Why are my tomato leaves curling up?

Green, curling upward and inward, worst on the lower leaves, during a hot dry spell, and still flowering. That is almost certainly physiological leaf roll, which is the plant protecting itself rather than anything going wrong.

The likely cause

When a tomato loses water faster than its roots can supply it, the leaf rolls to reduce the surface exposed to sun and wind. Heat, wind, uneven watering and heavy pruning all trigger it, and vigorous varieties do it more readily than others. The tell that it is harmless: leaves stay a normal green and the plant keeps setting flowers. Worry instead if new growth is twisted and distorted, if leaves are mottled yellow, or if the curling arrived with stunted shoot tips, all of which point toward virus or herbicide drift.

How to fix it

1. Water deeply and on a rhythm. Long soaks less often beat daily splashes. Swinging between drought and flood is the main driver of leaf roll.
2. Mulch the root zone. A thick layer of straw or compost holds soil moisture steady and keeps roots cooler through a heatwave.
3. Ease off the pruning. Stripping lots of foliage in hot weather makes rolling worse and leaves fruit exposed to scald.

What to expect

Leaves usually relax once the weather breaks, and yield is rarely affected. If curling carries on into mild weather, go back and look again at the virus and herbicide signs above.