You’re probably watering your trees wrong. Here are 6 things to know

Picture of Paul Cappiello Ph.D.

Paul Cappiello Ph.D.

Yew Dell Botanical Gardens Executive Director

When you think about it, for all they do for us, trees really don’t ask for all that much. Give them the right amount of sunlight, the right amount of rain, nutrients and some basic support, and they return the favor with a lifetime of shade, show and shelter. Heck, wind pollinated tree species don’t even need pollinators!

Short of mistakenly planting a tree in the basement broom closet, Mother Nature has pretty much taken care of the supply of sunlight. Sure, we have to make sure we put sun trees in the sun and shade trees in the shade, but when it comes to sunlight, trees are usually in pretty good shape without much input from us.

And as for nutrients, yes, we can maximize tree growth with keen attention to soil nutrition issues, but trees are remarkably adept at extracting important nutrients from even the most impoverished of soils. 

Then there’s the whole support thing. There are certainly some trees that are more forgiving than others when it comes to soil quality. But considering that roots of an average mature shade tree can occupy the area of an Olympic scale soccer pitch, how many of us can make a meaningful impact on even a modest fraction of the soil a large tree will occupy over its lifetime? 

So as lovers and growers of trees, what can we do to help . . . ? Water!

Watering trees is the one activity we can pursue to make meaningful contributions to long term tree health. And that’s true no matter what stage tree you’re tending. Let’s look at a few scenarios . . .

How often should you water newly planted trees?

Container grown trees have the greatest need for watering after planting. Most container grown trees are irrigated at least daily while they’re in the nursery. In fact, during production it’s not unusual for container plants to be irrigated multiple times per day to both supply sufficient water and to keep roots (in black plastic pots) at a reasonably cool temperature. 

Once you remove the tree from the container and plant it in the ground, it doesn’t suddenly lose its need for constant irrigation. Organic-matter-based container mixes hold very little water compared to mineral soils. The young tree’s roots are highly concentrated in that fluffy growing mix. The result is rapid drying out and daily need for irrigation until the roots begin to grow into the surrounding soil.

If you plant a new, container-grown tree in November, after the leaves have dropped and the rains have returned, one or two good waterings following planting might be sufficient to get it through the winter. But planting a tree in spring or summer, with rising temperatures and high water demand from the leaves, you’ll need to plan on daily watering for a few weeks and then a healthy 2-3 waterings a week through the return of fall rains.

Trees harvested from a nursery field can lose up to 80% of their root system and, most importantly, up to 98% of their fine roots and root tips that are responsible for most water absorption. And while the heavier, mineral field soil will hold much more water than will a container mix, the majority of water absorption will occur at the edge of the soil ball where the few remaining fine roots live and where new roots will be produced. So, the key for watering field-grown trees is to concentrate on the outside of the root ball and where it contacts the surrounding ground. 

For field-produced trees newly planted into the landscape, I recommend watering every other day for a month and then backing off to two thorough soakings per week through the rest of the growing season.

For both container and field-grown trees, it’s best to plan on one good soaking per week through the second growing season.

How often should you water existing, mature trees?

There are five basic concepts that are essential to understanding how to effectively water existing trees:

  1. Tree roots spread well beyond the so-called drip line of a tree – way beyond the spread of the branches. In fact, up to 85% of a tree’s water absorption can come from roots located well beyond the tree’s drip line.
  2. The majority of fine roots that are so important to water and nutrient absorption are located well away from the trunk, especially in large, old trees. Winding 100 feet of soaker hose around the trunk of a giant old oak tree and watering it for 3 days straight won’t help a whole lot.
  3. More than 95% of tree roots sit in the top 18 inches of soil. No matter how much the Facebook universe crows about tap roots, they’re among the biggest myths in the plant world. 
  4. It takes a long, long, long time to get irrigation water through those top 18 inches of soil . . . especially when that soil is dry. A good estimate for water penetration through a clay-containing mineral soil is about one quarter inch per hour. That means that if you ignore your trees long enough that you need to get irrigation water through that full 18 inches of your soil and roots, you’d have to run your sprinkler system for about 72 hours straight!
  5. Your soil loses way more water to evaporation during the day than it does to absorption by your shade tree. 

So, putting all the above into some kind of a system for planning your mature tree watering, here’s my list of watering recommendations:

How early should you start watering trees?

Start early. The sooner in the season you start watering and keeping the top few inches of soil from drying out, the more water you’ll keep in those lower layers of soil where the deeper roots live. If you wait too long to start, it’s easy to get to a point where you can’t catch up.

How long should you water trees?

Water long and slow. If all your irrigation water is running across the lawn, down the gutter and into the storm sewer, it’s not doing your trees any good. Adjust your sprinkler so you can keep it going without wasting water.

When should you water your trees?

Water early in the morning. Mornings are cooler and more humid so less of your irrigation water will evaporate before it reaches the roots.

What are the benefits of drip irrigation?

Use drip irrigation where you can. Where you have shrub or other planting beds near your big old trees, using a drip or trickle irrigation system is tremendously efficient. But coiling 1000 linear feet of drip tube around the front lawn to drip irrigate your favorite tree might not jive with the local HOA police!

This article was originally submitted to the Courier Journal on June 22, 2023.

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