By Leah Smith
Mention mulch and the first thoughts that spring to mind are probably moisture retention and weed suppression. And yes, mulch serves both of those functions well. But the benefits of using mulch go beyond those to other areas of importance. If you thought it a simple matter of spreading whatever you like on the ground whenever you like around whatever you like, think again.
What To Use
Mulch can be almost anything that slows the loss of moisture from the soil and suppresses weeds. It is often true that when you are in need of mulch you are likely to use whatever the best option is you have at hand; and this is fine as long as you are able to judge what the best option is.
Popular natural mulches include compost (including composted manure), hay, straw, shrub and tree leaves and grass clippings. These feed the soil to varying extents, with the compost, hay and grass clippings doing it best. There are procedures to keep in mind with most mulches. For example, leaves must be chopped or mixed with other materials so that they don’t mat down and prohibit aeration and water infiltration to the soil. Also, some leaves contain compounds that can inhibit plant growth. So either conduct your own trials or (preferably) age leaves for at least six months before spreading them. Grass clipping layers should be kept to about two inches deep at most so that they do not become putrid. Alternatively, clippings can be allowed to dry a day or two so that they are no longer overly damp and not as prone to become fetid.
Pine needles, sawdust, wood chips, chopped bark and rock are additional natural mulch options. Pine needles add acidity to the soil and should only be used when this won’t create a pH problem but rather is desired (or at least tolerated) by plants that enjoy a lower pH. Blueberries and hydrangeas are examples of that, though many people use them on strawberry beds with great success as well. Pine needles can also inhibit seed germination due to the terpenes they contain. Wood products should be aged at least a year prior to use to prevent the depletion of nitrogen in the soil. And rock as a complete mulch is naturally used in permanent or semi-permanent situations (such as fruit trees and bushes), unless it is a few larger rocks used in conjunction with another mulch (more below).
And there are further and creative natural mulch alternatives. For example, you can use the ferny (reproductive) growth of asparagus as the bed’s own mulch, bending it down and leaving it in place at the end of summer. Plants like comfrey and aquatic plants in general are especially nutrient rich. They will give you mulch that is a valuable nutrient source. What is more, pulled weeds that lack seed set have the potential to be used as mulch. Some people will have access to other plentiful and unique mulch sources — buckwheat, cottonseed, peanut hulls, peat moss, seaweed or even hops or coffee grounds. Use what you have access to.
Processed mulch alternatives include cardboard, newspaper and plastics. Make sure your cardboard is free of toxic or non-decomposable materials (e.g., glues and ink). The colored ink now used in newspapers tends to be water-based and non-toxic, making it a viable option. Black and clear plastic mulch, though it has multiple environmental minuses, are viewed by many with admiration because their coverage is so total and they are frequently used to warm the soil and keep it warm, an aid to season extension. Overheating of the soil should be avoided (a greater issue with clear plastic).
Reflective mulches are gaining popularity. Said to increase plant photosynthesis and productivity because they increase sunlight availability, they can especially aid those growing gardens in the partial shade. Also, they are known to disorient pests like the Mexican bean beetles, leaf hoppers, whiteflies and aphids. Though commercially made (from products like silver polyethylene), economic versions can be created with aluminum foil or cardboard painted white. Because they also increase air temperature and their reflectiveness is diminished as plants grow larger through the season, they are a means of getting a head start during the growing season; additionally, peak temperatures during the season could lead to overheating and plant burning, a reason to remove it prior to the summer heat. Their reflective ability is diminished by dirt and grime as well, often leading to their disposal after each season.
Why to Use It
As previously stated, weed suppression and moisture retention may be the principle reasons for mulching crops, but they are by no means the only benefits.
Aid Germination: Certain seeds are known for having especially temperamental germination. In many cases, keeping seeds and soil moist is of help; and, naturally, this is also the case for almost any seed you may be trying to germinate during a drought. A light sprinkling of mulch on top of a recently watered row will often do the trick. Carrots, peas, spinach and many flower seeds can be helped in this way.
