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Deep Watering

How to Water Your Trees During Dry Periods

Drought stress is the most common type of environmental stress we observe in trees, making it the primary cause to many tree diseases and insect problems. It can cause unsightly die-back and result in tree death, especially to younger trees and transplants. If your trees are exhibiting signs or symptoms of poor health at any time of year, its likely related to a lack of water and summer is the especially difficult for trees.


What is Deep Watering?


Trees have absorbing roots that take up water from the soil and grow close to the surface. Still, they are slightly deeper than the absorbing roots of grass, small shrubs, and other plants in your garden, and therefore require a different method of watering. 

The best way to water trees is by deep watering, which can be described as infrequent, long duration watering. By watering for a longer period of time, and then allowing the soil to drain for at least a day, you can ensure that the water is soaking deeper than the absorbing roots of your grass or garden and reaching the tree’s absorbing roots, and still maintaining a healthy root system with good pore space in the soil. The frequency and duration of the watering depends on the size and age of the tree (and of course the species of tree and many other factors, but let’s keep it simple).

Most sprinkler and drip irrigation systems fail to adequately water trees, even if they run intermittently everyday. This type of frequent, short duration watering is perfect for smaller plants and conserving water, but isn’t ideal when it comes to watering trees for three reasons: One, the water usually doesn’t soak deeply enough to reach absorbing tree roots. Two, when it does, the frequent waterings mean there’s not enough time for water to drain out of the soil causing roots to suffocate from lack of oxygen. Three, it encourages trees to grow a shallower root system that makes them susceptible to uprooting in high winds.



Steps for Deep Watering

Step 1–Remove grass and other plants competing for water

Consider removing or reducing grass and other plants within the root zone of the tree, which is a term for the entire area beneath the tree, extending beyond even the drip line, or width of the tree’s crown. If this cannot be done, or you would prefer to keep your garden or lawn intact, do as much as you can, and know that this will have an effect on your tree’s long-term health.


Step 2–Apply mulch to the root zone 


In addition to the nutrient-cycling benefits of mulch, which reduces the need to fertilize the soil, the mulch will deter weeds from growing and help retain moisture in the soil. Don’t go overboard, though. Only apply mulch about 2-3 inches deep and don’t let any mulch pile up around the trunk. It shouldn’t even touch the bark at all, which could cause rot. For small and newly planted trees, it can be nice to create a berm around the outer edge of the mulch area like a dam to prevent runoff while watering. Imagine a donut shape or inner tube. 


Step 3–Determine your tree size


Start by measuring or estimating the diameter of the trunk. On larger trees, it’s best to measure the Diameter at Breast Height (DBH) which is 54” above ground, a common method for measuring trees. On smaller trees, just measure a few inches above the ground. On trees with multiple trunks, pretend they’re multiple trees and measure each one and add them together. In any other special cases, use common sense to come up with an approximate DBH.


(Hint: The easiest way to measure is by wrapping a tape measure around the trunk to get the circumference, and then divide that number by 3.14 to get the DBH)


Step 4–Determine your tree’s water needs


Once you have an approximate DBH, use the chart below to determine how much water your tree needs and how often. A small tree should take 30 minutes to 2 hours for watering. A medium tree may take 2-5 hours and a large tree may take 6-10 hours.

Deep Watering Table_edited_edited.png
Step 5–Instructions for watering using a hose:

The best low-tech option is using a garden hose: Turn the hose on until water is coming out in a small, continuous stream or trickle. You can figure out the best flow rate with a simple experiment and basic math:  Measure how much time it takes to fill a 5-gallon bucket, and use that figure to determine how long you will need to water your tree. It should take about 30 minutes to fill a bucket for a small tree, about 20 minutes to fill a bucket for a medium tree, and only about 10 minutes for a large tree. 


Next, let the hose lay on the ground somewhere in the root zone. Periodically, move the hose to another part of the root zone to water evenly. You can also use a soaker hose, which is just a hose with small holes at regular intervals that you can buy or make yourself. This will allow you to cover more of the root zone at one time so you won’t need to move it as much.


(Note: Bubbler irrigation systems can be used for trees, but require some sophistication and knowledge when setting up to be effective. You can also use a 5-gallon bucket to pour but it’s likely to cause erosion and runoff, and is not recommended.)

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