6. Give extra water
Spring flowering usually tapers down as summer heat picks up, but you can keep the plants going a bit longer by giving them extra water. You will notice your plants wilting in the afternoon and then getting revived during the night. The repeated wilting shortens the lifespan of the plants, and they hurry onto the next phase, i.e. making seeds. That would practically put an end to the flower show. Increasing soil moisture and spraying the plants with water, especially in the afternoon, raises humidity and cools down the atmosphere.
7. Provide shade when necessary
Temperatures in the shade remain a few degrees lower than that of fully exposed areas. Providing shade can sometimes keep the plants blooming for a few weeks more. Shade screens can do it very efficiently, but they may mar the aesthetics of your garden.
A better alternative would be planting summer and fall flowering plants closer to the spring flowers. They will grow tall enough to provide some shade by the time it gets too warm for the spring plants. Similarly, summer-flowering plants can be protected from frosty, desiccating winds by cold-hardy plants around them.
8. Mulch around the plants
Mulch acts as an insulator. It keeps the soil cooler by a few degrees when the temperature rises in spring, and helps retain soil warmth for a little longer in fall. Before spring flowering annuals and perennials starts showing heat stress, apply a thick layer of mulcharound them. With extra watering and some amount of shade, you might be able to extend the flowering season for a few more weeks.
For summer flowers, warm soil beneath dark colored mulches may help them hold out a bit longer. Stone and rock mulches are especially good at absorbing the sun’s heat during the day and releasing them in the night. When you grow tropical perennials as annuals, a little bit of extra warmth can make a big difference.