It’s light.
When the tomato seeds first sprout, if they don’t have enough light, they grow very quickly and become tall and spindly, and are very light green in color.
To prevent that you need a grow light and I put a small fan on the plants to simulate winds and the harsher conditions of being outside.
That is true, however, I assume the poster knows that basic truth, and has a problem with growth even outside.
To prevent that you need a grow light and I put a small fan on the plants to simulate winds and the harsher conditions of being outside.
Yes, I have about 70 plants started from seeds, some 2 months old, 6'' high in 20oz cups (and that much need to get planted, once weather allows), that I placed in Windows near the back of the house that see little sun, using grow lights. However, I later moved some to windows that get more sun and in a cooler room and have no grow lights and the plants grow more, if slowly. Glory to God. The fan is a good idea, but what I do is have them in together in a box and put them outside on the porch on warmer sunny days, and which are often very windy, and put tape lines across the rows so that the wind does not push them over to breaking. It is after planting that the chances of that occurring is a bigger problem, and thus using rocks or stakes and string is necessary.
Ditto with the fan. I found a solar powered small one with two fans.
I bought a couple and set them.up on the grow beds in my greenhouse right at potting level.
The sun comes out and they blow across the tops of the pots.