Complete Guide on How To Grow Tomatoes Indoors

Tomatoes are one of the plants that represent summer. The aroma, the bright colors, and the yummy taste all remind us of a sunny summer day. Gardeners often can’t wait to start growing tomato seedlings. Today, we want to talk about how to grow tomatoes indoors. Can you, should you, and how do you do it? Here’s a complete guide on how to grow tomatoes indoors.

Can you grow tomatoes indoors?

Tomatoes are a great plant to grow indoors. All tomato varieties can be grown indoors, not just cherry tomatoes. You will not need any special varieties of tomatoes for indoor growing. Since tomatoes are perennials, you can actually grow a tomato tree indoors. What variety you choose depends on the space you have with the right conditions for growing and preference. Dwarf tomatoes like Tiny Tim are a good choice for a small space, but tall tomatoes pruned to the main stem can also be a good choice. Determine if you have room for a small or large pot to know if you should grow smaller varieties or have room for bigger indoor tomato plants. We have grown many heirloom varieties with the best results. 

Tomatoes also grow well from cuttings. If you don’t have room for a tree but want to grow the same great variety continually, take a cutting from the existing plant to start a new plant whenever the old one outgrows your room.

Tomatoes are pollinated by wind. Checking the plant every few days is enough to achieve pollination.

We have grown many heirloom varieties with the best indoor results, and we can assure you it works well.

Why grow tomatoes indoors?

Living in a cold, northern climate means we have to endure long, cold winters. With frost possibility till late May, it seems our winter lasts nine months. We can’t wait for spring. We want to start planting. We want to grow a garden. It might just be one plant or a whole indoor garden; it is so fun to grow indoor plants. Growing tomatoes indoors is a great alternative to starting seeds for the growing season way too early. 

Another reason to grow an indoor garden is extreme weather conditions outdoors. For us, it is a cool and short summer; for someone in a warm climate, it might be extreme heat… too much rain…, and hail… you get the idea. In order to grow tomatoes earlier and enjoy the delicious fruit longer, we grow some of them indoors. Also, if you do not have any outdoor space to grow tomatoes, your best bet to still enjoy homegrown fresh tomatoes is to grow them indoors.

How to grow the same tomato plant indoors and outdoors

You can grow tomatoes indoors for part of their life. Note: I’m not talking about seedlings but rather about mature plants. You need to plant the tomato plant in a movable container for this. Use larger pots or grow bags.

Start tomato seeds in late winter under grow light. Artificial light is a must because the hours of direct sunlight are not sufficient yet. Read more about when and how to start tomatoes from seeds.

Transplant that seedling into a bigger pot or a grow bag and give it plenty of light. Move that plant out as soon as the weather permits (after the last frost date). Make sure to harden off the plant first by either moving it out for a few hours at a time or covering it with a Plant Protection Blanket. This will give you an early start on tomatoes.

However, do not expect the indoor tomato plant to grow as quickly as it would outdoors. Also, especially determinate tomatoes will be done producing fruit at about mid-summer. At this point, you can start a new plant from cutting and have it ready for the fall indoor garden season.

Bringing in a plant in the fall and prolonging its life indoors is another option. As I explained above, it can be grown in the same pot from a cutting. If you bring a mature plant indoors in the fall, ensure that it does not take in many bugs. I find that it helps not to wait till the weather gets cold but to bring the plant in earlier in the fall. When the weather changes, bugs seek hiding places and love containers. So bring your plant in before that happens.

Note: It is difficult to transplant a mature plant from the outdoor garden into a container without damaging the root system. We do not recommend doing this; take a cutting instead. 

Personally, we do not like to move plants in and out. We have grown tomatoes indoors year-round at big south-facing windows. We often call our window room a greenhouse, but technically, it is not. Those are just windows we grow plants at. A sunroom or a bay window would also work very well.

Step-by-step guide How to grow tomatoes indoors

1. Enough Light

Tomatoes are a summer crop that needs full sun to grow and produce fruit. If they do not have enough hours of light, the plants will not die but will not produce good fruit either.

From October 28 till February 14, our daylight is less than 10 hours. Also, the much-needed winter sun is often behind neighboring buildings and trees. Growing year-round, we can see how much impact the “below 10-hour days” have.

We would recommend using grow lights for the seedlings to give them a healthy start. To grow only with natural light, start growing indoor tomatoes right at the edge of the 10-hour day. Read more on how to start tomatoes from seeds.

