I don't know about tomatoes in particular, but I can tell you that other warm-season crops (like cucumbers) do not do well under those conditions. They like warm temps to germinate, hot temps while they are growing. If you wait until July, it will be too hot for the seedlings, and then maybe too cool at night by the time they bear fruit.

It's worth a try though - for the cost of a few seeds you could wind up with some extra tomatoes. I would start them indoors where it is cooler and then give them some extra attention once you plant them out.

We tried second plantings in July last year. They produced fewer and smaller size tomatoes than the spring planted crop. In our garden this could have been due to less direct sunlight toward end of growing season, and also that we didn't pay as much attention to them as we did with the plants in spring. We were pleasantly surprised with cucumbers we planted in late July. They started producing in September and did much better than we expected. I did have to hand polinate them toward the end of the season when the bees slowed down though. We have a small garden so this is not a problem for us.

I always do a 2nd crop. The tomatoes are usually less abundant and smaller, but much of that may be due to neglect on my part. And I never start the 2nd crop from seed. Cut off a limb of the plant and stick it in a jar of water, in a week or two, it will have roots coming out all over the place. Then transplant it. If you have great soil and take special care, you can even skip the jar and go straight to the ground, but I tend to be neglectful at times, so I wait for a good root system to show in the water. Oh, and I always have tomatoes on the vine right up until the first frost.

I've tried this experiment using Cherokee Purple hierlooms for two years running now with good results. I start some plants and set them out in late May/Early June. About mid/late July I prune them back to an 18-24" stump. They re-sprout branches and go at it until frost on a nice thick truck and a deep root system. I have not been able to get a hybrid to do this, but use this techniques in my greenhouse with CHerokee Purps as well and the plants keep spitting out tomatoes through the entire winter. Go figure.

mcrean 1 Thank you. I started Ch Purps from seed last summer when the SC summer got too hot for the plants to blossum. Got some good toms but not nearly enough starting from seed. Then the early Thanksgiving frost killed them. I've got a volunteer and some plants from saved seeds ready to go now. I assume if I start from the plant I'll get more toms this year at the second crop. Thanks again.

mcrean1 thanks- that is really interesting! When you say you prune back to an 18-24" stump, do you leave any leaves? I think I'll try either your technique or jmzms' this year just to see what happens.
jg

Laxfan,
I left a few leaves. Honestly, first time I tried this I was bored, the plants looked like crap, and I actually set out to shovel prune them. On a whim, and just to cut the fool with my wife (a notorious black thumb) I pruned them like a fruit tree in January and stuck a handmade sign on them that said "Morticia's Tomatoes." Again, I've tried this with hybrids (Heatwave, Burpee Whopper thingy, big boy hybrid, etc.)and killed them. BTW, the purps are the only heirlooms I grow successfully, so not sure if others will work.