Although rainfall has been significant so far this summer, newly planted trees and shrubs need watering as daily high temperatures creep up and rain dwindles during the drier part of our summer. 

The next few months can be critical for woody plants struggling to become established in their new planting location and the water we provide can really make the difference.

Transplant Shock

Transplant shock is a term commonly used to describe the plight a of newly planted tree or shrub, which can often last for several years. When we install a new tree or shrub, it takes quite a bit of adjustment for the plant to acclimate to its new site and extend its roots beyond the initial planting hole. Everything from water to sunlight, to soil drainage and available moisture, differ from the nursery setting the plant experienced prior to being planted in the real world. 

It is safe to assume that a new plant may take up to 3 years - or sometimes longer - to adjust to its surroundings and become “established,” which is a term used to describe a tree that has grown out of transplant shock. 

Trees become established when they have fully adjusted to their new location and extended their root system beyond the planting hole that was initially dug at installation. 

Trees and shrubs in urban areas

The care the first three years after planting is critical to ensure a tree or shrub’s longevity in the landscape. Recent studies have reported alarmingly low survival rates among urban street trees in the years following planting. The myriad of stress that the urban environment puts on plants can really add up and watering is one of the few factors we may be able to control. Since some urban conditions can be so harsh, even trees that are 5 or 6 years old may still be struggling to overcome transplant shock, requiring irrigation during the hotter months. 

Mature trees are presumed to have a root spread up to three times the canopy spread, which provides an illustration of the desired root zone we might expect later. However, we know that a newly planted tree or shrub doesn’t have nearly that size of a root system relative to its above-ground parts. A huge part of overcoming transplant shock is the development of a more proportional root system.

Watering Basics

How much and how often should you water?

I am often asked how much and how often to water newly planted trees or shrubs. The simple answer is to plan on watering anytime we have received less than one inch of rain per week. 

• Watering should be done in one large application, about once a week, as opposed to smaller amounts more frequently. 
• The total quantity of water needed by individual plants varies considerably based on the size of plant.
• Many other factors but should be designed to provide the equivalent of one inch of rain to the entire root system of the plant.

To promote the expanding root system of your newly planted tree, plan to water an area larger than you might expect the roots to occupy. Over time, expand the area you water as your tree grows to draw root development outward and create the larger root system of an established plant. So, the area you should plan to apply water each week is likely much larger than the planting hole or the mulch ring around the base of the tree. 

While the area or root zone to water can seem simple enough, the actual application of water in the field is quite a bit more complicated. Homeowners tend to have a ton of questions as they try to determine exactly how long to turn on the hose to ensure your plant has adequate moisture.  

A watering example

To look at a specific example, consider a container tree in a 5-gallon pot. This size plant requires about a 3-foot diameter hole, if it was installed with a nice and wide planting hole, as is recommended. To encourage root growth outward, watering should extend beyond the initial planting hole, encompassing a circle about 5 ft in diameter. 

The quantity of water needed may vary if there was some rainfall to account for, but if you assume there was no rain then irrigation should provide one inch of water over the 5ft diameter that represents the root zone of our tree. That would actually require about 150 gallons of water. 

One inch of rain per week is commonly recommended because we assume that amount will add plant-available soil water down to a depth of about 12 inches or more, which is the average area that most tree roots occupy. While the total quantity of water to apply in this example seems astronomical, its much more reasonable when you consider the volume of soil it must occupy, which is actually quite large at 12 inches of total depth.

Common Questions and Calculations

I get a lot of questions each year about exactly how long to water in order to provide an adequate amount.

 In the example above, for a 5-foot diameter circle, it would take about half an hour for an average garden hose to apply one inch of water. That calculation assumes you are using a 1/2 inch diameter, 100-foot long garden hose and there is no flow restriction at the hose outlet. Add a sprinkler on the end and you add significant flow restriction, and a considerable amount of water loss to evaporation. 

While the very basic calculations above provide an illustration of the quantity of water supplied by an open flowing garden hose, they don't account for many factors such as soil infiltration rate and other aspects of soil drainage, as well as other environmental factors. My calculations also don’t consider the application method, which I commonly get quite a few questions about as well.  

We know that sprinklers have considerable evaporation loss, but with that aside, it would take almost 45 minutes to apply one inch of water to the soil over our 5 ft. diameter circle. So, they take longer than a direct hose, but need to have evaporation factored in. Some sources cite up to 50% loss from evaporation, but that calculation is quiet dependent on specific weather conditions and becomes tricky to tease out.

Soaker hoses are an excellent way to apply water and minimize evaporation.  However, it takes a little over 3 hours to apply one inch of water using a 5/8 inch diameter, 50 ft. long soaker hose. While the application rate is much slower, the delivery of water is much more efficient, making soaker hose my preferred method for most situations. They are relatively cheap and can easily be cut down or spliced to fit the area needed to be watered.

In cases where water is less accessible, tree watering bags are my preferred tool. These collapsible bags can be filled with water and carried to the plant where they slowly release water through tiny seep holes. The only drawback is that the bag has a limited radius it can water with gravity, so they cannot water as large of a diameter as other methods. However, they are quite efficient with delivery and make the best choice for locations your hose won’t reach.