, but it's a reasonably straightforward process. This guide discusses the supplies you’ll need, teaches you how to prepare for planting, gives you step-by-step instructions for sowing, explains how to care for seedlings, and helps troubleshoot common problems.

When to Start Tomato Seeds Indoors

Tomato plants have a longer growing season, meaning they take longer to go from seed to bearing ripe fruit, than many other vegetables in your garden. Because plants are susceptible to frost, starting seeds indoors approximately six to eight weeks before the last spring frost for your area is necessary. Once the threat of frost is low, you can transplant the young plants outside into the garden.

Supplies Needed

  • Seeds from the Tomato Variety Pack
  • Growing media: Coconut coir and commercial potting soils are the two most common growing substrates used in containers, and both are available online or at local retailers. A quality growing medium in your containers is essential; it holds on to moisture and nutrients, provides air space around the roots, and anchors the plant’s roots to keep it upright. 
  • Containers: Seed-starting trays are the easiest to use, especially with domes or covers. The trays are wide and shallow, allowing you to plant many seeds in one tray. You can also use individual pots to start your seeds or recycle plastic containers from your kitchen (clean yogurt cups, sour cream containers, etc.). Just ensure they are cleaned and sterilized.
  • Plant tags: Use plastic or wooden plant tags to label what seeds you have planted and where. Plastic plant tags are more durable, and the words don’t fade as quickly; wooden tags are biodegradable and more environmentally friendly.
  • Supplemental grow light (highly recommended): LED, fluorescent, and compact fluorescent grow lights are good choices for hobby growers. Grow lights emit different wavelengths or “colors” of light crucial for plant growth. 
  • Seed starting mat (highly recommended): A seed-starting mat can accelerate germination. It is similar in size and shape to a household heating pad and goes underneath a seed-starting tray to warm the growing medium.

Reusing Supplies

Reused items like potting soil and containers must be thoroughly cleaned and sterilized to remove pathogens or fungal spores. This is especially critical if you have had trouble with damping off in the past.

Sterilizing Potting Soil

To sterilize your potting soil, thoroughly moisten it and place it in an oven-safe dish no more than three inches deep. Cover with aluminum foil and place in an oven preheated to 200°F. When the soil's internal temperature reaches 180°F, bake for thirty minutes without opening the oven door. Shut the oven off and keep the soil inside until it reaches room temperature.

Cleaning Used Containers

Scrub the containers with soapy water and a soft-bristled brush to remove any potting soil, then rinse well. In a large bucket or a sink, mix one part bleach with nine parts water. Submerge the containers and let them soak for 15 minutes, then set them aside to dry. 

Starting Seeds

If you’ve ever started seeds indoors before, you will already be familiar with the basic instructions. The following steps have been tailored to tomato seeds specifically.

Pre-soaking Seeds

Before planting, many seeds are best soaked in water to speed up germination and improve the number of seeds that sprout. The water softens the hard seed coat, exposing the embryo to moisture and kickstarting growth. It is best to soak tomato seeds overnight and then plant them the next day when the seeds start to swell. 

Prepping Growing Medium

Pre-moisten the potting soil by putting some in a large basin or bucket and adding lukewarm water. Use a clean trowel, serving spoon, or your hands to mix it thoroughly. The potting soil should feel damp but not soggy. It should form a clump and hold together if you squeeze a handful, but excess water shouldn’t drip out.

Planting Instructions

  1. Fill containers almost to the top with pre-moistened growing medium.
  2. Plant seeds at a depth of ¼ inch, then cover gently with potting mix. 
  3. Mist the potting soil with room-temperature water.
  4. Cover the seed tray with the accompanying plastic lid to keep the humidity level high or create a dome for a container by placing a plastic storage bag over it.
  5. Set the tray on top of a seed-starting mat or somewhere slightly warmer than the air temperature in your home, such as on top of the refrigerator.
  6. Periodically mist the potting soil to moisten the medium without disturbing the seeds.
  7. If heavy condensation collects on the cover, open the vents slightly to increase air movement or prop the lid open with a pencil or dowel.
  8. Once seeds germinate and sprout, remove the cover to prevent damping off and move the container(s) to a spot with plenty of light.

Transplanting Seedlings to Individual Containers

About 30 days after planting, seedlings will develop the first set of true leaves, followed quickly by a second and third set. At this time, transplant them from the seed tray into individual containers to give the roots more room to grow. Handling the seedlings also helps to toughen them. Be careful when separating them to prevent breaking the stems or damaging roots. 

Caring For Your Tomato Seedlings


When grown outdoors, most tomatoes require a location classified as full sun, where they receive a minimum of six hours of sunlight daily. Indoors, they require twelve to sixteen hours of daylight. The best locations in your home are by south- or west-facing windows. South-facing windows get the most sun. West-facing windows stream direct sunlight for a good portion of the day and are often shaded from the intense sun in the late afternoon. 

There is a high likelihood the light inside your house isn’t sufficient for good growth, especially in the winter when there is less daylight. Plants that don’t get enough light will grow tall and spindly, lacking robust flavor. To compensate for low light and prevent legginess, purchase a simple and inexpensive grow light for your tomatoes. 

