Why should I mulch?
Mulch provides a pleasing visual benefit to nearly every garden. In addition, it provides a valuable service by reducing weed growth and conserving soil moisture by reducing evaporation from the soil surface. Mulch aids water penetration into the soil by slowing the full force of heavy rains, thereby reducing the effects of soil erosion. By slowing down the movement of the water, it is more likely to be absorbed by the soil than to run off.
Soil temperature is moderated as the mulch, acting as an insulator, reduces temperature fluctuations. Both low and high temperatures can cause injury or death of boxwood roots. On average, cold soil temperatures of 15oF (-9o C) result in the death of more than half of the roots. Extreme heat is also dangerous. Many boxwood roots stop growing when exposed to near 90oF (32o C), and they begin to die as temperatures increase to 95oF (35o C).
Mulch greatly inhibits weed growth. Weeds will consume water and nutrients in the soil that would otherwise be available to the boxwood. In addition, weed vines will shade the leaves, reducing vigor in the plant. Soil fertility is enhanced as nutrients leach from the mulch, and the organic matter content of the soil is increased as the mulch decomposes and enters the soil.
Which mulch is best?
Shredded hardwood bark
Because of its availability, shredded hardwood bark mulch is the most popular in the Northern United States. It is an effective and visually pleasing mulch, which also retains soil moisture and inhibits weed growth. Having a high cellulose content, it decomposes in about two years, adding important organic matter to the soil.
Relatively inexpensive and readily available, this mulch is often chosen for gardens in the Gulf States. The medium and fine grades of this mulch, however, tend to blow away with the wind and wash away with rain, while coarse grades of pine bark do little to discourage weed growth. Having a high lignin content, they decompose slowly, without adding much organic matter to the soil.
The needles from various species of pines make a very effective and attractive mulch. The needles interlock with each other, blocking most weed growth. Limited quantities of needles can be raked up from under pines in fall. Pine needles are commercially available on a limited, usually seasonal, basis.
Professional tree companies grind up tree branches into chip-sized pieces. This by-product, if properly managed, can become a useful coarse mulch. “Green” or fresh hardwood chips have a high carbon-nitrogen ratio. As it begins to decompose, green mulch removes large amounts of nitrogen and should not be placed directly into planting areas. After it has aged and begun to decompose, chip mulch can be used in garden beds. The hardwood chips also release manganese, to the soil. If the chips are applied too heavily, they will release excessive amounts of manganese which is toxic to boxwood and other plants.
Some materials not suitable as mulch
Available in limited areas, sawdust should only be used after it has become well decomposed. If fresh sawdust is applied to the garden, it will remove large amounts of nitrogen from the soil as it decomposes. Pressure-treated sawdust, which contains toxic wood preservatives, should not be used. Black walnut sawdust should not be used due to its allelopathic effects. Any type of sawdust has a tendency to cake and harden, which causes it to repel water.
Used as a mulch, peat moss will reduce soil aeration, lower the soil pH, harden, and repel water. However, when the shredded peat moss is used as a soil conditioner and lightly cultivated into the soil, it greatly improves the organic matter content of the soil. This method of incorporation can only be used in new or unplanted areas as it will greatly endanger the shallow roots of established plants.
Stone chips and gravel
Stone chips and gravel are a durable, permanent mulch that is available in a wide range of sizes, shapes, and colors, providing an effect not possible with organic mulches. Used by itself, however, stone is not effective in controlling weeds or improving the soil. Marble and limestone gravel have the advantage of being quite alkaline which will benefit boxwood grown in a low pH soil.
Black or white plastic effectively eliminates weed growth by blocking sunlight, but plastic will not permit rain water to pass through and severely impairs normal water movement in the soil. While black plastic is more durable than clear or white plastic, it will cause elevated soil temperatures. When plastic is placed on even a slight slope, the mulch placed on top tends to slide downhill from rainfall.
In an effort to dispose of an excess supply of pallets, companies are shredding and incorporating them in traditional mulches. When constructed, the pallets are treated with chemical preservatives to prolong their life. These preservatives can adversely affect the health of boxwood and many other plants.
How should mulch be applied?
Before applying any mulch make sure that there is a good supply of soil moisture. Ideally, mulch should be applied after several hard frosts in late fall. Mulch may also be used in early spring after the chance of hard frosts has passed. Apply mulch only to a depth of 1" (2.5 cm). If the mulch is being used to top dress existing mulch, remove some of the existing mulch and apply only enough to bring the total depth to 1" (2.5 cm). Avoid mounding mulch under branches, which will encourage adventitious rooting. Over a period of time the organic materials used in mulch will decompose, adding humus and plant nutrients to the soil. As this occurs, it will be necessary to add additional mulch to maintain the 1" (2.5 cm) mulch depth.
Can mulch cause problems?
Nitrogen deficiency in the soil
By using freshly chipped wood or bark mulch, including sawdust or straw, the soil nitrogen is rapidly depleted by the mulch as it decomposes, leaving little for the surface-rooted boxwood. This is easily avoided by using only decomposed mulch. If fresh mulch has already been applied, broadcast high nitrogen fertilizer to offset the loss caused by the freshly decomposing mulch.
This occurs most often in poorly drained soils and in areas where surface water collects. Mulch compounds the problem by restricting normal evaporation. Several remedies include: discontinue mulching, divert surface water, and improve the water drainage.
It is difficult to overstate the problems associated with applying too much mulch or when repeated applications of mulch are permitted to accumulate. Mulch that is deeper than 1" (2.5 cm) will encourage voles. The lack of soil air inhibits proper root growth. Deep mulch encourages the branches in the mulch to layer, producing adventitious rooting. Soils with excessively high organic matter content will not be improved by applications of mulch.