What, When, Why and How to Mulch Your Garden
As soon as the ice-cream trucks disappear in the fall, taking their incessant recorded “HELLOOO” with them, the mulch trucks fill the void, and homeowners across the region repeat the ritual application of fragrant, brown carpets around their trees and flowerbeds. For most, the motivation is strictly esthetic; they like the way it tidies up the beds and brings order to the chaos of the long, hot summer. But there are practical, horticultural benefits to mulching, as well as ways to do it and ways not to do it. If you’re mulching without knowing why, then read on!
Mulch offers many benefits to the esthetics of a landscape and plant health. Mulching retains moisture and protects root zones from heat and cold, suppresses weeds, increases soil organic matter, increases disease and insect resistance, protects plants from injury, and controls erosion.
When to Mulch?
For most of its benefits, mulch can be applied anytime. But to help prevent plant damage due to heaving during periods of freezing and thawing, timing is more important. For this purpose, mulch should not be applied until the ground has frozen. Also, mulch applied to early in the fall may delay plant dormancy, leading to winter damage and dieback. In the early spring, mulch should be lifted, and can be reapplied after soils warm up, if desired.
How to Mulch.
The amount of mulch to apply depends upon the type used. Coarser mulches can be applied to a greater depth – 4-6″ – than finer mulches – 2-3″. Apply in flower beds around plants, but not touching the crowns. Apply around trees as far out as the drip line, but again, not up against the trunk. Think donuts, NOT VOLCANOES! The common practice of piling mulch to a depth of a foot or more around trees damages bark, creating attractive entry points for insects and disease, and interrupts water and oxygen exchange to the root zone.
What is Mulch?
It may seem like a stupid question with an obvious answer, but it’s not. The majority of mulches used by homeowners are variations on a theme of shredded wood; but the devil is in the details. What kind of wood is it, and where did it come from? If you’re buying your mulch from a passing truck, you don’t really know. There were rampant urban-legend rumors after Hurricane Katrina that storm debris containing Formosan subterranean termites was making its way into the mulch supply nationwide. Experts assured us it was unlikely the termites could survive the violent mulch-making process, much less survive our colder climate. But other concerns, including the presence of arsenic from pressure-treated wood, pesticide residues and other less sinister chemicals and debris could cause harm to plants and gardeners.
Buying your mulch from a reputable source, either in bulk or by the bag, will alleviate most of these potential problems. Many mulch-producing companies are now having their products certified by the Mulch and Soil Council to verify that the products meet accepted industry standards for content, including volume (you’d be surprised at how many bags sold at gas stations and convenience stores are light of the labeled volume). Others gain certification through the US Composting Council. Not having these certifications doesn’t necessarily taint a product, but they are added assurances of quality.
Finally, if mulch has a vinegar or ammonia smell, it hasn’t been composted properly and should not be used. It can kill herbaceous plants overnight and cause damage to trees and shrubs!
Here are the most common types of mulch and uses for which each is best served:
- Bark Mulch. Decorative mulch for around patios, on walkways, etc. Doesn’t compact or mat like finer mulches, but is very slow to decay, adding virtually nothing back to soil.
- Chipped or Shredded Cedar/Cypress. These barks are similar in appearance, with a reddish/brown color, and resistance to decay. The upside is it lasts longer; the downside is it doesn’t readily decompose, so is slow to benefit the soil. It’s also more expensive than hardwood mulch. It has some insect repelling qualities, so might well be used around homes and patios.
- Chipped or Shredded Hardwood. From bark of hardwood trees, may also include lumber scrap or wood pallets, and yard waste. Dark brown or black color, use around trees and shrubs and perennial gardens. Should be cultivated occasionally to avoid “matting.” Ranges in texture from coarse chips to single-, double-, or triple-ground (fine) shreds.
- Pine Needle Mulch. Used more in the South where supplies originate, but an excellent mulch for garden paths and underneath acid-loving rhodies, azaleas and other plants.
- Leaf Mulch/Composted Yard Waste. Perhaps not as decorative as other mulches, these can make good mulches, if from a reliable source. Breaks down faster than hardwood, so needs to be reapplied more often (1-2 times a year), but delivers more benefits to soil. Use in annual and perennial flower beds.
- Straw/Hay. An inexpensive mulch for vegetable gardens. Discourages weed growth and helps prevent soil-borne pathogens from splashing up on plants.
- Grass Clippings. Definitely not decorative, but function satisfactorily to control weeds and conserve moisture around vegetables and out-of-view trees and shrubs. Make sure herbicides were not applied to the grass.
- Nut Shell Mulches. In some cases pecan shells, cocoa hulls and other nut byproducts are used as mulches. They decay very slowly so are best used as path materials (avoid cocoa hulls in area frequented by dogs, as they are poisonous).
- Inorganic Mulches. Plastic sheeting, landscape fabric, rocks, gravel and shredded tires all qualify as mulches, suppressing weeds and to some extent, conserving moisture. They last indefinitely, but have no benefit to the soil, and in some cases can interrupt water and oxygen exchange to plants. Use in pathways or other non-planted areas.