As the gardening season moves toward fall and its memory is still sharp, make plans to improve the landscape or ease its maintenance for next year. Mulch may be worth considering if it was not used this past season. Many urban gardeners do not have a readily available source of mulch and must purchase it. The cost of mulch is usually not excessive, but does add substantially to the cost of the landscape and garden.
Mulch provides several benefits which are important at all times of the year, but particularly during the heat of summer. It helps to keep the soil surface cool and also helps to retain moisture in the soil. In areas where heavy clay soils are common, it prevents crusting of the surface during hard rains of summer thunderstorms. It prevents the germination of many weed seeds because of the added depth above them. While it also suppresses perennial weeds, some of them, if they already exist, may push through the mulch. Trees seeds, such as acorns, are also able to germinate and push up through a mulch. Still, it greatly decreases the need for weeding. As mulch gradually decomposes where it contacts the soil surface, it adds organic matter to the soil which slowly improves the soil over years of use.
The normal depth of mulch application should be between 2 to 4 inches. More likely, two inches would be used among annuals or perennials in a bed while four inches would be used under trees or shrubs or in covering areas which are not planted heavily. To prevent the emergence of perennial weeds and grasses through a mulch, they should be dug out or killed with a herbicide before the mulch is applied.
When a mulch is established, it may not be necessary to add new mulch every year. However, it will often improve the appearance to place some fresh mulch over the surface of existing mulch. When this is done, it is not necessary to add this top dressing of mulch at the same thickness as the base. If too much mulch is added, the depth of the bed may gradually rise too high for existing plants. Basically, the 2 to 4 inch depth should be maintained, but not increased. A new depth equal to the amount that decomposed during the past season should be added.
Mulches will need nitrogen as the organisms break it down. In preparing to add a mulch, add a high nitrogen containing fertilizer on the soil surface. After the mulch is established, high nitrogen fertilizers may need to be added during the season. There is no general recommendation for this, but gear sidedressings to plant growth. Lack of nitrogen will usually mean lighter green leaf color, weak or slow growth, and perhaps yellowing and loss of lower leaves. Nitrogen problems are likely to be greatest in future years if new plants are set into the mulched area. Mulch may drop from the surface into the planting holes which provides more organic material for decomposition and nitrogen tie-up in the root zone as well as on the surface.
Sour mulch has sometimes been a problem in recent years as gardeners use more and more mulch. Basically, sour mulch is one that has been composted improperly. As a result, when this mulch is used, symptoms such as leaf scorch, defoliation or death may result. This may happen very quickly and is very difficult to diagnose since it mimics many other common plant problems and leaves no residue or other indication. If damage occurs within 24 hours of mulch application and no other reasons for similar damage exist, remove or check the mulch.
While sour mulch is not common, when using mulch, smell it. Good mulch generally smells like fresh cut wood or good garden soil after it has been composted. Sour mulch will not have a good smell and may smell of ammonia, sulfur, vinegar or perhaps silage. If the smell does not give you an indication, check the acidity of the mulch. Sour mulch may have a pH of 1.8 to 3.5. Good mulch will normally have a pH slightly above or below the neutral point of pH 7.0.
The benefits of good mulch greatly outweigh the concern of getting sour mulch. However, be aware of the problem and check mulches before applying. Toxic mulches develop in the absence of oxygen during decomposition. This material can be properly composted by turning and inoculating it to prepare it for later use when it becomes a sweet mulch again.