Gardening with Laurie: Marvelous Mulch

By Laurie Brown
Reader Columnist

Mulch is something most gardeners know about but may not know how to use. There are several reasons for using it; in winter, it insulates the ground and prevents frost heaving and protects plants from low temperatures. In summer, it smothers weeds and reduces water use by slowing evaporation. It also keeps plants clean — especially nice with spinach or lettuce — and prolongs the life of soaker hoses or drip lines by keeping the UV rays off. You can get a longer asparagus harvest by pulling the mulch off half the bed early, allowing that part to warm up and uncovering the rest a couple of weeks later. On the downside, critters like rolly-pollies, earwigs and slugs find straw or leaf mulches a nice home.

Image courtesy rockystone.com.

There are two classes of mulch: organic and inorganic. In this case, “organic” doesn’t refer to pesticides or lack of, but that the mulch has carbon in it – bark, leaves, straw, newspaper and cardboard. “Inorganic” includes gravel, lava rock and rubber mulches. Organic mulches will break down and eventually turn into soil; inorganic ones won’t (at least, not within our lifetime). 

There are a few mulch basics. Large-size mulch, like rocks and bark, should be avoided in areas you will be digging in frequently, like vegetable beds that are tilled every year. You will have to rake these mulches out before digging, then put them back when you’re done. That gets a big “nope” from me – I’m lazy. Don’t let mulch touch the stems/crowns of the plants; you want moisture held in the root zone, not against stems, where it can lead to rot. Don’t mulch over areas you have seeded, or the seeds may not germinate. Mulch will stop most annual weeds from germinating, but does nothing against perennial weeds like quack grass and thistles. Don’t put mulch on warm weather crops until the ground has warmed up — keeping the soil cool retards the growth and ripening of crops like tomatoes and squash. You’ll need about two to four inches of mulch to smother weeds (blocking all light from the seeds) and to keep soil moist. Don’t bury the plants! Especially don’t bury trees with mulch; it WILL kill them to have the trunks buried past their normal soil level. The various mulches all have pros and cons.

Rocks, like gravel or lava rock, sink into the ground over the years and is very painful to kneel in to work. It absorbs heat and might be a good choice around heat-loving plants. Rock lasts for years. “They” say you should put it over ground cloth so it doesn’t sink into the ground, but I hate that stuff. Unless it has very deep mulch over it, weed seeds that blow in germinate. They either live in the soil that works up through the ground cloth or sink their roots down through it and become impossible to remove without cutting the cloth. 

Wood chips/bark will decompose slowly and work down into the soil. It has a lot of nutrients in it, but it has to decompose for plants to use it. I like the look of it, but many don’t. 

Leaves are free in autumn. Whole leaves can catch air and blow away; they are easier to use if you run over them with a lawn mower before raking them into the beds. Do not use them if the plants they are from have disease. 

Straw is cheap, but is very inflammable so I wouldn’t use it against buildings. Great for vegetable gardens because it decomposes quickly. Do NOT use hay, as it has seed heads in it. As soon as it gets wet, a bale of hay can turn into a Chia Pet. 

Cardboard and newspapers need to be held down to keep them in place. It will decompose; modern inks use no lead and are safe in food areas. It can create a solid barrier, so leave room around the stems/crowns for water to get into the soil. It’s ugly, so most people cover it with leaves or straw. 

Grass clippings may have weed seeds, but it decomposes quickly and is a good source of nitrogen – so good that it can actually start heating up if piled too deep. DO NOT use if the lawn had Weed and Feed on it!