FAQ: How Much Should I Water My (insert tree/shrub here)?

One of the questions I get asked the most from customers is how much water their landscape needs. It’s a surprisingly challenging question without a straightforward answer. Firstly, every tree and shrub has different water requirements. We see this easily in how a Hydrangea (from Latin hydro, meaning water) wilts on a hot sunny day while roses and trees may continue to look fine. Beyond that, there are many other factors that affect how often you should water: Are the plants/trees potted or in-the-ground? Do you mulch regularly? How mature are the trees or shrubs? How sunny or shady is the area? How close to the home is the tree or shrub located?

You get the idea. There isn’t a one-size-fits-all answer to this question. Yet watering is one of the most critical (and controllable) elements to maintaining a healthy landscape. We all know that water is essential for plant life.

I am sure you came here for practical advice on how often to water, so I don’t want to bore you with detailed research that demonstrates how even a minor lack of water can weaken a tree or shrub and lead to its ultimate death, but I do want to give a brief description of Dr. Paul Manion’s “Spiral of Decline”. Feel free to skip ahead!

Manion’s “Spiral of Decline” illustrated here separates factors that contribute to a plant’s death into three groups: predisposing factors, inciting factors and contributing factors. On the outer most ring, predisposing factors are elements in the surrounding environment or the genetic makeup of the tree/shrub itself that will make it slightly weaker or less likely to overcome challenges.

For example, perhaps the DNA of the tree or shrub simply does not make it tolerable to our heavy clay soil. Or perhaps you were unlucky and the particular tree or shrub you picked out from the nursery had poor root formation that you couldn’t see buried in the potting soil. There are some things we can do minimize predisposing factors such as amend our soils and making informed choices about where to put plants based on their preferences for sunlight. There isn’t too much we can do about other things like whether or not our neighbor’s stormwater runs off into our property or if the plant genetics aren’t strong.

In the middle ring, we look at inciting factors. Inciting factors are the impetus for the plant’s decline. If you had a perfect tree specimen planted in the perfect location (think of trees growing in the woods) it might be able to withstand certain inciting factors. However, in our urban landscapes, this is not often the case. Our trees and shrubs are often surrounded by grass, patios/walkways/driveways, with little protection from wind and other harsh environmental conditions.

Let’s look at the list of inciting factors to see what types of things start are trees and shrubs on their “spiral of decline".”

• Defoliating Insects: Tree and shrubs feed themselves via photosynthesis. Defoliating insects that destroy the leaves or inhibit chlorophyl production cause the plant to starve. Compare this to a nutrient-deficient person that hasn’t eaten, they would be much less likely to overcome a slight illness that an otherwise healthy person might handle with ease.

• Excavation: Both transplanting the tree or shrub itself, as well as ongoing construction around the root zone of the tree or shrub can cause it to severely weaken by severing its roots.

• Drought: even two weeks without rainfall or water can cause stress to a tree or shrub. There are parts of the Mainline that are currently in a moderate drought and have not received rainfall in over two weeks!

• Excessive Salt: This is typical of trees or shrubs located next to a walkway or driveway that is heavily salted in the winter…not so much of a problem this year when we hardly had any snow or ice!

• Frost: Frost damage is usually outside of our control, but well-mulched and well-irrigated landscapes are more resistant to frost damage than those that are not. Did you know that water acts as an insulator? Thus, moist soil with stay warmer than dry soil. With so many of our Skip Laurels appearing burned after the harsh freezes in December and February, this is important to consider for next winter!

• Air Pollution: Photosynthesis and Chlorophyll production are inhibited by air pollution, leading to the same type or decline caused by defoliating insects. Planting full, evergreen trees alongside the street can help mitigate air pollution caused by cars and trucks.

A tree or shrub with one or several predisposing factor that is then exposed to an inciting factor is well on its way to decline. Many fungi, insects and bacteria are attracted to weak, declining trees and finish them off. There is little we can do to mitigate harm and prevent death at this point.

Takeaway: A regularly watered tree or shrub is a stronger tree or shrub. It will be more resilient to disease and pest pressure and will be more likely to overcome disease and pest pressure should it occur. According to the EPA, heat waves and short-term drought in some areas are more common. In the greater Philadelphia area, we have experienced weeks on end with no rain followed by extreme one-day precipitation events.

Luckily for us, drought is a controllable inciting factor!

Basic Watering Guidelines and Tips:

• In lieu of 1” of rain in the previous seven day period, water at minimum once a week. Ideally you will water 2-3x per week especially when temperatures are consistently in the 80s and 90s. Purchase a rain gauge and get in the habit of checking it on a regular schedule ie. every Monday morning when you have your coffee. If it isn’t full to 1”, you know you need to water! You can purchase a simple rain gauge for $5.98 or you can invest in a decorative brass rain gauge. Alternatively, you can use the internet to easily look up precipitation totals at your address for the last 7 days.

• Newly planted trees and shrubs that are 3-years-old or younger should be watered more frequently BUT established trees and shrubs still need to be watered! YES, even that extremely large, old Oak tree in your yard.

  • Consider this: Bacterial Leaf Scorch, a bacterial infection transmitted by insects that targets Red Oak and Pin Oak in our area, is much more likely to infect a poorly irrigated tree. In this case, an Oak tree in the urban landscape is a predisposing factor, drought is the inciting factor and Bacterial Leaf Scorch is the contributing factor. Combined, this is enough to kill Red and Pin Oaks in 2-4 years time.

• Ideally, water at the ground level to prevent wet foliage that favors fungal disease. I use Rainbird Drip Irrigation from Lowes at my house which I installed myself in a few hours. My hydrangea receive 1-hour of drip irrigation in the morning and 1-hour of drip irrigation in the evening. My roses and trees only receive the morning water. If you are interested in having drip-irrigation installed, please reach out and I will recommend a company in your area that can do it for you!

• If you are watering by hand, use a hose attachment with a soaker setting such as I have in the photo below. Set it at the base of your trees and shrubs and let it run for several minutes depending on the size of plant. For reference, 3-5 minutes should be OK for a Skip Laurel depending on it’s size and location. Set on the soaker setting, my garden hose can fill a 5-gallon bucket in 1 minute 30 seconds. Try your hose out and adjust watering time accordingly.

• For larger trees, move the hose around several times to water the entire critical root zone of the tree. Speaking from experience…set an alarm on your phone or you might get started on another project and forget to turn off the hose!

• If you already have pop-up sprinklers installed, set them to run in the very early morning so that tree and shrub foliage has all day long to dry out. Wet foliage favors the spread of fungal disease such as Leaf Spot! Some sprinkler heads may be adjustable to lower the angle of the arc, wetting less foliage.

• Mulch, mulch, mulch! The benefits of mulch are many, but one of the most important is it’s ability to hold moisture. If you water dirt, you will often observe runoff. This is inefficient and your tree or shrub won’t absorb the water to the root zone. Mulch holds water in the root zone and slowly releases it to the roots of the plant as-needed. It also helps regulate ground temperature and delivers nutrients to the trees and shrubs as it breaks down!

There are a ton of online resources to help you determine how much and how often to water your landscape. I hope this post gave you a bit of practical information on how to water but that it also conveyed how critical regular water is to the long-term health and vitality of your trees and shrubs.