Mulch. This should be quite thick, maybe three or four inches deep or greater if very loose (like straw). The mulch helps to keep the roots moist and cool (most desirable)

Heat. It's a good idea to devise some sort of shading to protect the plants from the worst heat of the day. This shading usually took the form of trees which cast a shadow over the plants. The tree could be quite loose leafed. Adjacent trees help cool the ground too.

Watering. Never let delphiniums get either bogged or dried out (particularly when growing well) or they will die. Keep only barely moist immediately after cutting down after the first summer flowering (July) or they may rot. Increase water as the plants grow back. Be sure the soil is moist going into a hot spell.

Cold. Delphiniums are generally very hardy. Winter mulching benefits here and folk often use pine branches to provide a loose protection from severe frosts.