If you planted a new tree this spring (or even last fall), it will need careful attention to make it through the heat of summer. Young trees expend a lot of energy making leaves and their root systems are still small. As a result, the stress of hot summer weather on newly-planted trees can be deadly without regular watering to help them get established. Learn how much water is needed, how often to water, and which factors affect a tree’s water needs.
Water = Life for a Newly-Planted Tree
The first few years of a tree’s life are when it develops its anchoring and feeding root systems below ground, as well as the trunk and branch structure above ground. The more vigorous a young tree, the faster it can put out new leaves for photosynthesis (to generate internal energy stores) and develop its root system.
This early growth requires significant amounts of water, which isn’t always easy for a tree to “find” during the hot, dry summer months.
During a summer heatwave, a young tree that lacks sufficient water can become stunted and susceptible to pests and diseases at best, and die within days at worst.
Start Watering Your Tree BEFORE You Plant It
Proper watering starts when you plant the tree, beginning with the tree’s rootball. Be sure to thoroughly water the entire rootball when you remove it from its nursery container or unwrap the burlap from around it. Often, the soil within the rootball is dry (and sometimes compacted) and doesn’t easily absorb moisture from the surrounding soil unless it’s wetted before planting.
The goal with this first watering is to ease your new tree into its new home with as little stress as possible, ensuring a smooth establishment period.
Water Immediately After Planting
One good rule of thumb is to immediately irrigate a newly-planted tree with 2 to 3 gallons of water per inch of its trunk diameter. So a tree whose trunk is 2 inches in diameter when you plant it should be given 4 to 6 gallons of water right away.
A newly-planted tree’s roots only extend as far as the rootball. Any water in the soil that’s beyond the reach of the tree’s rootball can’t be absorbed. And since that rootball isn’t very large when the tree is first planted, it’s critical to provide enough water around the rootball. Without that nearby water, a young tree in a summer heatwave especially vulnerable.
Sufficient watering is also important if the soil you’ve planted your young tree in is already on the dry side. The dry soil will automatically pull water from the wetter rootball to balance the water distribution where the two materials (rootball and native soil) meet, leaving the tree without enough moisture.
If your young tree is planted among other trees or shrubs, watering is important because the roots of these other, established plants will compete for water with your new tree.
And most importantly, you’ll want to water low and slow. This slow infiltration rate gives the young tree’s roots a longer period to take up water and allows water to move deep into the soil, which is where you want your tree to develop its roots. Shallow watering encourages shallow root development, leaving the tree unstable and susceptible to dry conditions.
Rule of Thumb for Watering a Young Tree After Planting
How much water a young tree needs will depend on several factors. There’s no single answer to this question but there’s a helpful rule of thumb that serves as a good starting point –
Give your young tree 1 ½” to 2” of water a week.
But how do you measure that?
This is where calculating how much to water your young tree gets more complicated.
Here comes the math!
To figure out how to get 2” of water to your new tree each week, you need to know two things:
- the flow rate (in gallons per minute or GPM) at your hose bibb or irrigation emitter, and
- the size (in square feet) of the area you’re going to irrigate (this should be the area under the tree reaching all the way out to the edge of the canopy).
Then plug those numbers into this formula –
# of minutes of watering = (0.62 x area of coverage (sf)) / flow rate (GPM)
- The first number, 0.62, is the irrigation constant. It represents 1” of water over 1 square foot of soil.
- To turn that amount into gallons, multiply the irrigation constant by the size of the area that you’re irrigating (for example, 100 sf of planting area x .62 = 62 gallons)
- Then divide that number by the flow rate (GPM) of your hose bibb to get the length of time you’ll need to run water (example: 62/2 GPM = 31 minutes of watering).
If your head’s already exploding, don’t worry! An experienced irrigation designer will do the calculations for you as part of installing a new system, or as an addition to your existing irrigation system’s valves.
If you want to DIY your tree watering with a garden hose or by placing a soaker hose around the tree, you can easily get a flow meter for your hose bibb to calculate its flow rate. IMPORTANT: Set the flow rate you want for your tree (a slow trickle) before measuring it – do not measure at its maximum flow.
Whatever method you use to water your newly-planted tree, remember that it will need irrigation until fall’s cool weather and rainfall arrive with the shorter hours of sunlight. If dry weather persists through fall, continue to water.
