There are 2 possible definitions for "dripline" or "drip line" within horticulture: the dripline of trees and a drip line in irrigation. Let's explore what each of these means.
The Dripline of Trees
The dripline is the area directly located under the outer circumference of the tree branches. When the tree canopy gets wet, any excess is shed to the ground along this dripline, much like an umbrella. This is also known as a tree’s Critical Root Zone (CRZ), sometimes also called the Root Protection Zone (RPZ), and is defined as a circle on the ground corresponding to the dripline of the tree.
The most active water absorption area is at the dripline and beyond, not close to the center or trunk. This is where the tiny terminal feeder rootlets are located that take up water and nutrients from the soil for the tree. Trees should be watered here, not by the base of the trunk, or the tree may develop root rot.
Know your plants and trees; some have shallow roots near the surface, others have deeper roots that require more water to be able to soak down to the depth of their feeder roots. The soil surrounding the plant's roots, called the “root zone,” serves as a storage tank from which the plant draws moisture and nutrients.
When you apply fertilizing nutrients to a plant, it is essential that you put them within reach of these feeder rootlets, or your offering will be leeched away and mostly lost.
Drip Line in Drip Irrigation
Drip irrigation is low-pressure, low-volume watering system that saves water and fertilizer by allowing water to drip slowly to the roots of plants, either onto the soil surface or directly onto the root zone, through a network of valves, pipes, tubing, and emitters. By keeping the roots moist but not soaked, you use less water than other irrigation techniques.
Drip irrigation is done through narrow tubes that deliver water directly to the base of the plant. It is chosen instead of surface irrigation for various reasons, often including concern about minimizing evaporation. The drip line is where these tubes and hoses are laid.
Most of the micro irrigation is accomplished through this drip tubing along these drip lines, 1/4-inch or 1/2-inch hose fitted with tiny plastic nubs, called emitters, that allow water to drip out at a regulated pace without clogging. The tubing snakes around and among plants and trees to get water into the soil at the roots. You can buy that tubing either pre-punched, with emitters factory-installed under the surface every 18 inches, or unperforated, which requires you to punch the holes and attach the emitters to the outside of the tubing yourself.
If your irrigation is inadequate, you will encourage the roots of young plants to grow up near the surface, which will make them more dependent on frequent watering to satisfy them and give them poorer holding power in the soil, which may result in uprooted trees and shrubs in a storm.