That depends on where you live.

In California, plant growth is mostly limited by the availability of water, which means that the growing seasons run from mid-winter to early summer. In deserts where winters are cold, plants grow most after the thaw, in early spring, but before the desert can dry up. In most other places, plants grow more in summer either because it's too cold in winter (northern/southern climates) or because the summer is when the rains come (tropical climates).

During the summer, there is more direct sunlight on Earth in the Northern Hemisphere. Plants depend on sunlight to grow. So in the Northern Hemisphere during the summer, plants grow more.