Mulches are materials placed over the soil surface to maintain moisture and improve soil conditions. Mulching is one of the most beneficial things a home owner can do for the health of a tree or shrubs. Mulch can reduce water loss from the soil, minimize weed competition, and improve soil structure. Properly applied, mulch can give landscapes a handsome, well-groomed appearance. Mulch must be applied properly; if it is too deep or if the wrong material is used, it can actually cause significant harm to trees and other landscape plants.
Mulch can be beautiful and if used properly provides many benefits.
Benefits of Proper Mulching
• Helps maintain soil moisture. Evaporation is reduced, and the need for watering can be minimized.
• Helps control weeds. A 2- to 4-inch layer of mulch will reduce the germination and growth of weeds.
• Mulch serves as nature’s insulating blanket. Mulch keeps soils warmer in the winter and cooler in the summer.
• Many types of mulch can improve soil aeration, structure (aggregation of soil particles), and drainage over time.
• Some mulches can improve soil fertility.
• A layer of mulch can inhibit certain plant diseases.
• Mulching around trees and shrubs helps facilitate maintenance and can reduce the likelihood of damage from “weed whackers” or the dreaded “lawn mower blight.”
• Mulch can give planting beds a uniform, well-cared-for look.
Trees growing in a natural forest environment have their roots anchored in a rich, well-aerated soil full of essential nutrients. The soil is blanketed by leaves and organic materials that replenish nutrients and provide an optimal environment for root growth and mineral uptake. Urban landscapes, however, are typically a much harsher environment with poor soils, little organic matter, and large fluctuations in temperature and moisture. Applying a 2- to 4-inch layer of organic mulch can mimic a more natural environment and improve plant health.
Beautiful forest with natural mulch ground cover
The root system of a tree or shrubs is not a mirror image of the top. The roots of most trees can extend out a significant distance from the tree trunk. Although the guideline for many maintenance practices is the drip line—the outermost extension of the canopy—the roots can grow many times that distance. In addition, most of the fine absorbing roots are located within inches of the soil surface. These roots, which are essential for taking up water and minerals, require oxygen to survive. A thin layer of mulch, applied as broadly as practical, can improve the soil structure, oxygen levels, temperature, and moisture availability where these roots grow.
Types of Mulch
Mulches are available commercially in many forms. The two major types of mulch are inorganic and organic. Inorganic mulches include various types of stone, lava rock, pulverized rubber, geo-textile fabrics, and other materials. Inorganic mulches do not decompose and do not need to be replenished often. On the other hand, they do not improve soil structure, add organic materials, or provide nutrients. For these reasons, most horticulturists and arborists prefer organic mulches.
Organic vs Inorganic, rock vs wood chips
Organic mulches include wood chips, pine needles, hardwood and softwood bark, cocoa hulls, leaves, compost mixes, and a variety of other products usually derived from plants. Organic mulches decompose in the landscape at different rates depending on the material and climate. Those that decompose faster must be replenished more often. Because the decomposition process improves soil quality and fertility, many arborists and other landscape professionals consider that characteristic a positive one, despite the added maintenance.
More is not always better!
As beneficial as mulch is, too much can be harmful. The generally recommended mulching depth is 2 to 4 inches. Unfortunately, many landscapes are falling victim to a plague of over mulching. A new term, “mulch volcanoes,” has emerged to describe mulch that has been piled up around the base of trees. Most organic mulches must be replenished, but the rate of decomposition varies. Some mulches, such as cypress mulch, remain intact for many years. Top dressing with new mulch annually (often for the sake of refreshing the color) creates a buildup to depths that can be unhealthy. Deep mulch can be effective in suppressing weeds and reducing maintenance, but it often causes additional problems.
Bad techniques can kill a tree.
Problems Associated with Improper Mulching
• Deep mulch can lead to excess moisture in the root zone, which can stress the plant and cause root rot.
• Piling mulch against the trunk or stems of plants can stress stem tissues and may lead to insect and disease problems.
• Some mulches, especially those containing cut grass, can affect soil pH. Continued use of certain mulches over long periods can lead to micronutrient deficiencies or toxicities.
• Mulch piled high against the trunks of young trees may create habitats for rodents that chew the bark and can girdle the trees.
• Thick blankets of fine mulch can become matted and may prevent the penetration of water and air. In addition, a thick layer of fine mulch can become like potting soil and may support weed growth.
• Anaerobic “sour” mulch may give off pungent odors, and the alcohols and organic acids that build up may be toxic to young plants.
Symptoms of Decline
Death from over-mulching is gradual, with symptoms sometimes taking 3-5 years to express themselves. It starts with the decline of plant vigor and rate of growth.
• Off-color leaves (pale or marbled)
• Abnormally small leaves
• Poor twig growth
• Die-back of older branches
• Rotting, pealing trunk bark under the mulch.
These trees died due to root collar damage from insects at the base of the tree.
It is clear that the choice of mulch and the method of application can be important to the health of landscape plants. The following are some guidelines to use when applying mulch.