Crow Confounder: Mulch, especially bright green grass clippings, can be used to camouflage young shoots such as those of corn when they are emerging from the ground and are especially tempting to birds like crows. But really, mulch can help to hide many a plant from many a curious garden trespasser.
Control Your Temperature: The application of mulch can be used as a method of season extension, so to speak, due to the ability it has to create a temperature microclimate. Though air temperature changes rapidly, soil temperature does not, so using a mulch to blanket the soil will insulate it well. This can be done to extend the season of both cool and warm season crops. On our farm, we use mulch at the beginning of the season around arugula, spinach and turnip plantings, applying it before the ground begins to heat up for summer so that it can “keep its cool.” Rocks increase the thermal mass of garden beds. Though seldom welcome willy nilly, strategic placement of rocks can hold onto heat in the garden where you want it to keep your heat-loving crops cozy for as long as possible.
Keep it Clean: Using mulch to grow a clean crop is nothing new. For strawberries, it would be quite impossible to grow a fruit with a decent shelf life without the use of a mulch, as those delicate fruits and dirt really don’t mix well. We also use mulch to keep our husk cherries clean and easier to handle. But the cleanliness of mulch doesn’t end with fruits. Basil is always a very popular herb. Clean leaves of any sort have better storability than dirty ones and so a dip in cold water is always good. But basil leaves, unless they dry just right, can develop brown/bronze spotting where moisture clings, leaving you with an unattractive product. Deciding one summer it would be better to grow clean basil that didn’t need to be washed at all, we turned to mulch. A nice hay mulch, stretching out eight inches on either side of the basil row, protects the plants from rain splattering dirt onto the leaves and gives you just that.
Harvest Helper: Spinach is another crop where mulch can be used to produce a clean leaf. Savoyed spinach leaves, in particular, hold on to soil they pick up by growing so close to the ground. Mulching prevents this, but it also does something more. It causes the spinach to grow in a more upright orientation as it reaches for the sun. This orientation makes harvesting clean spinach leaves quite easy.
Good SPF: Bulbing onions, as they are putting on size and nearing harvest time, are susceptible to sunburn. This is particularly discouraging when you are so close to harvest. An airy layer of mulch removes this threat by giving it protection from the sun. Onions are one of our top priorities when it comes to mulching. Weed suppression and moisture retention are particularly important to the plants, and they are sensitive to having their roots disturbed as well as having sensitive skin!
Shelter from the (Wind)Storm: In nature, and therefore the garden, everything is connected and everything has an impact; it can be easy to forget some of the perceived minor players when the major ones take center stage. Though moisture and temperature levels have a great impact on plant health, the common wind does as well. The air itself may be cold, but it is often that wind that is really driving it home. And though drought conditions are bad enough, a wind harassing the air layers around leaves’ stomata where gases are exchanged and moisture loss can take place will make it all the worse. Using mulches early (or late) in the year to protect them from a chilly wind or the sapping nature of a harsh, dry wind during a drought and you have mulch-as-windbreak.
Nab Nutrients: The extra soil protection provided by mulch slows the rate at which water enters the soil. This allows for better water absorption, and also prevents nutrients from being leached out of the root zone by excessive water flows and therefore better nutrient absorption as well.
Mulch for Your Microorganisms: We are hearing more and more about the different ways in which happy microorganisms will create happy plants. Many of the conditions that make microorganisms happy are aided by the presence of mulch. A steady soil temperature (during the heat of summer as well as the chill of winter), moist conditions, and raw materials with which they can work (“food” sources to utilize) can all be produced when using mulch and will encourage microorganisms to operate nearer to the surface of the soil and in your plants’ rhizospheres. And don’t forget, the maintenance of a steady population of beneficial soil bacteria and fungi means a steady defense against destructive nematodes. And then there are the earthworms. They’re not microorganisms, but definitely fans of mulch (though not straight sawdust mulch).
