Primetime Newspapers

By Calvin Finch, PhD, SAWS Conservation Director, and Horticulturist

Week of July 5, 2004

PROBLEMS WITH MULCH AND ORGANIC MATERIAL

            Mulch is an essential part of gardening in South Texas and organic material added to the soil improves the performance of all plants, but there can be some problems with mulch and organic material during wet periods.

            Tree bark and plant stems evolved to protect the plant innards from drying out. As good as they are at protecting plants from drying, they are susceptible to wet environments. Situations where plants have soil or mulch mounded against the stems and trunks often result in rot and girdling of the stem. This problem is especially prevalent in wet periods like we have had this spring. Move mulch away from the stems. Leave an air space of a few inches between the mulch and the stems.

            In some severe situations, even mulch over the bed surface can be a problem. If you have a soggy flowerbed, rake the mulch into piles until the weather changes to our usual hot dry summer pattern. Iris and other plants with shallow roots or bulbs are especially vulnerable to rot if the soil does not dry out between rains and/or irrigation applications.

            Many gardeners have learned this spring why I and many other horticulturists recommend that compost and other organic material not be added to the planting hole in clay soils. It is always desirable to enrich the soil with 2 to 4 inches of compost over the entire planting area, but that is not the same as filling a planting hole with compost. One strategy improves the drainage and soil texture of the planting area and the other kills newly planted shrubs and trees.

The problem occurs because the water from long spells of rain or excessive irrigation gets into the planting hole easily but does not drain quickly. The planting hole stays soggy resulting in the roots rotting in the hole. Ceniza, hollies, Texas red oak, Texas mountain laurel, and other drought tolerant plants seem to be especially vulnerable. New plants are most likely to be affected because they have a limited root system and it’s all confined to the soggy planting hole.

Sometimes the affected plant would recover but, because root rot symptoms mimic drought symptoms, we are inclined to water the plant more. If your newly planted tree, shrub, or perennial shows a leaf drop, yellowing leaves or burnt leaves, the first task should be to test the planting hole. If it is dry, add water; but, if it is soggy, do not add more water.

There are some tactics that help solve the soggy planting hole problem, if you catch it fast enough. If the native soil is well drained just let it dry naturally. In clay soil you can dig a trench on the downhill side on the planting hole. The excess moisture will drain away from the hole. Leave the trench long enough for the hole to drain and for new roots to develop. If it is inappropriate for the trench to be left open, fill it with a course material like bark or rock.

            Another tactic that often works is to dig the plant out of the hole and place it back in the container until the roots repair and the rains quit. The process of root repair may take all summer. Replant in the autumn.

It is not a good idea to automatically water plants when they show wilting or leaf drop, especially in wet periods. It is equally inappropriate to add fertilizer. Plants wilted by either drought or root rot cannot utilize nutrients efficiently and the salty nature of the fertilizers contribute to root damage. As a general rule, fertilizers should only be applied to healthy growing plants.

Compost is decomposed organic material such as leaves, brush cuttings, and sawdust.

The raw material makes a good mulch (insulating layer on the soil surface layer over the roots), and it will decompose with desirable results in the long term. In the short term, however, the raw organic material may cause a nitrogen deficit in your garden. The plants do not grow and are yellow. The decomposition process requires nitrogen to begin and proceed. This phenomenon is experienced when large amounts of leaves are tilled into the soil. It also occurs when you buy soil mixes for your raised bed gardens. They are often one-third sand, one-third top soil, and one-third commercial compost. The problem is that commercial compost includes large amounts of sawdust or shredded brush.  It is easy to hoe and rake but is not fertile. If you use a commercial compost, soil mix, or you till in dry organic material, always prepare the soil with two extra cups of slow release lawn fertilizer spread over each 100 sq. ft. of bed surface.