How Much Water do Trees Need?
Trees rarely need extra watering once they are established and can rely simply on rainwater. In areas of scarce water resources, understanding the variables and type of tree you choose will go a long way to reducing the need for gallons of water. In this article, gardening expert Wendy Moulton explores how much water trees really need.
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Trees go a long way to improving the environment by sucking up carbon dioxide, cooling the air, and preventing erosion. They also reduce the effects of extreme weather like flooding and give us much-needed clean air. In fact, there is an argument for saying that trees could save the world. It is essential that we plant new trees and, meanwhile, use the planet’s dwindling water resources with care.
In this article, we explore just how much water trees need and how to be water-wise while still growing big, beautiful trees.
The Short Answer
Trees need a lot of water, but it doesn’t necessarily have to be provided by you. When you first plant a tree, you must keep it watered for up to two years while it gets established. Eventually, it will be able to ‘root’ out underground water sources on its own. So, exactly how much water does a tree need? That depends on many variables, which we will discuss further in this article.
The Long Answer
To figure out how much water trees need, we need to delve into how a tree uses water and how they get to water on their own.
How Do Trees Absorb Water?
Trees use their roots to keep upright and stable in the soil. Roots also take up water and nutrients that keep the tree alive and growing. Water is absorbed by the roots in the soil by osmosis. Then, it travels up the trunk and into the branches and leaves using capillary action.
The tree uses what it needs and then releases the liquid into the air through the stoma in a process called transpiration. This process keeps the water balance in the tree at the optimal rate, just like our bodies do.
Nearly all of the water sucked up by the roots is released into the atmosphere. You can, therefore, understand why the air in a tropical forest is very humid and full of water vapor.
The rate at which transpiration occurs is determined by four factors: temperature, humidity, light, and wind.
A tree transpires to keep cool. The higher the humidity, the less the tree needs to cool down and trigger transpiration. The same is the case for temperature. At higher temperatures, a tree transpires more quickly to cool down. In cooler temperatures, it holds onto water because it doesn’t need to cool down.
A mature tree can lose several hundred gallons of water on a hot and dry day and virtually none on a cold wintery day. This is all due to temperature and humidity.
Light is another important element in the life cycle of a tree. Sunlight (energy), combined with water and carbon dioxide, transform via photosynthesis into chemical energy that provides fuel for the tree.
In windy conditions, tree leaves often close their stomata to preserve water. These four variables all affect how much water a tree needs.
How Do the Roots of Trees Work?
We can see that roots are the most important part of getting water into the tree. This is where another variable comes in – what types of root system does a tree have?
A tree starts with a tap root, which grows straight down into the soil. As the tree grows, the tap root diminishes, and the lateral roots take over and spread out. However, in some species, tap roots remain quite prominent, even going so far as to tap into underground water systems.
The lateral roots start at the surface of the soil and move outwards, eventually becoming large woody roots that hold the tree upright and provide a decent network that sources water for the tree to survive and thrive.
Feeder roots then form a pattern underground that can vary from species to species. The framework of roots underground can be two to four times as big as the aboveground canopy of the tree and, in some species, as much as eight times bigger.
All Roots are Not Equal
While it’s good to get a picture of what roots look like under the soil of your tree, there are another set of variables that may impact what they look like.
- When the conditions are such that the soil is fertile with plenty of organic matter and there is plenty of rainfall or access to water, tree roots are more fibrous and plentiful.
- Dry conditions and compacted soil result in fewer roots, but they are bigger and longer as they try to reach a better spot.
- Trees that are planted together have a smaller root system than those in an open space with plenty of room to expand.
- Root systems are blocked in some way by things like buildings, large boulders, rocks, or a high water table. The root system then grows any way it can.
How to Tell if a Tree Needs Water
Interestingly, the problems of underwatering and overwatering have the same effect on a tree. Roots need oxygen to perform their duties. If the soil has too much water, the roots will not have enough air around them and effectively drown. This leads to problems like root rot and other fungal diseases and puts serious stress on the tree. The result is that the leaves will begin to wilt and die off.
On the other hand, when a tree has limited water, the roots are also unable to function properly. The result is that the leaves begin to wilt and die off – same result, different problem.
There are a few other signs to look out for, like wilted leaves turning brown at the ends, early leaf drop, yellowing leaves, scorched leaves, small leaves for the species, and a small canopy.
