What’s the Best Mulch?

If done correctly, the best mulch is the plants themselves, followed by straw.

Using Plant Foliage as a Living Mulch

It’s possible to let plants’ foliage aid their roots by casting enough shade to keep the soil moisture that’s necessary for themselves and all of the microbial life that makes them thrive, while also protecting the soil from the compaction and erosion caused by the direct hits of rain. 

By planting in “perfectly” spaced blocks as opposed to rows (discussed more in this post and others on garden layout into beds and plant spacing in beds), the foliage of many garden plants—e.g. carrots, spinach, lettuce, squash, and bush beans—will relatively quickly shade the soil, making applied mulch unnecessary. For plants with less spreading or dense foliage, like corn, it’s often possible to let another crop shade their roots. Squash and cucumbers, for example, are perfect for vining between the rows of corn and shading the soil and will benefit from the arrangement in their own ways, as well. 

Plants’ differing foliage, even at different times of the season, then, causes varying needs for supplemental mulch similar to that caused by varying temperatures and rainfall. While some plants either benefit from it for most of the season or not at all, others need a more nuanced approach—often only a little while they’re young and not yet large enough to shade and protect their own soil.

Young broccoli seedlings, for example—a tiny fraction of their future size and surrounded by 18 inches of otherwise-bare soil—often benefit from a thick supplemental mulch that can be left long after they’re garden giants. Bush beans, in contrast, as mentioned earlier, need nothing. They require so much more heat in the soil to get going that mulch early is often detrimental, and, since they grow so quickly and spread their foliage so vigorously, late mulch is unnecessary. You’d scarcely be able to find your way between their thick bushy vines to espy any soil to mulch.

Carrots, spinach, and lettuce, on the other hand, only need a thin, crushed-up sprinkling of mulch—maybe only covering ½ to ⅓ of the surface area, just enough to help keep the surface moist and protected from direct hits of rain while they’re sprouting—that will be almost indiscernible by the time the plants are even half-grown and providing most of their own shade and soil protection.

What, then, is the best applied mulch when it is needed?

The Best Applied Mulch

The best applied mulch for vegetable garden plants is straw. Hands down. 

Hay typically contains too many weed seeds, adding them back to your garden just when you’re attempting to get them—and their seed banks in your garden soil—under control. Plus, its typically clumpier nature often makes it more difficult to spread and more difficult for small seedlings to find their way through. 

Leaves can be problematic for the same clumpy smothering reason: They end up forming a dense impenetrable matted layer. Even the big strong leaves of garlic, powerfully probing their way through even the thickest mulch to find the sun, have trouble getting through the densely-matted nature of leaf mulch. One might think this tendency would make leaves a great mulch. Isn’t that what we’re going for after all? Sort of. It depends. Leaves definitely don’t work over planted seeds. We’ve also learned from failed efforts that compromised or ruined hours of work or killed precious garden seedlings, however, to only use leaves on paths and around heavily mulched seedlings, like broccoli and cauliflower—and then only once they’re large and more established. Even on plants, like these big brassicas, that like dense mulch, if leaf mulch is put on too early, the leaves tend to blow around and get caught on the seedlings in ways that end up shading their tiny leaves at a time when they are in such desperate need of sun. Now, imagine spreading that over your freshly planted rows of carrot seeds. It would completely smother them. Leaves, therefore, unless they’re thoroughly chopped up, only tend to work well on areas in which one wants nothing else to grow.

Grass clippings are fine—actually great in many ways—but they will likely introduce weed seeds into your garden since yard weeds are prevalent in all seasons mowing occurs. Additionally, if grass clippings are free of weed seeds, they likely contain some herbicides you definitely don’t want on your garden plants. Plus, their own seeds will be abundant for much of the year and will become weeds in your garden. Furthermore, your lawn actually greatly benefits from leaving the clippings there. Studies done at the University of Connecticut Agricultural Experiment Station found that the nitrogen molecules in grass clippings become available to the lawn very quickly, starting to reappear in the grass plants within a week. They’re natural fertilizer for your lawn that are best left there.

What about wood chips and sawdust? 

Meh.

Any mulch you add that is dried vegetation (as opposed to green, like grass clippings) or pieces of wood is going to pull nitrogen from your soil to decompose. It’s just what microbes require for decomposition. But it also results in less nitrogen being available for your plants. Just because of their density compared to other mulches (especially their much greater carbon density), wood chips and sawdust pull a lot more nitrogen out of the soil without any added benefits. In other words, they take more of your soil’s nitrogen to break down all of their carbon-rich material without doing anything any better than straw. Plus, they don’t provide that great of a barrier for weed growth unless they’re applied very thickly, which then pulls even more nitrogen from your soil.

Straw, as the hollow stalks of previously harvested grain on the other hand, provides ease of spreading, little, if any, weed seeds, great soil coverage for protection from rain and sun, good breathability of the soil, and relatively little lost nitrogen for the amount of coverage provided. Being hollow, it just doesn’t need to pull as much nitrogen from the soil to break down and is easy to fluff up. Plus, the few seeds that do sprout are almost always the wheat or oats from which the straw is made, are usually so few in number that you’ll barely notice, and make similar great mulch if you have to weed a few out. Straw, additionally, has the benefit of being able to be easily crunched up and sprinkled lightly over even freshly planted seeds—even on those most delicate and tiny seedlings like carrots, spinach, and lettuce. If done lightly— covering only about ⅓ to ½ of the soil surface—the way it lays on the soil in a cross-hatch pattern with spaces in between still allows sprouting seedling a way to find their way through.

If I’m mulching heavily or regularly, I’ll usually give a good soaking of water mixed with fish emulsion to both give the young seedlings a much-enjoyed boost and to help counteract the nitrogen pulled out by the decaying straw. Fish emulsion contains immediately available nitrogen, meaning it’s used or it leaches away. The straw absorbs some of the excess, giving a storage bank for what isn’t absorbed by the plants and would otherwise be lost. The straw turns gray with light charcoal spots and stripes within a day or so, the seedlings perk up with the boost, and the excess nitrogen is held in the decay process of the straw, slowly releasing itself into the soil and plants—greatly benefiting both of them. Hose-end sprayers make this job a cinch, but mixing it up in five-gallon buckets and sprinkling it on with watering cans works almost as well.