Early Garden

Secret to starting tomato transplants indoors

Despite starting late with my early tomatoes this year, my first set of tomato transplants seem to be right on time. In fact I began the hardening off process and am quite proud. The tomato transplants I started indoors were already quite hardy!

Last year after starting seedlings indoors, and just as I was beginning the hardening off process we were struck with hot weather. As soon as I took my tomatoes outside they began to wilt - even in the shade. It was just too hot. But there were other problems. The transplants were too leggy. They waited a week too long before starting hardening off and they were generally babied inside. Extensive top growth in a 3 inch pot meant that as soon as the wind started to blow there wasn’t enough root to feed the top and they transpired and wilted.

What made my tomato transplants hardy indoors this year? Here’s my secret to short, stocky and well rooted indoor tomato transplants that are hardy before they even get outside.


Last year my tomatoes were started in a basement under fluorescent lights. That’s fine but the temperature down there is a few degrees cooler than upstairs. In comparison this year I brought my whole works upstairs where it is at least room temperature. I also had it on an outside wall so maybe even warmer. For the last week before I started hardening off I began bringing them downstairs in the cool and dark for the night to help them get a very small temperature change.

Light Intensity

If you are starting tomatoes indoors it is crucial you get the lighting right. I used fluorescent plant grow lights no farther than 1 to 2 inches away. That’s what I did last year but the difference is in the lighting itself. I used 4 bulbs in 2 fixtures placed side by side and touching. As those tomatoes grow larger the leaves grow horizontally away from the plant and it is hard to capture enough light on these leaves without 4 bulbs. I also noticed that one of the newer indoor plant light fixtures that I purchased last year despite using the same 40 watt bulbs doesn’t throw off as much light. I assume this is because it was a bargain fixture and uses a more efficient electronic ballast instead of the old style magnetic. Light is important and it should be reaching all parts of the plant equally. Put some reflective material like aluminum foil or better yet mylar on the wall to reflect back as much light as possible. Spend 12 hours a day or more (I choose 15 hours) under those grow lights.

Watering Frequency

Not….much. Last year I thought I kept things evenly moist but I think they were too wet. This year I let them dry out and even wilt just a little bit before lightly watering them. This was a trick I read in an old Organic Gardening magazine and it worked like a charm! I think this may be the most important difference from last year. The roots spread out to look for water but you don’t get so much top growth. My transplants are wider then they are tall!

Simulate Wind

Last year I used a fan but it was too steady and cooled the plants off further. This year that old Organic Gardening magazine came in useful because they said “tickle your tomatoes.” Brush the dry leaves for a minute and a half a day. After about a week I got tired of this and instead when moving my tomatoes to the basement for the night I put them in a tray all at the same time and on my way downstairs I wiggled them like I was sifting for gold. You could visibly see them getting tougher everyday.

As much as you want to, don’t baby your tomatoes! Look after them and give them what they need, but like a mommy bird you need to prepare them for that moment when you kick them out of the nest. If you mix your light, water, wind and temperature just right you’ll lessen transplant shock when they go into the garden and that will help you harvest those tomatoes extra early. For tips on tomato seedlings, see our page on starting tomatoes indoors.