Tree and Tree Root Facts
If you’re planting or caring for a tree, here are some facts you need to know about protecting tree roots, a tree trunk and ensuring the tree’s overall health.
1: Most Tree Roots Grow Outward
When you think about tree roots you may think of a large root system going deep down into the ground. That’s the case for some trees with long tap roots, but most tree roots grow out horizontally from the tree at a depth of 6 to 18 inches. Soil conditions are usually best here and this is where water and nutrients are most readily available for the small, fibrous feeder roots to collect.
2: Tree Root Systems Can Cover a Large Area
A mature tree’s roots can spread to a distance two to three times the diameter of the tree’s crown or canopy. A tree that’s 30 feet tall can have roots that reach out 60 to 90 feet from the trunk. That’s far past the dripline, the outside edge of the branches. While the extensive roots anchor the tree, the top of a mature tree is far heavier than the root system.
3: Trees Roots Need Oxygen
The oxygen tree roots collect is as important to a tree’s health as water and nutrients. Adding even a couple of inches of soil on top of the existing topsoil that covers a root system can smother a tree. Compacted soil also prevents oxygen — and water — from reaching the roots.
4: Trees Compete with Grass
Competition for water and nutrients is one reason that grass often doesn’t grow well underneath trees. This competition with the tree roots and the shade itself mean the tree will usually win. Shade-tolerant grass is an option, but even these types of grass need some periods of direct sunlight. Get more ideas for planting under a tree from our article on growing grass in shade.
5: Planning and Planting Matters
Choosing a shade tree, flowering tree or feature tree that’s right for your area and planting it correctly give the tree and its roots the best chance for success. Plan for the tree’s size at maturity. A plant tag gives you information about mature tree size as well as tips for planting and care.
In addition to the tree height and width, consider the amount of space a healthy root system will need. Otherwise, the roots may damage sidewalks, driveways, etc. See our article on planting trees for step-by-step instructions.
6: Tree Roots Collect Chemicals
The feeder roots will absorb fertilizer and other chemicals like pesticides and herbicides in addition to nutrients. Runoff from deicing agents can also harm a tree. Be cautious when using any chemical around trees and follow application directions. Any damage may take several seasons to become visible. By then it may be too late to prevent a problem.
7: Good Mulching is Good for Trees
Mulch helps retain moisture, maintain consistent soil temperature and protect roots from damage. A mature tree needs a maximum of 2 to 3 inches. More can be harmful. The exception may be in extremely cold regions where frost heave can disturb roots, pushing them upward when the groundwater in the soil freezes.
Keep mulch away from the trunk. Piling mulch against the trunk can cause problems with disease, insects and rodents. Our mulching guide will show you how to mulch around a tree.
8: Digging and Mowing Can Damage Trees
Be aware of the root system when digging and aerating around trees. Don’t cut too many roots. Use power equipment carefully around the base of a tree. Avoid nicking or slicing exposed roots with your mower or string trimmer. Don’t use a string trimmer against the bark as it can split this protective layer and make the tree vulnerable to destructive fungi.
You can purchase plastic tree guards as an added measure of tree trunk protection. Mulching around the tree can keep you from needing to cut grass and weeds near it, but remember, don’t pile the mulch against the trunk.
9: Wraps Provide Tree Trunk Protection
Tree wrap can protect young and delicate trees from damage caused by insects and rodents. Look for a wrap that can stretch as the trunk grows. Some tree wraps can protect trees from sunscald, a condition where the protective bark cracks in the winter due to cycles of warmth from the sun and cold air.
10: Trees Need Protection from Home Renovations
The heavy equipment, machinery, stacks of material and foot traffic that home renovation brings to your landscape can harm your trees. The weight of the material and the activity can compact the soil in your lawn and prevent oxygen and water from reaching tree roots. Compacted soil also causes water runoff and erosion. Soil compaction is difficult to reverse, so prevention is the key.
Put up a tree protection fence at or just beyond the dripline before work begins. Extend it out to the drip line or slightly beyond. You can purchase temporary fencing and posts or you can construct your own with stakes and rope. If you can’t construct a fence, spread a six-inch layer of natural mulch over the area to act as a cushion. Remove it when the project is complete.