IN ADDITION TO WATERING …
Don’t forget to mulch! Mulching your newly-planted tree is one of the best things you can do for it.
A 3” layer of organic mulch (organic means made of organic matter like wood chips or ground-up bark) helps your tree through hot summer weather in several ways:
- Insulating the soil and regulating its temperature
- Slowing water evaporation from the soil
- Suppressing the weeds that would otherwise steal water for their own growth
- Enriching the soil’s water-holding capacity as it breaks down
And, as you know, always keep mulch away from your tree’s trunk.
Other Factors Affecting a Young Tree’s Water Needs
Beyond the rules of thumb listed above, the water needs of your young tree are affected by many factors, including:
Tree Species and Size
It’s a good idea to know the general water needs of the trees you choose before planting them.
- Different species of trees have different transpiration rates, meaning they need different amounts of water and take up water by their roots at different rates. Trees that have evolved to withstand windy, coastal conditions are very different from trees that evolved in humid, rainforest, or subtropical conditions.
- Rootball size at planting determines water use. If you are planting more mature trees with larger rootballs, remember that in addition to their larger rootballs they will most likely have larger crowns with more branches and leaves to support. The stress of transplanting can cause trees to drop some or all of their leaves, so be sure to provide enough water to minimize transplanting stress.
Is the tree in full sun or in a windy location? Is it in a low spot or where there’s shade? Are you planting your new tree on a slope? The specifics of a tree’s surroundings are important to its water uptake needs.
- A low spot in part shade will most likely lose water more slowly than a spot that’s in full sun, because of slower evaporation.
- The top of a slope will lose water most quickly because of gravity pulling the water down the slope and through the soil.
- A windy spot will increase the transpiration rate of a tree, as the water that’s released by a tree’s stomata (openings on the underside of leaves that release water) will evaporate more quickly when wind blows it away. This means the tree needs more water, faster, to be transferred from its root system to its crown to keep its transpiration balanced.
Are there many other trees and shrubs around your new tree? How about lawn or flower beds?
- Competition by your plants for water is real! You’ll want to supply enough water to keep everyone’s roots healthy.
Outdoor temperatures and hours of sunlight are directly related to how much water a tree of any size needs.
- The hotter it is, the more water a tree’s roots will take up.
- The longer the hours of sunlight, the more time that tree will spend taking up water each day.
The kind of soil your tree is planted in is also important. Soil has a natural texture, defined by the percentages of its components, and that texture determines how fast water will run through it.
- Sandy soil drains water very fast, sometimes too fast for tree roots to be able to take up all the moisture they need.
- Clay soil can be slow to absorb water and can hold water for long periods of time. Clay soil can become waterlogged, essentially “drowning” your tree.
- Loam soil has the best water-holding capacity for most landscape plants and is ideal for most trees.
- Urban fill soil may be made up of any or everything, so its water-holding capacity isn’t known without a soil analysis.
If you’re curious about your soil, the University of Connecticut’s soil testing lab offers soil testing that will tell you what your soil is made of and how it’s classified by texture. Understanding your soil type will give you a more precise idea of how much and how often you’ll need to water your newly-planted tree.
To add to the complexity of water, soil, and trees, your soil’s depth is also important to how much water you’ll need for your young tree. Shallow soil just can’t hold the volume of water that deep soil can so your young tree’s roots will need more frequent watering.
Testing your soil will also tell you about its nutrient levels and if, or how much, you’ll need to amend your soil to ensure healthy tree growth. Soils in our area of Connecticut (and New England in general) can be shallow and nutrient-poor.
As you can see, there isn’t one simple answer to the question “How much water should I give my newly-planted tree?”. When you water and how much you provide will vary depending on a wide variety of factors.
However, the general tree watering guidelines are:
- Water the rootball before planting
- Give 2 to 3 gallons of water per inch of trunk diameter right after planting
- Provide 1 ½ to 2 inches of water each week throughout the summer and fall until temperatures cool
If you’d like help figuring out the best way to water your new tree, give us a call!
Our arborists can arrange a soil test for your property, explain how and why you might need to amend your soil, suggest ways to set up or change your irrigation schedule or method to best suit your tree and its location, and ensure your newly-planted tree receives all that it needs for long-term health.
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