To Mulch or Not to Mulch?

by Debi Borden-Miller

Mulch discourages weeds and it looks good, but did you know that it also helps retain moisture and keeps soil temperatures even? (Big advantage here in the high desert!)

Mulch is a material spread on the surface of soil, like a covering. There are two basic types of mulch: organic and inorganic or synthetic.

Organic Mulches

Organic mulch helps curtail the growth of weeds, looks natural, retains moisture, helps keep soil temperatures even, encourages worms and can improve the soil as it decomposes. Bark, compost, grass clippings, straw, and shredded leaves are all types of organic mulch. Two to four inches of depth of organic mulch is recommended.

Problems with organic mulches: slugs like to live in organic mulches. If it is too close to the plants it can cause crown or stem rot, and if it is put down too early in the season it can slow the sun heating the soil. As organic mulches decompose, they use nutrients like nitrogen from the soil that competes with the availability of those nutrients for the plants. Organic mulch is not suitable around plants that like good drainage and drier growing conditions like succulents, some native plants, iris, carrots, garlic, onions, lavender, rosemary and sage.

There are many types of organic mulches.

Bark is best used around trees and shrubs and in beds where there won’t be much digging. There are shredded or chipped types. Bark looks very natural in planting beds. Colored bark is also available but is not as natural looking.

Large wood chips blow around easily and are best used around larger plants. Smaller wood chips are better suited to perennials or small plants. Shredded bark sticks together better and doesn’t blow around as much. These aren’t suitable to mix into the soil.

Soil Pep is a product made from very fine bark particles, and can be used where it will be dug into the soil at the end of the season, or even used to mulch containers to help retain moisture.

All of these organic mulches decompose at different rates (the larger the particle the slower the decomposition). They will use nitrogen and nutrients from the soil as they decompose, so be sure to fertilize your plants where you use bark mulch.

Keep mulch 6 to 12 inches away from the trunks or stems of the plants to prevent rotting, and critters like voles from damaging the trunks/stems.

There is some concern about the colored wood mulches having dyes, preservatives or other contaminants. These mulches are generally made from recycled wood products. Be sure to buy these products from reputable dealers and look for the seal from the Mulch & Soil Council (www.mulchandsoilcouncil.org) to ensure there are no questionable ingredients in. The Mulch and Soil Council tests for things like lead paint, chromated copper arsenate and other contaminants. Compost needs to be well-composted and weed free.

Compost will release nutrients into the soil and can be easily incorporated into the soil later. Compost can be used as a light mulch and should only be spread in a thin layer and then topped with shredded leaves, straw or bark to create a thicker layer. It is especially good for vegetable gardens, or under fruiting trees and shrubs.

Leaves are nature’s favorite mulch! Shredded leaves are even better for mulching purposes. They add nutrients as they decompose. They are perfect for a woodland garden or to spread on a vegetable garden (particularly in the fall so they can decompose over the winter). They also work well to spread around perennials in the fall to help keep the soil from the extreme freeze/thaw cycle that can cause heaving of soils. If the layer is thick and partially decomposed, it can bind close enough to repel water – if that happens just fluff up the mulch to separate.

Grass clippings are best left on the lawn to decompose if a mulching mower is used. They can be used as a mulch, but do not use clippings that have had herbicides or pesticides used in them. Because grass clippings are tan/brown in color and mat down quickly, which doesn’t allow effective water penetration. They are best for walkways of vegetable gardens or in remote areas where you wish to suppress weeds, and not as a decorative mulch. They will decompose rapidly.

Straw/Hay is a popular mulch in vegetable gardens, or to top off an area when it is seeded (like a new lawn area). Be sure to choose a weed free type. Wheat straw may also contain wheat seeds which can sprout. Straw, when used as a mulch, can keep soil borne diseases from splashing up on lower leaves of vegetable plants or can be used in pathways to make them less muddy. It decomposes slowly and is easy to rake up or can be worked into the soil later.

Newspaper is useful to smother grass or weeds. It can be used over an area where a new bed or garden is going to be started. Use 4-8 sheets and soak it until it’s wet. Lay it over the area and weigh it down with some rocks or even soil and keep it wet until the grass/weeds are dead. Newspaper can also be used shredded to keep roots moist. Keeping it moist in our arid west climate may be a challenge.

Synthetic or Inorganic Mulches

Inorganic or synthetic mulches are materials like gravel, fabrics, or plastic. These also curtail weeds. They can heat up the soil, but only the gravel looks natural, and none of them add any nutrients to the soil. However, they do have their place, such as around foundations or beds where no other planting is to be done.

Synthetic or Inorganic products also have appropriate uses.

Gravel is great for plants that like heat and good drainage. Many of our native western plants or plants of Mediterranean origin would prefer gravel or no mulch. Gravel as mulch is appropriate under drought-tolerant plants or in rain gardens. The size of the gravel depends on the size of your plants. Smaller plants look better will smaller gravel, larger plants could take a little larger size gravel. Cons: it is hard to move, it’s heavier than other materials, and it retains heat. Pros: Lots of choices of colors, sizes and types. 1-2” of gravel is the recommended depth.

Weed fabrics or landscape fabrics do restrict weed growth and generally allow water penetration so they are useful under trees or shrubs where no other digging or planting will take place. Fabrics don’t decompose and can’t be incorporated into the soil, so they are a more permanent option. To add other plants later, holes will have to be cut into the fabric. Fabrics are generally not attractive on their own, so bark or rock can be used on top for better aesthetics. Weed seeds can blow on top or can be dropped by birds and animals and can germinate and grow on the top of the fabrics, so it does need to be cleaned or the top material removed and replaced periodically.

Plastic should only be used where no water penetration is needed. It is also very hot. Farmers use it in strips under heat loving vegetables like melons, cucumbers, eggplants, tomatoes and peppers. Drip lines or soaker hoses are run under the plastic to water the plants. Plastic is usually put down on the row first, secured with rocks or soil on the edges. Then, holes can be punched into the plastic with a bulb planter to make room for the plant to be planted. Red plastic has been used with tomatoes to increase the yield. Plastic is not generally recommended for other types of planting or even around foundations as roots will grow close to the surface seeking water and oxygen. Shallow roots not only are deprived of oxygen and water but can be damaged by extreme temperature, and shallow-rooted plants can blow over easily in high winds too.