11: Slow Watering Is Best
Apply water slowly to prevent runoff and ensure that the water reaches the tree’s deep root system. Thoroughly water a tree’s entire root zone during periods of drought with the equivalent of 1 inch of water. Water once every week or two, depending on soil type. Loose, sandy soil drains quickly; trees in sandy soil need water more frequently than those in heavy, clay soils. A drip irrigation system or soaker hose is great for slow, direct watering.
12: Tree Roots Need Water for Winter
Water one last time in the fall to help your trees survive the winter in good health. The cold of winter can dry out plants, especially when it’s combined with wind. Apply water at or near the dripline, watering slowly to avoid runoff. Watering early in the day is best, especially when there’s a chance of freezing evening weather. Never water when the ground is frozen.
13: Fertilizer May Be Necessary
Decide whether the tree needs fertilizer. If growth seems stunted or the leaves look unhealthy or small, first rule out pests and disease.
It’s best to fertilize trees in early spring when roots are actively growing. Follow up in summer with light fertilizing if it’s not too hot or dry. Test the soil to see what’s lacking and determine soil pH. Adjust the pH as needed.
If a tree is growing in your lawn and you use lawn fertilizer, it shouldn’t need additional fertilizer. Otherwise, consider a slow-release granular fertilizer formulated for the tree and appropriate to the soil test results. Apply it around the tree and water the granules in. Follow the package application instructions.
14: Pruning Requires Timing and Technique
Maintain your tree’s health by using proper pruning techniques. Because there are so many different types of trees, it’s impossible to list every pruning technique here, but we’ve got a few helpful suggestions for the most common trees:
- Trim flowering trees within three weeks of final blooming to avoid removing buds containing next year’s flowers.
- Prune live branches with a bypass-style hand pruner or lopper for a cut that promotes healthy regrowth. Use an anvil-style pruner or lopper on dead branches to seal the end of the branch.
- Avoid topping a tree — removing the upper portion of the tree’s central stem and cutting back the topmost primary branches — when it becomes too big. Topping creates a hat rack appearance and can permanently ruin the branching structure of the tree. In most cases it’s better to move or remove a tree altogether than to top it.
- Don’t use a ladder to trim branches that are out of reach from the ground. Use a pole pruner with a long telescoping handle. Most pole tree trimmers have a lopper blade for thin branches and a saw to cut through thick branches. Wear eye protection. Depending on the size of the branches, you may need a hard hat.
Tree Root Protection Frequently Asked Questions
The answers to these common questions about tree roots can help you give your trees the best chance to grow.
What Is the Best Way to Protect Tree Roots?
Covering the roots with a couple of inches of natural mulch is the best way to protect them. You won’t need to mow the area so you’ll avoid damage from lawn care equipment. The mulch will reduce foot and equipment traffic over the root system, minimizing soil compaction while allowing oxygen and water to enter the soil.
Is It OK to Cover Exposed Tree Roots?
It’s okay to cover exposed tree roots as long as you use a material that allows moisture and oxygen to reach them. A couple of inches of mulch that drains well is a good option. Don’t simply cover the roots with topsoil. If it’s too deep or becomes compacted, it can smother the tree.
What Is the Best Material to Cover Tree Roots?
A couple of inches of bark chips or shredded wood mulch is the best material for covering tree roots. The mulch allows moisture and oxygen to reach the roots and helps hold in moisture. You won’t need to mow the area so you won’t risk damaging the roots with a mower. Mulch also reduces weed growth and the need for using a string trimmer near the trunk.
Should You Put Rocks Around a Tree?
You can put gravel around a tree as an alternative to bark chips or shredded wood mulch, but rocks can trap heat, which can be harmful to some trees. Rocks can also compact the soil, making it difficult for tree roots to pick up moisture and oxygen. If you use gravel, first put down landscape fabric and add only a couple of inches of the stone.
Can I Cut Exposed Tree Roots Without Killing the Tree?
Avoid cutting tree roots if possible. Removing roots reduces absorption of nutrients, water and oxygen and can make the tree unstable. Consult a professional. If you decide to remove roots, cut as few as possible. Leave at least 85% of the above-ground system intact. Don’t cut roots wider than a couple of inches and don’t cut any near the trunk.