When shoot growth ceases in early summer, root growth begins again. The rate of root growth increases through the summer reaching a peak in early fall and then declines as the soil cools. So, two of the main restraints on root growth are soil temperature and competition with shoots and fruit. The other factor determining root growth is soil moisture. Roots grow were the conditions for growth are favorable.
Roots grow where the water is.
Roots in dry soil show very little growth. In the spring, when the soil is moist and the surface is warm, roots near the surface show the most growth. As the soil dries out, roots near the surface die and new roots grow deeper in the soil. As the season progresses, the depth of active root growth moves progressively deeper and deeper in the soil. With each rain or irrigation new roots appear near the surface.