There are a lot of mulch options out there.
Why Use Mulch?
Mulch is a regulator. It regulates moisture, temperature and weeds. Most plants like consistency in soil moisture levels and soil temperature and they don’t like competition from other plants. Mulch helps control all these things. Applying mulch will improve the growth and productivity of your plants and protect them from temperature extremes.
What is a good mulch?
A good mulch should allow water to pass through from above but minimize evaporation from the soil. It should be biodegradable and heavy enough to stay in place in moderate winds, and, if used on a hillside, it should stay in place. Most people want it to look nice too.
Let’s look at some materials commonly used as mulch and examine the pros and cons of each.
Plastic sheeting controls evaporation and suppresses weeds for a while but it doesn’t allow water to permeate and isn’t biodegradable. There is plastic mulch around my house put down by the previous owner that is just as tough as the day it was applied (been in the house 28 years). It is very difficult to remove and once it has been down a few years it accumulates enough dirt from runoff and airborne dust to support weed growth whose roots will penetrate the plastic. When you go to plant something else you will have to remove it or cut an opening. This stuff is not recommended.
Many of the same issues as plastic although it does allow water through. It’s main use is to suppress weeds and can be hidden by another mulch. But weeds still grow through it after a few years and they are very difficult to remove when they do. In addition, it keeps mulch from “sticking” so is problematic for use on slopes. Not recommended.
Rocks, Stones, Gravel, Sand
They allow water through and regulate moisture loss and temperatures but tend to heat up hot days and radiate the heat back to the plant. They don’t suppress weeds well (recommend using Preen or other pre-emergent to prevent weeds from taking hold). Rocks sometimes get buried in the soil and if not put down generously, and take effort to move away or remove. Rocks do have a place in xeriscapes and desert environments or next to structures. They are an outstanding choice for fire mitigation around your home. You can read a comparison between rock and biodegradable mulch here. Recommended with reservations.
Cardboard & Newspaper
Biodegradable and suppresses weeds well until it breaks down, which it does in two to three years. Allows water through and reduces evaporation fairly well. Usually free if you use flattened cardboard boxes or old newspapers. Is best covered with a thick layer of another mulch as it isn’t pretty and tends to dry out and blow away if left exposed. Recommended in place of plastic or fabric.
Editor's note: Recently, cardboard has become controversial when used as a mulch. Here are two recent articles that address the science of cardboard used as mulch. LA Times What's the fastest way to kill your lawn? and The Garden Professors™ Cardboard Controversy.
Perform well as a mulch but wet clippings tend to form a moisture impermeable mat so it’s best to wait for them to dry before applying. It’s free but there is also the possibility of introducing weed seeds. Recommended, but prefer compost to clippings.
Whether store bought or homemade, compost is a great mulch with the added benefit of adding organic material and nutrients to the soil as it breaks down. Homemade compost can introduce weed seeds if it doesn’t reach a high enough temperature to kill them. Free if you make your own. Not a fan of the plastic bags with store bought, but usually high quality. Recommended.
Straw & Hay
Straw is a good mulch but it tends to introduce weed seeds - especially wheat seeds. Run it through a leaf shredder which makes it easier to spread and improves evaporation control by being denser. Adds organics and nutrients as it breaks down. Best used in the garden if you don’t want the bright golden color in your yard. Recommended straw mulch. Note: Hay, even spoiled hay, has too many weed seeds to make a good mulch. Don't recommend hay for mulch.
Includes wood chips, shredded bark, wood shavings, and sawdust. Each makes a good mulch and lasts a long time but be aware that wood tends to leach nitrogen out of the soil as it breaks down (which it does very slowly). Wood mulch that has not been dyed is preferred. Sawdust tends to form a mat much like wet grass clippings because it is so fine. These products are generally flammable so keep them away from structures. Recommended, but be aware that they are not fire mitigation safe.
Also called leaf mold, these are fallen leaves from deciduous trees. They make a great mulch and are free but best shredded with a leaf shredder or a mulching lawn mower as the large leaves tend to form an impermeable mat when wet. They also tend to blow away if not contained, and are flammable.
Pine needles make a very good mulch especially if they are run through a leaf shredder. They stay in place, last a very long time, keep weeds down, and blend in nicely with the landscape. They can also help reduce the alkalinity of soil as they are somewhat acidic. You can purchase pine straw in bales, but chances are you can get locally bagged pine needles from neighbors in our area. One caution for pine needles is that they are highly flammable so don’t apply them close to structures. Recommended.
Final Notes About Mulches
- Most mulches are most effective when applied thickly - to a depth of 2-6”. No mulch lasts forever (except rock mulch), so plan to refresh it over time.
- Mulch applied right up to the trunk of a tree (called a "mulch volcano") causes problems over time (stressed roots + rotting trunks = unhealthy trees = increased susceptibility to (otherwise minor) insect and disease problems and structural instability = more money to treat + increased probability of tree death). Here's a helpful article about mulch volcanoes.
- Many experts advise mulching perennials and shrubs in the fall to prevent frost heave over winter. Wood chips, compost, straw, shredded leaves and pine needles are good for this but should be contained with a collar or cage to keep it from blowing away.
- Flammable mulches can be treated with a boric acid solution to reduce their flammability.Recipes can be found on the internet. Here’s one we've found useful.
- You can purchase many of these products from big box stores, but unless you only need a couple of bags, do yourself and the environment a favor and order in bulk. It is less expensive and keeps the single use plastic bags out of the landfill.
Thanks to guest writer Dennis Volz for this article.