Q. Last year many of my tomatoes developed large deep cracks. I grew Better Boy, Jet Star, fourth of July, and Sweet 100, all of which cracked–Better Boy was the worst, Jet Star the best. I removed the suckers from all these and grew them up a string. I also grew Celebrity, Romas, Better Boys and Jet Stars, from which I did not remove the suckers, nor did I train them up a string. Both groups were grown in the same area with the same amount of water and fertilization, but the later group had very few tomatoes with cracks. The vertically-grown plants produced MUCH more, but I am trying to figure out why the determinate varieties and the indeterminates that were not grown vertically didn’t crack.
A. Larger tomatoes are more prone to cracking, so the Better Boys – being bigger than Jet Star – tend to have more problem. The same holds true with the determinates – Celebrity and Roma are smaller, plus they have strong skins (especially Roma), so they will crack less. And the Sweet 100 cherry tomatoes have very thin weak skins, and thus are among the worst for cracking. I have changed varieties and am now growing Grape tomatoes. They are firm, have strong skins, taste great, and will last at least a week on your kitchen counter.
We teach and encourage vertical growing of tomatoes and other vining crops because your yield is many times greater. However, you must to it properly, so as not to reduce the quality of your fruit from cracking.
The difference between vertically-grown and tomatoes on the ground comes from at least three things – often in combination.
Cracking is caused by rapid changes taking place in the fruit, that the skin is unable to accommodate. Those changes can be heat related, moisture related, growth related, or any combination of the three.
Let’s talk about ways in which you can moderate the extremes of heat, moisture, and growth, and thereby greatly reduce the cracking.
Your tomatoes growing on the ground had more leaf cover and were cooler than the ones growing vertically. The moist ground and leaf cover, along with the proximity of the fruit to the ground, moderate the temperatures in the fruit. Also, there is a different ratio of plant material to fruit (less fruit and more vines & leaves), so when fertilizers come into the plants there is less of a surge of growth in the fruit of those plants.
You also have your plants near the wall of your house. The wall captures and reflects the heat of the sun onto the tomato plants. That’s great in the spring and fall, when you want all the heat you can get, but it contributes to the problem on the hottest days of summer.
Make sure you do not remove the leaf cover from your vertically-grown tomatoes. Sometimes people see pictures of vines in a greenhouse with almost no leaves, and they remove too many leaves. Do not remove the healthy leaves growing on the main stalk. Only remove leaves from the bottom, when they touch the ground, or as they become old.
In the heat of the summer we will sometimes feed our tomatoes 1/2 the amount of fertilizer twice per week, instead of all at once. This can reduce the surge of growth which contributes to cracking.
Providing partial shade during the hottest part of the day can also help. If you’re near a building wall, even shading the wall may help enough.
And finally, making sure there are not big changes in the water supply to the plant roots is important. Never allow your ground to become dry – even on the surface. We will even sometimes water twice each day in the extremely hot weather.