An Insect’s Worst Enemy: Mulch is frequently regarded as an insect pest deterrent. The laying of onion maggot, cabbage root maggot, bean beetle, and cucumber beetle eggs at plant bases is lessened and/or eliminated by the presence of mulch, and the likelihood of egg/larva survival in mulch is poor as well. Insects overwintering in the soil will also find less success if they emerge under a thick layer of mulch (as is the case for thrips).
A Slug’s Best Friend?: A word of caution here. As well as making plants and microorganisms happy, many of the forms of organic mulch also create environments that are very pleasing to slugs. Mulch avoidance isn’t the way to go. Just keep an eye open for them; if an outbreak occurs, opt for using a commercial product like Sluggo (a snail/slug bait that contains iron phosphate, a naturally occurring soil mineral), drowning them in beer bait traps, or calling upon nature’s slug collectors — a flock of ducks. Also, note that oak leaves are said to repel slugs (as well as cutworms and the larvae of June bugs), so use this to your advantage when possible.
The Sick Bed: The number of ways in which a mulch can help to combat diseases in the garden appear to be legion. Mulch is essential for certain crops that are susceptible to diseases which are spread by contact with the soil. Many fungal diseases are spread to plants when water splashes off the ground and up to them, and mulch will reduce or even eliminate this splashing of fungal spores. For example, mulching bean plants provides protection against bean rust. Bacterial diseases are impeded by mulches as well. Further, leaves (chopped, of course) and straw mulch show the ability to mitigate the effects of certain harmful soil fungi and nematodes by creating a chemical environment which either repels or kills these potential pests. In other words, it is not the physical barrier itself but rather the chemical changes created by the mulch that is key. Root rots of pea and bean plants have been shown to be reduced by wheat straw mulch is this way.
Soil Improvement: Many of the features of an improved soil that organic growers aim for can be achieved by using mulches — reduction of compaction, soil pore destruction, and nutrient leaching, for example. They also prevent the overly hot and dry environments so destructive to soil life. Further contributing to an improved soil is the addition of nutrients including nitrogen and hefty quantities of organic matter when using various plant matter and animal manure mulches. Additionally, you can think of these as slow-release fertilizers for throughout the growing season. We find this particularly beneficial for plants that are heavy feeders with a long season — crops like cucumbers, summer squash and winter squash.
And Back to Weed Suppression: Though the focus has been on the other benefits of mulch, weed suppression is virtually essential with certain crops. This is the case with plants that are very sensitive to having their roots disturbed by the pulling of weeds (both large and not so large) as they grow — pea and cucumber plants being very good examples of this. Without mulch, keeping them happy can be hard.
Things to Watch Out For
Though there are clearly many benefits to using mulch, there are a few things to remember. Very thick mulch can make it difficult for moisture to get through to the soil. “Too thick” varies with the type of mulch, but be aware of that possible pitfall and check the soil itself when in doubt. Remember when mulching heat-loving plants that the soil must be suitably warm before you mulch them. You may be trying to get such plants out early for a longer season, but you must not prevent the soil from heating up by mulching too early. As mentioned, mulch can potentially harbor slugs and there is the need to be mindful of the rules involved with specific mulches. Additionally, special attention needs to be paid to the weather during the winter season. Though roots and the crowns of perennials appreciate the protection of mulch in cold weather, mild winters can lead to dampness and potentially crown and root rot under that layer of mulch. Be vigilant in milder years and check.
Whether dealing with orchards and small fruits or garden annuals, the need for mulch is great indeed.
Leah Smith is a freelance writer and home and market gardener. She works on her family’s farm in mid-Michigan called Nodding Thistle (certified organic 1984-2009, principally by Organic Growers of Michigan). A graduate of Michigan State University, she can be reached at noddingthistle@gmail.com.