Choosing the Right Tree
In some respects, choosing the right tree is the most important element of planting a new tree in your garden space. A tree is an investment and deserves proper treatment. In return, you may well be drinking a cocktail under a canopy of healthy leaves one day.
The first thing to consider is what you want a tree for – shelter, as a windbreak, for shade, for fruit or nuts, for vertical height in the garden design, because they are pretty, etc.
Then, consider planting a native tree. Our environments need replanting more than ever before. By planting native trees, you build corridors for wildlife and other plants to move and flourish.
There is also a practical reason for planting native and endemic trees, and that is because they are able to survive and thrive better in your environment than a tree imported from another country. A good nursery should have trees that do well in your particular area.
A note on native trees: just because they are native does not mean that they don’t require water. All trees need water, especially in hot, drought conditions.
A lot of the reasons a tree becomes invasive in an area is often because of the amount of water they suck out of the water table. Exotic tree species replace native plants in the area and become enormous, water-guzzling forests.
Looking After a Newly Planted Tree
Trees should ideally be planted in the dormant stages (usually autumn or winter in milder climates) so that they have enough time to get acclimated in their spot before spring. Make sure to make a good-sized basin about the drip line of the tree. This is so that when it does rain, the water soaks in around the base of the tree to the roots.
Adding a two-inch-thick layer of mulch around the tree goes a long way to saving water. Make sure not to get too close to the trunk, as this can cause diseases. Wood chips are ideal as they break down slowly, adding nutrients back into the soil while keeping the roots cool and inhibiting the evaporation of water.
At the planting stage, consider placing a length of pipe or a plastic milk bottle with the base cut off at an angle toward the roots. This allows you to irrigate directly to the roots with the minimum amount of water. Or bury an olla system close to a tree.
There is also the option to add irrigation – drip lines, bubblers, or soaker hoses – at the planting stage. Run yours on timers on a schedule for even more convenience. Some even come with weather monitors that will not run if it’s raining.
Watering Schedule for New Trees
Most newly-planted trees need a good deal of water to get established for up to two years.
Step 1: Water deeply after planting.
Step 2: Water daily for one to two weeks.
Step 3: Water every two to three days from three to 12 weeks.
Step 4: Water every week after 12 weeks. Each tree is different and may need only six months to be well established. Others may need up to two years.
Step 5: Water only when you see the tree is stressed and on very hot days after the tree has established.
How Much Water to Give a Tree
It is generally fine just to water deeply according to the schedule, but you can also calculate the amount of water a tree needs based on its age. As the trunk size increases, so does the need for water. However, as it becomes established and during cooler weather, it needs less water.
As a general rule, follow this calculation for trees up to the age of nine to ten years:
- For tree trunks less than two inches in diameter, use one gallon of water per inch, up to two gallons of water.
- For tree trunks between two and six inches in diameter, use two gallons of water per inch. For example, a six-inch diameter tree will need 12 gallons of water.
For older, more established trees, there should not be a need for water. However, in cases of extreme drought, for example, a 25-year-old tree with a trunk diameter of 12 inches would need up to 120 gallons of water.
How to Test if Your Tree Needs Water
There is a way to test if your tree needs water, especially if you are not sure by the look of the leaves on the tree. Simply dig into soil six to eight inches deep under the tree.
At this level, the soil should be cool and still have some moisture. If the soil is pooling in the hole, the tree has too much water, and if the soil is dry and crumbling, it needs water.
How to Water Correctly
By watering correctly, you save water in the long run. The first rule is to water at the correct time of the day. The mornings and evenings are cooler than the middle of the day, which results in less evaporation and waste due to heat.
Water slowly and deeply rather than short, frequent bursts for the best results. The water needs to get to the roots of the tree. If you water too quickly, the water doesn’t get to the bottom of the roots and is only used by the shallow roots. In time, shallow roots are unable to grip into the soil enough to provide stable support for the tree. If you water deeply, the roots are encouraged to grow downwards to secure the plant in the soil.
Water into the basin you created while planting. Watering the leaves and branches provides minimum water to the soil and the roots.
Final Thoughts
We know that many variables determine how much water a tree needs. The quick answer is a lot. But once established, that tree will give you decades of pleasure and, in some cases, feed you for a season. It’s a good investment of time and resources, and I urge everyone to plant a tree sooner rather than later. Even if you have no space, there are always community projects looking for volunteers to plant a tree. You can even plant a tree in memory of a loved one.