Temperature Requirements

Most tomatoes prefer daytime air temperatures between 75°F and 85°F or soil temperatures between 70°F and 80°F. Keep nighttime air temps between 60°F and 75°F. This fluctuation mimics the drop in temperatures outdoors once the sun goes down. Never allow the air temperature to drop below 50°F. 

Beware extreme temperature variations when growing tomatoes indoors. These fluctuations cause internal stress, hindering plant growth and yields. To minimize variations, avoid putting your tomatoes where they can feel cold drafts from a door or window or register vents that blow heat during the winter.

Relative Humidity Levels

Because tomatoes are native to tropical or semi-tropical regions, they grow best when the relative humidity is high. Ideal daytime humidity levels are between 80 and 90%, which isn’t always feasible indoors. Lower levels are sufficient, but anything below 45% relative humidity stresses the plant as water evaporates from the leaves. 

You can increase the relative humidity around your tomatoes by grouping plants together, misting them periodically with a spray bottle, or setting containers atop a pebble tray that holds water. To minimize fungal problems, create space between the plants for air to circulate or set up a small fan nearby to move air through the foliage.


Once seedlings have sprouted, keep the growing medium moist without overwatering. The preferred method is to water containers from the bottom. To do this, place the seed tray or container(s) in a shallow pan of water. The growing medium will only wick up the moisture it needs to fill the pore spaces. This helps prevent overwatering and won’t dislodge the seeds or seedlings like overhead watering may.

Tap water is usually acceptable for your plants, but there are a few things to keep in mind:

  • If your home has hard water, you will see a white buildup on the soil surface over time. This buildup comprises calcium and magnesium salts from the limestone in the water. Periodically flush the soil with filtered or tap water to remove.
  • If you have softened water, collect water from your pipes before it goes through the softener or use distilled or bottled water.
  • If the local municipality treats your water, it may have a reasonably high chlorine content, which can be problematic in sensitive plants. Before watering your plants, fill a watering container and let it sit for 24 hours to allow the chlorine to dissipate.


There is no need to fertilize your seeds before they sprout. Inside every seed is an endosperm that stores food, usually starch and some nutrients, for the developing embryo. This is enough to get the seedling started, especially since the plant’s needs are low because of its size. However, you’ll need to fertilize your tomatoes once their growth takes off.

When your seedlings develop the second (or third) set of true leaves, which is when you should transplant them into individual containers, you need to start fertilizing them. Tomatoes are heavy feeders, so they need to be “fed” regularly. Feed your seedlings a half dose of your chosen fertilizer when transplanted and then every two weeks. Options include fish emulsions; liquid seaweed extracts; tomato-specific blends; or a balanced, water-soluble formulation.

Hardening Seedlings Before Transplanting Outdoors

About a week before you hope to move the seedlings outside to the garden, start acclimating them to outdoor conditions. This process, called “hardening off," helps minimize transplant shock from severe temperature variation and light-exposure differences. Start by setting the planting trays or containers outside in a sunny spot protected from the wind for a few hours. Gradually increase the length of time the plants are outside every day, bringing them in at night, until it’s time to transplant.

Transplanting Seedlings Outdoors

When it’s time to plant your tomatoes in the garden, space plants about two feet apart within rows, leaving three to four feet of space between rows. Dig planting holes about twice as wide as the container and at least twice as deep. 

Tomatoes grow roots anywhere the stem is covered in soil. Plant the bottom 6 inches of the stem in the ground or up past the first set of leaves. Burying them deeply like this creates a more extensive, robust root system.

Troubleshooting Common Problems

Damping Off

Damping off is a problem caused by fungus or mold that thrives in cool, damp conditions, resulting in seedling death. The most effective way to avoid damping off is to increase air circulation by setting up a small fan nearby. Keep the potting soil moist while avoiding overwatering, and keep containers in a warm location. 

Leggy Plants

When seedlings grow tall and spindly, it’s a sign they aren’t getting enough light. If you are using natural light, try moving the plants to a spot that gets more sun or supplement the lighting with a grow light. If you are using grow lights, they may be positioned too far from the plants. Place shims (books work well) under the trays so the plants are closer to the light source. The lights should be four to six inches above the tops of the herbs. Some seedlings grow faster than others, so stacking smaller shims works well, making it easy to maintain the proper spacing.

Discolored Leaves

Because the seed only contains a small amount of food, young plants can discolor when these reserves deplete. A lack of phosphorus causes leaves with a reddish-purple hue. Yellow leaves can indicate your seedlings need nitrogen. To fix discolored leaves, apply a half dose of diluted plant food.

Insect Pests

Indoor plants have fewer pest problems than outdoor plants, but fungus gnats are problematic indoors. You can have problems with aphids and spider mites, but they typically prefer larger plants with bigger leaves. Pest problems can be discouraged with increased air circulation and by avoiding overwatering; insecticidal soap or neem oil can also be used as a treatment.