Different groups of plants grow best at different times of the year.
In my general observations, I have noted that:
Perennials from cold climates (zones 3-4) prefer to grow most actively when temperatures are cool. Oriental poppies are a prime example. They begin growing in early spring as soon as the soil thaws. After flowering in late spring, they will go dormant in the heat of summer. Then they sprout new leaves in the cool of fall to begin the cycle once more.
Perennials from hot summer regions, such as the Great Plains and the Southwestern United States, wait for the warmth of late spring before they begin growing. Leary of frost, these plants wait for the nights to begin to warm and the days heat up before their season starts. Flowering occurs during the heat of summer and early fall.
Salvia Amistad
It's these perennials and warm season ornamental grasses are excellent choices for planting in June. They grow well when the summer heat is on. To ensure success, follow these suggestions:
Be sure to water summer transplants regularly. And soak the soil long enough to push the water deep where the roots will follow.
Mulch where appropriate. In the Intermountain, California, PNW, Great Plains and Mid-Western states, summer mulching is an effective technique for keeping the soil moister and cooler for roots to grow their best. In hot, very humid, wetter climates, mulching may not be beneficial because of the potential for crown and root diseases.