Think of the produce that we strongly associate with summer and you are likely thinking of a fruit or seed: Zucchini, Melon, Tomato, Bean, Pepper, Peach, Corn, etc. The summer months are the fruiting months of the year. For plants to produce fruit they usually require time and warmth. The other plant parts naturally precede them, ripe tomatoes don't just pop out of the ground. First there are roots and shoots, leaves, flowers then fruits. It has taken a succession of success' for our little tomatoes started in March indoors (check out the photo gallery link on this page to see them as babies in our late winter nursery) to get to the point now 4 months later that they are just beginning to ripen a couple of their fruits. It was a cold, wet, late spring and it has been a pretty cool summer, this delays ripening which is why people in mild coastal climates sometimes have a hard time with tomatoes.
Understanding this helps us as gardeners and cooks to grow and eat seasonally. Lettuce for example is a plant who's leaves we eat, so it grows quick and easy most months of the year. Stick a seed in the ground, 60 days later you can have a mature head of lettuce. If you wanted to eat the flower or seed of the lettuce (has anyone done this?) you would have to wait longer. This is actually one of the problems we have with lettuce during the summer, the heat tells the plant that it should flower and form seeds and this causes the part we like to eat (the leaves) to become more bitter, this flowering is often referred to as "bolting". Of course because we can harvest leaves as baby greens at a very young age, and we can grow bold resistant strains of lettuce, and we can plant successions of lettuce (every two weeks here), we can enjoy lettuce all year. Lettuce is extremely winter hardy even here in Julian well into the low 20's. Even if tomatoes could survive this (and they can't) they would never produce fruit in these temperatures, nor would anything I can think of. Even in the warm winter parts of San Diego, I am told tomatoes are not very productive, fruits of plants need heat!
These biological premises are at the heart of why we eat seasonally and how gardeners and farmers can hitch their wagon onto nature's train and grow successfully.
Yours in Plant Positivity,