How to Mulch a Vegetable Garden

Learn the best methods for obtaining and using vegetable garden mulch, as well as the many benefits of mulch.


| June/July 1998



168-066-01

Compost is one of the best vegetable garden mulches ever.


PHOTOS: WALTER CHOROSZEWSKI

Mulch, as a verb, is the act of applying some covering to the soil, usually for the purpose of controlling weeds. As a noun, it is any material that will serve the purpose. If that sounds like a very broad range of materials, it is. I have used mulches that were living and dead and ranged from dust to oil. What vegetable garden mulch accomplished—with any luck—was: 

Controlling Weeds

I can't think of a mulch that doesn't do this if used properly. When I first heard of clear plastic being used as a mulch, I questioned how it would control weeds. After all, I used clear plastic to help plants that needed heat to grow. Clear plastic used as a mulch works to kill weeds by cooking them when the sun comes out. Some weeds may get started in cool cloudy weather, but one sunny day will shrivel them in a hurry. Unfortunately, I didn't learn this by using a clear plastic mulch. I learned it when, one sunny morning, I got to the garden a little late to open the plastic row covers over my eggplant and pepper seedlings. Baked seedlings were a sad lesson but one not soon forgotten. I paid for my laziness that year and didn't use plastic row covers for several years after that. Then I found slitted row covers which open up to release excess heat without my attention. 

Retaining Moisture

All of the mulches I'm going to discuss retain moisture with the arguable exception of a living mulch. A living mulch like clover will draw moisture from the sod for its own growth and some of the moisture drawn by the plants will be lost to the atmosphere through the leaves. On the other hand, a carpet of leaves on low-growing plants that catches morning dew and holds a cushion of air beneath it may snake up for the moisture it uses. By cushion of air I mean that low-growing plants, such as clover grown between rows of corn, will shelter the soil surface from drying breezes and from the baking sun. Whether or not the beneficial influence offsets the moisture used is something for scientists to investigate.

All mulches, except dust, inhibit rainfall or irrigation from above from reaching the soil. For this reason, it is best to mulch a soil that is already moist. Here in New England, the spring soil is usually moist. There has been a time or two when I held back on mulching and waited for rain. Once an impervious mulch like plastic is put down, there is no chance of additional moisture reaching that soil in any quantity. It amazes me when I check the condition of soil under a plastic mulch in mid or late summer and find it moist below the top half inch. Where are the plants getting sufficient water to fill out eggplant and tomatoes? The roots have probably extended beyond the plastic, but still, the soil under the plastic has received no additional moisture since the end of May.

Fertilizing

All organic mulches do this. Grass clippings provide nutrients more quickly than other organic mulches. Sawdust is at the other end of the scale, as it will require nitrogen from a source outside itself to break down into usable nutrients and humus. When we put sawdust in contact with the soil, the sod organisms recognize a big job ahead and they multiply rapidly. To multiply, they need nitrogen. Since it is not available in the sawdust, they use nitrogen that is already in the soil. If the nitrogen supply in the soil is insufficient, the sawdust is converted more slowly. Of greater importance, the soil is deficient in nitrogen for the plants and they will suffer.

The most important thing to keep in mind regarding soil fertility and mulch is the carbon-nitrogen ratio. The carbon-nitrogen ratio of sawdust is 400:1, for example, while young sweet clover is 12:1. An average bale of hay might be 80:1, while rotted manure might be 20:1. A substance that has a C:N ratio below 17:1 will actually add nitrogen to the soil while a ratio above 33:1 will take nitrogen from the soil. Between those two figures the result is neutral. Grass clippings might have a ratio of 16:1 but if the plant is allowed to grow to maturity, the ratio might go to 30:1 or 40:1. If the plant starts to dry, the ratio goes up even more. Once dried as hay the ratio will be in the vicinity of 80:1.





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