Though mulching is usually prescribed in cultural publications for rhododendrons and kindred plants, little choice is left to the gardener as to just what constitutes suitable material. Often printed, and repeated through force of habit alone only such material as peat moss, oak leaves or pine needles are recommended. The above group has proven entirely satisfactory under almost all conditions, but why not use many of the other materials usually more available and readily at hand. The average gardener can easily obtain grass clippings, rotted wood, sawdust, straw, and usually huge mounds of leaves from deciduous trees as magnolias, chestnut, walnut and maple. The principle difference in these materials is the degree to which they tie up available nitrates during their decomposition. Aside from the fact that some of the decaying materials as leaves change their acid reaction but mildly, the effect is not permanent and is hardly to be considered here. Materials that decompose rapidly enough that the soil organisms demand all the available nitrogen during their decomposition can literally starve a plant. Oregon State College according to N. Roberts, assistant horticulturalist has carried on an extensive program with various mulch materials. Rhododendrons have not been used as test plants because of their slow reaction, but results with other species might indicate the relative values of some of these materials.
The materials now under test include the following:
Chopped fir bark
Cedar tow, chopped fir bark and redwood bark decompose so slowly that there is little if any depression of available nitrates for the plant. Chopped fir bark adds considerable acidity to the soil much as peat moss. If the soil already has abundant nitrogen or by the addition of nitrates many materials can be used for mulching that are usually overlooked. The importance of mulching hinges not so much on the availability of plant food, but on moisture conservation during the drought of summer. If the mulching material does not pack tight, but will remain in an aerated condition all of the above materials may be used. Even lawn clippings if used sparingly with some other coarse substance may be considered if it is not piled higher than four inches. It must be remembered that the question of which material also depends on the acidity of the soil. Soil with a low pH, i.e. acid, could receive a plentiful application of mulching material that at some time during its decomposition actually reacted alkaline. Such a condition of neutral or nearly neutral soil would harm the plant, as was mentioned earlier in this article. Because rhododendrons and kindred plants react slowly, and most rapidly decaying material in decomposing changes its own pH from its original state to alkaline and then to neutral or acid, the temporary effect on the plant would in most cases be less harmful than no mulch at all.