Best Mulch for a Vegetable Garden

For most vegetable beds, weed-free straw is the best all-around mulch, and it's the one extension services recommend most often. Shredded leaves, dried grass clippings from untreated lawns, and a thin layer of compost also work well. Black plastic is the better choice for heat-loving crops early in the season because it warms the soil, while organic mulches cool it. The two biggest mistakes are putting organic mulch down before the soil has warmed and bringing in straw, hay, or manure contaminated with persistent herbicide.

What mulch does in a vegetable bed

UMN Extension's page on mulching for soil and garden health lists the main benefits: less evaporation from the soil surface, more consistent moisture, fewer weeds, and a moderating effect on soil temperature. Mulch lowers peak soil temperatures in hot weather and slows heat loss when it's cool. It also keeps soil from splashing onto produce, and organic mulches add organic matter as they break down. UMN suggests 2 to 4 inches for most organic mulches, balancing weed suppression against soil aeration.

The options, compared

Depths for grass clippings, compost, and newspaper come from Colorado State's mulches for the vegetable garden; straw and leaf depths follow UMN's general range.

Straw

Colorado State calls weed-free straw excellent mulch for vegetables, and UMN describes it as widely recommended for keeping moisture in, weeds down, and produce clean. It's cheap, easy to spread, and easy to push aside at planting time. Both sources stress weed-free straw; bales full of seed trade one weeding job for another.

The real risk with straw is herbicide. See the section on herbicide carryover below before you buy.

Leaves and grass clippings

Leaves are free and effective, but Rutgers notes that they tend to mat down and break down slowly unless shredded first. Running a mower over them before spreading solves most of that.

Grass clippings are also free, but they need care. Colorado State advises applying fresh clippings in thin layers, up to ¼ inch at a time, and letting each layer dry before adding more. Rutgers notes they get moldy if piled wet. Use them only from lawns that haven't been treated with herbicides. Colorado State points out that some lawn herbicides carry over for years.

Compost as mulch

Compost and leaf mold make good mulch at 1 to 2 inches, and a layer on the surface feeds the soil as it goes. The catch is that compost carries phosphorus, and mulching with it every year adds up fast. If your soil test shows high phosphorus, use straw or leaves instead. The reasons are in can you use too much compost?

Wood chips

Colorado State and the University of Maine both recommend keeping wood chips out of annual vegetable beds and using them on paths and around perennials instead. The nitrogen question is more nuanced than its reputation, and it's covered in full in do wood chips steal nitrogen?

Plastic mulch

Plastic doesn't build soil, but it earns its place for warm-season crops in short or cool seasons. Utah State Extension reports that soil under clear plastic is 15 to 20 °F warmer than bare soil, and about 10 °F warmer than under black plastic. It recommends laying plastic 3 to 6 days before planting to get the warming benefit.

The effect on harvest can be large. Colorado State reports that black plastic along the Colorado Front Range moves production about two to three weeks earlier. The University of Alaska Fairbanks reports that zucchini harvest started 10 to 14 days earlier with yields two to five times higher when grown with plastic mulch and row covers. Alaska is an extreme climate, so treat that as an upper bound, not a typical result.

Clear plastic warms soil most, but weeds grow underneath it. Black plastic blocks weeds. Either way, plan to run drip line or soaker hose under it, since plastic sheds rain.

Timing: wait for warm soil

Organic mulch insulates, which is useful in July and a problem in April. Rutgers advises that natural mulches shouldn't be applied before the soil warms in spring. For tomatoes, peppers, squash, and melons, wait until the soil has warmed and the plants are established, then mulch. For cool-season crops like lettuce and peas, mulch earlier to keep soil cool as the weather heats up.

Rutgers also notes that organic mulches may need extra nitrogen early in the season to feed the microbes breaking them down.

Herbicide carryover in straw, hay, and manure

This is the most serious risk with organic mulch, and it's easy to miss. NC State Extension's publication on herbicide carryover covers three herbicides, aminopyralid, clopyralid, and picloram, used on pastures and hay fields. They pass through hay, straw, the manure of animals that eat it, and compost made from any of these.

• Persistence. They can break down in as few as 30 days, but field reports suggest complete breakdown can take several years. Hay has kept its herbicidal activity after three years in storage, and breakdown is especially slow in manure and compost piles.
• Sensitive crops. Tomatoes, beans, peas, peppers, potatoes, lettuce, carrots, and strawberries are all on NC State's list.
• Symptoms. Poor germination, twisted and cupped leaves, misshapen fruit, and reduced yields.

Ask your supplier what was sprayed on the field. If you can't get a clear answer, NC State describes a simple test: mix the material 1:1 with commercial potting mix, plant three pea or bean seeds per pot, and grow them for two to three weeks until they have at least three sets of true leaves. Twisted or cupped growth in the test pots means the material is contaminated.

Mulch is one piece of a larger approach to soil covered in building garden soil organically. If you want a living alternative for the off-season, see cover crops for the home garden.