To Mulch or Not to Mulch – That is the Question
Fleurette Huneault

Why You Should Mulch

You can use mulch to lower watering needs. Mulch keeps water from evaporating and helps the soil stay moist even through a hot, dry summer. It also helps to suppress weeds by blocking the sunlight thereby preventing weeds from germinating and ‘greening up’. Mulch can help minimize soil temperature fluctuations especially when applied after the soil warms up, usually mid to late June in the Ottawa area. It also creates an insect barrier which helps break the life cycle of garden pests such as Colorado Potato Beetles and Cucumber Beetles. It keeps produce clean. For all these reasons, mulch is a great time saver. But best of all, mulch helps to add fibre to the soil, especially when it is allowed to break down over winter and is plowed under in the spring.

What Can Go Wrong

In a very wet year, it can cause excessive moisture and encourage mould. Some mulch, hay for example, can bring its own weeds to the garden. When used as an insect barrier, mulch can be so effective as to prevent even beneficial insects notably soil nesting pollinators from completing their life cycle. And although organic mulches do enrich the soil, they can, over time tip the soil acidity/alkalinity balance (test the soil ph from time to time).

Some mulches, such as straw and paper, may cause a slight nitrogen deficiency in the soil. You may need to add a little extra nitrogen fertilizer or compost to the garden to offset that which is used up by the micro organisms in the soil as they break down the materials. Newspaper can be used either shredded or in sheets, but be sure to weight down the paper if sheets are used with a mulch cover such as straw.

Which Mulch to Use

Compost can and should be used every year. In addition to the all the benefits attributable to mulch, compost is a mild, balanced fertilizer. It is the icing on the cake - an indispensable topping for any organic garden.

1. For Weed Control

Since most garden weeds are annual, their seeds are carried on the wind and they will always be with us. You will never eradicate annual weeds – many weed seeds remain dormant for up to 40 years and only germinate when they are brought close to the surface. However, any mulch that successfully blocks the light will be effective in controlling weeds.

My preference for weed control is clean straw. I apply three large bails around the third week of June concentrating the most build-up, as much as 6” (15cm) near the stems of plants especially potatoes, tomatoes, corn, and Squash family plants (melons, pumpkins, zucchini, cucumbers). It is 
very beneficial around shallow rooted plants where it is easy to harm plant roots by hoeing, e.g. corn and melons. As well, squash family plants are often difficult to weed because of their large, prickly foliage and surface covering vines. Also, when straw is applied around potatoes and leeks, it lessens the need for hilling.  Although straw can include grain seeds that may sprout over the season, grain plants are often used as green manure crops in vegetable gardens.  Moderate grain growth will not likely shade your plants or deprive them of moisture, and will add fibre to the soil in the spring when the gardens are tilled (even so, they are very easy to pull from the soil, if necessary).

Last year I also used a product called “Biofilm”, a 100% compostable and biodegradable black film made from cornstarch. It was very effective. It served all mulching purposes including keeping the produce clean. It can be left in the garden to biodegrade. I applied it early in the season before the soil warmed up and used it for peppers, lettuce and brassicas (cabbage, Brussels sprouts, bok choy, cauliflower and broccoli). Every plant in the bed did extremely well. It would not work well if it were walked on often because it is thin and tears easily. Newspaper can also be used as mulch to suppress weeds but inorganic ink and glossy pages should be avoided (almost all newspapers use organic inks, even for coloured pages). Newspaper is easier to set down when it is wet and up to ten pages are needed to form an effective light barrier. Covering it with straw is a good way to make sure it stays down. The drawback with newspaper is that it mats and can take several years to break down when too many layers are used. When excessive layers are used in annual gardens, it could cause tilling problems in the spring.  Newspaper is optimal for areas where the soil will not be disturbed for several years, e.g. when starting new perennial beds or to suppress persistent weeds along fences. It is also a good option for fallow soil when laid down under black plastic for several years.

Other organic mulch can be just as effective as straw and can be used if you have access to it, e.g. 100% organic cloth, human and animal hairs, dry pine needles (takes a very long time to break down) and deciduous tree leaves. However, there are downsides. If you have access to these organic mulches a good rule of thumb is that the more finely shredded they are, the more easily they will break down into compost on your soil. Organic clothing may cause tilling grief if it is not shredded into very small pieces. Whole leaves and un-composted pine needles will tend to mat, therefore shed water instead of letting it through to the plant roots that you are trying to moisten. There is also some concern about the allelopathic properties (toxins produced by some plants that suppress root growth in other plants) of some of these products e.g. pine needles and maple leaves. Other allelopathic plants are black walnut trees and sunflowers.

Plant residues, straw, leaves, grass clippings, wood chips, sawdust or hay must be free of pesticides and other contaminants.  Do not use glossy paper or non-organic coloured ink.  Plastic mulches may not be incorporated into the soil or left in a garden to decompose.  In annual gardens, it must be removed at the end of the growing season. Plastic mulches in perennial plots may be left for more than one season but must be removed before the plastic decomposes. Use of polyvinyl chloride as plastic mulch or row cover is prohibited.

2. For Soil Heat

I use clear plastic to warm up the soil for plants such as sweet potatoes, jicama, Malabar spinach, watermelon and okra. I usually apply the plastic two weeks before transplanting out. Clear plastic heats up the soil and keeps the heat in the soil. However, it transmits most of the incoming light and must be topped with straw or some other light blocking mulch to control weeds.

In our community gardens, plastic mulch may be used but it cannot be incorporated into the soil or left in the garden to decompose. Care must be taken not to use plastics that contain polyvinyl chloride as this chemical may leech into the soil and is prohibited. For a complete list of permitted/prohibited mulch substances see Section 4.4 of the Organic Production Systems Permitted Substances Lists published by the Canadian General Standard Board in September 2006. 

3. As An Insect Barrier

I have always used clean straw as an insect barrier. It is especially effective around
potatoes, eggplant, squash and cucumbers.  Last year I used the corn product mentioned above (i.e., Biofilm). This year I will see if there is a marked difference in the population of Cabbage Loopers and Imported cabbageworms.

Mulching walkways certainly reduces the amount of time spent weeding. On the other hand, we know that bees and other pollinators nest in the soil, so it is good to leave some open soil in your garden, although in a large community garden, there is bound to be plenty of uncovered soil.

I leave the walking paths free of mulch preferring to slice off the growing tip of annual weeds at soil level. This can be done quite easily and effortlessly with a shuffle hoe or as seen in catalogues a variety of designs and names such as Dutch hoe, Collinear Hoe or Loop Hoe. All of these hoes allow you to slice weeds off at soil level with very little effort. If this is done before the weeds go to seed, the cut plant can be left on the soil to return to the earth. In a perennial garden, where the pathways are in the same place year after year, deciduous leaves applied every fall works very well to keep the weeds down.

I used to say: “Mulch, mulch, mulch”. Now I say: “Consider the pros and cons. Use mulch where appropriate

The web site CAN/CGSB-32.311-2006 contains the entire document entitled Organic Production Systems Permitted Substances Lists published by the Canadian General Standards Board in September 2006. The relevant section is 4.4 Weed Management, Mulch.  Go to  http://www.tpsgc-pwgsc.gc.ca/ongc/on_the_net/organic/scopes-e.html