Roots of Plants Vary in Depth

Q. How deep are plant roots?

Plant roots vary in depth from less than 12 inches to more than three feet. Large trees are watered to the deepest roots get water (guessing its three feet).  Citation is lost.

A. It depends on the size of the plant. This is because the roots of plants extend beyond their drip line. Roots of plants have the potential of growing anywhere under a plants canopy. When a plant is surrounded by dry soil, applied irrigation is very important for root growth. So is air. Roots must “breathe”. If you visually lay a plant on its side spin a circle with it, that will roughly describe where its roots can grow when there is plenty of rain. That is not true in the desert. Even tall lawn grasses have shorter roots if they are watered and then mowed closer to the soil.

Plants generally follow a "40-30-20-10" rule when their roots pull water from the soil. This means as the top quarter begins drying out, the water is used or pulled from deeper in the soil. That is why is it so important not to water every day unless they are very small plants like lawns, vegetable beds and annual flowers. Those roots at the bottom of medium to larger plants cant get the water and their roots suffocate or "drown". Citation lost.

            The functions of roots are not just to supply the top of the plant with water, but this plant must stay upright and resist the pushing by wind. This is one reason why tall plants have water that’s applied deeper than shorter plants.

            If the plant is small, water it only to 12 inches. Small plants don’t need as much support and the water travels smaller distances. Large trees and shrubs are watered as deep as 24 to 36 inches! Their roots must carry water a lot further as well as keep these big plants upright under their weight and in the wind. The water they need is applied to the soil deeper than when watering short plants. They need deep watering because the soil under the roots is dry. Deep roots are needed as the tree gets bigger because of its ever-increasing canopy size and weight.

            The taller the plant the deeper are the plant’s roots needed for transporting water and support.

            As plants get larger, they need an increasing number of the same drip emitters to apply water to the soil. By adding more drip emitters, you can keep the minutes the same. With very large trees and shrubs at some point you may need to increase the size of the emitters, as well as their numbers, to keep the minutes the same.