• Inspect plants and soil in the area to be mulched. Determine whether drainage is adequate. Determine whether there are plants that may be affected by the choice of mulch. Most commonly available mulches work well in most landscapes. Some plants may benefit from the use of slightly acidifying mulch such as pine bark.
• If mulch is already present, check the depth. Do not add mulch if there is a sufficient layer in place. Rake the old mulch to break up any matted layers and to refresh the appearance. Some landscape maintenance companies spray mulch with a water-soluble, vegetable-based dye to improve the appearance.
• If mulch is piled against the stems or tree trunks, pull it back several inches so that the base of the trunk and the root crown are exposed.
It doesn't matter if it is a tree or raspberry plant the technique is the same.
• Organic mulches usually are preferred to inorganic materials due to their soil-enhancing properties. If organic mulch is used, it should be well aerated and, preferably, composted. Avoid sour-smelling mulch.
• Composted wood chips can make good mulch, especially when they contain a blend of leaves, bark, and wood. Fresh wood chips also may be used around established trees and shrubs. Avoid using non-composted wood chips that have been piled deeply without exposure to oxygen.
• For well-drained sites, apply a 2- to 4-inch layer of mulch. If there are drainage problems, a thinner layer should be used. Avoid placing mulch against the tree trunks. Place mulch out to the tree’s drip line or beyond.
What to Do?
The best way to determine if you have a mulch problem is simply to dig through the mulch layer to see how thick it really is. If it is excessive (over 4 inches), spread it out to the drip line or remove much of it. Sometimes a light raking of existing mulch is sufficient to break up any crusted or compacted layers that repel water.
Uncovering damage can be disheartening, but it must be done.
A visual inspection of the root flare zone or trunk collar (where the spreading base of the tree just goes into the soil) is the best way for you or an arborist to check the condition of the trunk for possible rot, pest chewing or diseases. If detected early on, removal of mulch to allow drying out may help curb more serious problems.
Complete root flare zone excavation may be necessary and is best performed by a professional arborist.
Remember: If the tree had a say in the matter, its entire root system (which usually extends well beyond the drip line) would be mulched.
Comparing Mulching Options
- Type of Mulch
It may not look tidy but it is the cheapest and works well
- Grass clippings
- Is cheap, readily available, and easy to apply
- Decays quickly, so you must replenish often. If you use weed killers on your lawn or nitrogen-heavy fertilizer, it may adversely affect other parts of the garden; can turn slimy if you apply more than an inch or so at a time; if the grass goes to seed before you cut it, the grass seeds can germinate in your garden beds (yikes!)
wood chips come in many colors and consistencies
- Wood or bark chips
- Looks neat and attractive; stays where you put it (except in high wind areas);
- Is slow to decay Pine bark mulch is fairly acidic, which you may or may not want for your garden; if you apply too deeply (over 3″) or apply a deep layer up against tree and shrub trunks, you may create a hiding spot for a bark-damaging rodent, especially during winter
Leaf mulch if maintained well can look great
- Decaying leaves
- Smothers weeds very well; helps hold in soil moisture
- Is not especially attractive; if it contains seeds, they can germinate and become a weed problem; if the leaves are soft, like maple leaves, the mulch can mat; if it’s acidic (oak especially), it can lower your garden soil’s pH
Compost mulch looks very similar to mulch but has advantages over mulch
- Compost Is free and plentiful if you have your own compost pile;
- Aadds nutrients to the soil as it breaks down
- Makes a good place for weeds to take hold; fresh compost (especially if it contains manure or grass clippings) can burn plants
This picture shows peat moss in the garden bed and wood chips around them. Great way to look nice and save money.
- Peat moss Looks neat and tidy;
- Is versatile — also functions as a soil amendment
- Can be expensive; if dry, will repel water; becomes crusty over time
Straw mulch, very simple.
- Is cheap and easy to apply
- Is so light it can blow or drift away; may harbor rodents, especially over the winter months; isn’t very attractive for ornamental plantings
Hay is very similar to straw just a different consistency
- Is cheap and easy to apply
- May harbor rodents, especially over the winter months; isn’t very attractive for ornamental plantings; probably contains weed seeds!
This is a great example of using stones in a garden. Weather it be larger stones or flagstone it still accomplishes the purpose of mulching.
- Gravel, pebbles, or stone
- Has a nice, neat look (though not “natural”); is easy to apply; won’t wash away easily and will last a long time; doesn’t need to be replenished over the course of a season
- In colder climates Can allow weeds to sneak through; provides no benefits to the soil
Plastic does the job here.
- Landscape fabric (garden plastic, black plastic)
- Keeps weeds at bay; holds soil moisture and warmth in
- Watering and feeding is hard (you need to cut openings for plants); can be difficult to apply unless you’re doing an entire area at one time; isn’t very attractive
Rubber mulch looking identical to wood chips here.
- Rubber (shredded recycled car tires)
- Very long lasting, available in many colors, looks like shredded wood mulch
- Can smell strongly of rubber; provides no nutritional benefits to the soil, some studies suggest this can leach arsenic and lead into the soil, not recommended around vegetable plants or fruit trees.