I have some tomato plants I started after my current adult plants were half grown. This is to give us a longer harvest later. Tomato vines begin to die back after they produce fruit. This is a natural part of their life cycle. By having plants maturing at different times, we spread out the availability of fresh tomatoes.
This year I am experimenting doing this with cucumbers as well. I have adult vines but I also have started new seeds in the ground.
The green house also has seedlings that we may try to plant in the hoop house in late fall.