Just like for the outdoor garden, the seedlings will be ready to be planted just before putting forth buds. (6-8 weeks after starting). After being transplanted, the tomato plant can grow without grow lights at a sunny south-facing window if the daylight hours are ten or more. A young plant in the middle of winter will get very leggy without bright light. However, a mature plant does survive the dark months quite well; for example, Winter indoor garden update.

Learn more about additional light for an indoor edible window garden here.

2. Growing medium

• Growing in soil

Plants are best grown in good organic soil out in the garden. We can’t provide that in our indoor garden; we only do the best we can.

We have had good experiences growing in soil indoors; however, to be honest with you, there are more challenges. You will need a good soil mix, such as a good quality tomato potting mix. Or make your own Mel’s Mix from the Square Foot Garden: Mix together

1/3 compost
1/3 vermiculite
1/3 peat moss

You can find all of these ingredients in a local garden center or Home Depot. Read more about soil for container gardening here. 

Since tomatoes are heavy feeders, you will need to fertilize them. Using larger containers or a wicking bed, you can use natural fertilizers like worm casting, fish emulsion, or Bone Meal. Use a balanced, water-soluble fertilizer in a small pot, or if you prefer smell-neutral fertilizer like NPK 10-10-10. Fertilize again with a high on potassium fertilizer after you see the first fruit.

Watering is also very important indoors. At a sunny window, plants dry out very quickly and might need a lot of water. Plus, containers do dry out quicker anyway. You will have to make sure to be able to water regularly, every day, in the heat of summer or when using a heat mat in winter. Make sure the container has good drainage holes so the plants do not sit in water. Use room temperature water.

• Growing hydroponically

Personally, from our experience, growing indoors is easiest in hydroponics. A hydroponic system is a clean solution that provides the plants with all needed nutrients.

You can use whatever nutrient brand you want or is available at your local supplier. For tomatoes, you will need, for example, both MaxiGrow and MaxiBloom. 

Read more about hydroponic fertilizer, growing the Kratky hydroponic way, high-pressure aeroponics, and growing a Tower Garden.

Growing in a hydroponic setup is not the same as growing food in organic soil, but indoors, we do not have any organic soil anyway. Still growing tomatoes at home, without any pesticides, in a high-quality mineral blend is a great choice.

3. Temperature

Tomatoes are heat-loving plants that like it warm. A cooled-down indoor room is not the right place for them. Even though the sunny window might have a different temperature than the room itself, make sure to measure the temperature. Nighttime temperatures can get very low at the same window with cold drafts. For growing tomatoes, you want to have around 17C (62F) at night and 26C (79F) during the day. Use an oscillating fan to have good air circulation.

The grow room at the back of our garage gets really warm, up to 30C (86F), and the plants love it. In the winter, for tomatoes, the room needs to be kept above 12C (53F). We can’t provide optimal conditions for tomatoes in the winter months; the plants are usually less productive because of that, but still, it is so rewarding to harvest a ripe homegrown tomato here and there in the middle of winter. 

Grow tomatoes indoors in succession

This end-of-August picture is a great example of succession indoor tomato planting. The first three plants are plants that have been started in late winter and grew here all summer, producing lots of fruit. One has a sucker that will be there after the plant is done with the old fruit. I just cut back the plant and leave the sucker. Note: I do prune suckers of the indeterminate varieties (except the dwarf varieties); this is an exception because I can have a new plant from the old root. See more about pruning tomato plants here.

The next three plants in the picture are young plants that have been started from cuttings in the summer. They have fresh fruit growing. After that, there is empty space because some of the spring tomatoes were done, and I will use the space for new plants. I will not start new tomato plants at this point (late summer), but I’ll start greens for winter growing.

In order to grow tomatoes year-round, they need to be planted mid-summer. After some experience with succession planting, it turns out that planting mid-summer and then anew in January/February works great. This way, the tomatoes start to produce in late fall, after the outdoor tomatoes are done, and again in late spring before the outdoor tomatoes start producing.

There you have it, a complete guide on how to grow tomatoes indoors. It is easy if you can provide good lighting, soil (or hydroponics), and temperatures that tomato plants love. Try it and let us know how it goes. For us, it really has been worth it. We harvest lots of yummy tomatoes from our indoor garden every year.

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