Plump, juicy tomatoes are always a favorite at harvest time during the height of summer through the early fall. When growing tomatoes in your vegetable garden, you have a choice of starting them from seed or buying tomato seedlings or buying transplants. Sowing seeds gives you the opportunity to for the whole experience from seed to fruit to your plate. 

We spoke to two gardening experts about when to start seeds indoors, what is the hardening off process and why it’s important, and when to transplant. 

When To Start Seeds Indoors

Tomatoes love hot weather and need plenty of heat and sunshine to grow and produce fruit. If you want to sow seeds to have tomato seedlings, you can grow them indoors for about two months before you plant. Start sowing tomato seeds indoors a month and a half to two months before you anticipate planting them outside. “It normally takes about 6 weeks to grow a tomato from seed to a viable plant,” says Greg Key, founder of Hoss Tools, based in Georgia and National Garden Bureau member. Knowing the last frost dates in your geographical region are important so you can time when to start seeds indoors. “We plant our tomato seeds 6–8 weeks before the last frost in the spring,” he says.

Before you purchase seeds, check if they are a variety that grows well in your region. “When selecting tomato varieties, make sure you buy seeds that are proven to thrive in your area,” says Key. Knowing what varieties do well equals a better chance at a healthy plant that produces a bountiful harvest. 

How To Successfully Get Tomato Seeds To Germinate

Heading to the local nursery and buying seedlings takes out all the legwork and time of sowing seeds, waiting for them to germinate and grow so they’re ready to transplant. But growing tomatoes from seeds is rewarding, allowing you to participate along the whole process–from seed to harvest.

There are several factors to consider when sowing seeds for a higher success rate and producing hardy plants. The medium to sow seeds makes a difference. “Use a good seed‐starting mix—not potting soil,” says Key. “A quality mix should be fine and consistent, without large pine bark chunks.” Seeds shouldn’t be covered in too much seed-starting mix, either. “Don’t plant too deep; normally, ¼ to ½ inch is ideal,” says Key. He adds perlite to help seeds germinate. “I like to cover my seeds with perlite, which increases humidity around the seed and makes it easier for the plant to emerge,” he says. 

Seeds need some water to germinate but too much can saturate the soil, causing it to become soggy or waterlogged. Instead of using a watering can, consider using a spray bottle to mist the soil. A quick mist 3–4 times a day will keep the seeds moist, but not overly wet,” says Key. 

Temperature and humidity are also essential for successful germination.  “You want to keep steady heat on your tomatoes to get them to germinate, as well as keep the soil moist,” says Bailey Van Tassel, author of Kitchen Garden Living and host of The Garden Culture Podcast, based in Tennessee. High temperatures are necessary to encourage germination. “Keep the seedling tray at 80–85°F until you see the plants emerge,” says Key. Consider wrapping the tray for the first day to increase humidity and moisture. “Use a seed dome or wrap the tray in plastic wrap for 24 hours after seeding to boost humidity,” says Key. 

How To Know When Seedlings Are Ready For Transplanting

After seeds germinate, they’ll begin growing into small seedlings. Aside from waiting until the average last frost date has passed, the seedlings will give signs when they can be transplanted outside. Check to make sure the roots are established. “When the plants can be pulled from the seed-starting tray with the soil intact, [for example], they’re well-rooted, they are ready to be planted outside,” says Key. Before you get ready to transplant, the tomato seedlings need to be hardened off. 

What Is The Hardening Off Process?

Seedlings that have started indoors are used to a protected environment. When it’s time to transplant, tomato seedlings need time to adjust to the outside elements before living outside full time. Hardening off is the process of acclimatizing or transitioning plants from a protected area, such as indoors or a greenhouse, before being exposed to the elements full time. “Hardening off is key because you’re acclimating the plants to the outdoors, strengthening them bit by bit, more and more each day,” says Van Tassel. 

Begin hardening off the tomato seedlings a couple of weeks before the frost date in your geographical region. Place them outside in the sun for a couple of hours per day, gradually increasing the amount of time over two weeks, according to the North Carolina Cooperative Extension. Temperatures should be a minimum of 45ºF or above before placing them outside, per North Carolina Cooperative Extension. Temperatures at night need to be considered, too. “The night time temps should be solidly at 55ºF or above,” says Van Tassel.

Slowly exposing tomato seedlings to the elements, such as wind and cooler temperatures, will make the plants hardier. Tomato seedlings that aren’t hardened off can struggle to grow or adapt to the outdoor climate after transplanting. “If you don’t harden them off, then they can go into shock and die, having had too much exposure to wind or cold too quickly,” says Van Tassel.

When To Transplant

It’s time to transplant after the seedlings have been hardened off and have been timed to coincide with the average last frost date or after. Transplanting too soon, such as when temperatures aren’t at least 45ºF or before the average last frost date, could impact the growth of your tomato seedlings. “Tomatoes really love the warmth of summer, so I often recommend you want as long as you can before transplanting seedlings,” says Van Tassel.

Rushing to transplant tomatoes before the ideal time doesn’t translate into an earlier harvest because tomatoes need specific conditions, such as plenty of sunlight and hot weather to grow. “Timing is so important because you want the long hot summer days to ripen your fruit fully, with a late summer heat being most ideal,” says Van Tassel. 

Tips For Transplanting

Make sure to provide the tomato seedlings ample room underground to establish roots and spread out. “Set the tomato plants deep—up to the first set of true leaves,” says Key. “This allows the plant to develop roots along the buried stem, resulting in a stronger root system.” Also pay attention to how much air can circulate around the tomato plants. As they grow, consider pruning them to ensure there’s ample air flow. Van Tassel prunes her tomatoes throughout the season for better air circulation.