by Sarah Jay, Modern Farmer
March 16, 2026
Tomatoes are the best thing about late spring and early summer markets. Seeing large heirlooms in multiple colors lining stands is exciting for customers and farmers alike.
Tomatoes should be started indoors in most regions of North America, where winters linger long enough to prevent the direct sowing that growers in temperate areas do. Indoor growing begins four to six weeks before the last frost, and direct sowing occurs one to two weeks after the frost ends.
Most of the tips mentioned below are for those who have to start their tomatoes indoors. One applies to both indoor and outdoor starting. Try them out, and the results will convince you to put them into your annual tomato-growing practice.
There are two basic types of tomatoes: indeterminate and determinate. If you live somewhere where the spring and summer are short, determinate tomatoes are easier to grow, as they produce a harvest all at once and have a defined max height. Many within this category don’t need support to grow. They also produce earlier than indeterminates.
Indeterminate tomatoes, on the other hand, tend to provide small harvests over the entire season. These are best for longer seasons, or those in areas where spring and fall are punctuated by hot summers, when tomatoes can’t set flowers due to high heat. Indeterminates need support and consistent tending, even when they’re not producing in hot weather. Plant multiples of these for a market-worthy harvest.
Aside from these overarching considerations are the types of tomatoes you want to grow. Do you want large beefsteaks or slicers for sandwiches? What about sauce tomatoes or canning tomatoes, which have rich, thick flesh? Don’t forget cherries, either. Maybe red tomatoes are already plentiful at the market, and you need to bring something interesting like a Yellow Pear or Green Zebra.
Before you start tomato seeds, take some time to figure out which ones are best for your farm, whether urban or rural, and for your purposes. Choose open-pollinated varieties, and know that heirlooms tend to be indeterminate.
If you start tomato seeds in solo cups and it has worked for you, that’s great. Of course, you can continue with the same setup each year, but a few minor improvements will increase germination and make transplanting easier.
Use seed cells and ensure there’s a grow light. Seed cells often have the drainage and the perforations needed for healthy root development. These are designed to air-prune roots as they grow, helping your tomato plants grow stronger as they mature. Locate seed cells that you can wash and reuse every year, so you don’t have to start new plants in new cups annually.
Grow lights give developing plants the light they need to develop strong and healthy foliage and stems. If you have to grow indoors for the entire life cycle of your tomato plant, find a light that provides both rays that support vegetative growth, flowering, and fruiting.
Akin to adding better reusable equipment to your indoor setup is adding a heating element to increase germination rates. Tomatoes need warm soil to germinate effectively, in a way that won’t make it so you waste seeds when you sow them. Keep the soil warm (at 65 to 85°F or 18 to 29°C) for best results.
Heat mats are the best way to ensure future harvests when you start tomato seeds. Coupled with lights, warmth simulates the natural environment of tomatoes, which originated in western South America. This climate is warm and humid, and stays temperate year-round.
While you will still see germination without a heat mat, rates will be significantly less than they would be with one. If you don’t have time to locate a heat mat, use a chicken heater in your seed starting area. These provide ambient warmth that isn’t drying and harsh like it would be from a space heater.
As mentioned above, the native area of tomatoes is consistently warm and humid. Therefore, the soil should be moist when you start tomato seeds. After your sprouts come up, keep the soil moist but not waterlogged. Tomato plants are fleshy and require lots of moisture to thrive.
Because tomatoes grow rapidly, they use a lot of water as they mature. Keeping the soil full of the moisture they need assists them with their quick maturation process. If you’re sowing in an environment that’s super-dry, use a seed starting soil with water retention materials.
Worm castings, peat or coco coir, shredded leaves, and well-rotted compost are all great materials for water retention in soils. Keep a good balance of moisture-retentive materials with drainage materials, like perlite, agricultural sand, or agricultural grit.
After you start tomato seeds, they grow quickly and take over the seed cells. If you grew more than one seed per cell, you might have two or three tomato plants in each cell. If you live somewhere that has frosts into springtime, they need to stay indoors until frost danger passes. Once they grow their first set of true leaves and reach a few inches tall, it’s time to up-pot your tomatoes.
One way to maximize your harvests is to gently separate the multiple plants growing in one cell, rather than thinning them and getting rid of the smaller ones. Just use your hands to untangle the seedlings and up-pot each into its own container. Seed starting soil isn’t necessary at this time. Use a basic potting soil or a garden soil amended with drainage materials.
Finally, to give your seedlings a boost, fertilize them. At this stage, don’t use a tomato fertilizer, or they’ll start to flower too early. Instead, a full spectrum organic fertilizer is best. Use a diluted liquid (to ¼ strength), like fish emulsion, or the long-standing combo ferts that include multiple elements that support growth. Apply when you water next.
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