Whole or shredded…leaves are good mulch

Flower & Garden Magazine, Oct-Nov, 1985 by Janette Blackwell

Autumn means falling leaves. Depending on your point of view, you might see them as a nuisance, or as a treasure. Perhaps they are both. Many gardeners dislike using whole leaves as mulch because they tend to mat and are slow to break down into soil.

Take a careful look at your garden situations, however, for in some of them these qualities would be an advantage. Do you want a long-lasting mulch for trees and shrubs? Leaves will serve. They will smother weeds in neglected corners. They will improve the ground in compacted shaded areas under trees where grass won't grow.

How can you keep leaves where you want them? It is easy. Scatter a little earth over them. Not much is needed. If you distribute the small amount evenly, so some falls on each leaf, a large pile will be held amazingly well. Those leaves around the edge of a pile or in the path of wind are especially important to anchor this way. Although a few may fly off in a gale, most are there to stay and will hold down all beneath them.

Whole leaves may take a year to break down. This slowness is an advantage. I pile at least a foot, preferably more, of loose dry leaves under shrubs and around small trees. Anchored with a little soil, they mat down quickly. The appearance is acceptable. The larger the leaves, the better they look over the long term, in my opinion.

Leaf mulch will rejuvenate depleted areas under tree shade where nothing, not even weeds, will grow. I had such a corner. A few shrubs competed with tree roots for nourishment and were losing the struggle. Five years ago I began mulching this area with whole leaves, adding about 15 inches of them each fall. Shrubs became healthier; the area looks good all year around. The accumulated leaf mold has improved the soil so much that soon I expect to put in some woodland plants there.

Leaf mulches also control weeds. Where existing weeds are tough and pushy (perennial types, maybe), the leaves may need help. Lay a thick layer of newspapers over the weeds, overlapping the strips. Top with a few inches of leaves, anchored with a sprinkling of soil. This arrangement allows moisture to seep through to shrub and tree roots, yet smothers most weeds. A few minutes spent laying such a mulch in fall or early spring can keep the area neat and weedfree all year, at no cost.

Leaves do tend to make the soil more acid. On the pH scale they register on the acid side. Some such as oak leaves are more acidic than others. Over a long period, a heavy leaf mulch could increase soil acidity, benefiting acid-loving shrubs such as azaleas and rhododendrons, and most woodland plants. On the other hand, plants preferring neutral to alkaline conditions, such as most vegetables, might be harmed unless lime is also added. An occasional soil test will tell you what to do.

Research at the Connecticut Agricultural Experiment Station has shown that leaves contain growth inhibiting substances. These compounds, gradually leached out as leaves decay, affect chiefly the germination of seeds, according to Dr. George Stephens, of the Dept. of Forestry and Horticulture. Established plants are less likely to be affected than emerging ones.

My own experience with using leaf mulches on perennial beds has shown them to be beneficial in both winter and summer. In spring, they can be a disadvantage, as they delay the warming of the soil. Whole leaves will mat down and hinder or cripple the emergence of spring growth, so I much prefer using chopped leaves for mulching perennials. Chopped leaves don't mat, they stay put, and they break down into leaf mold much faster than whole leaves.

You can chop your own leaves with a shredder, or by running a lawn mower over them a few times. Chopped leaves are excellent erosion preventers on slopes and they insulate soil to reduce alternate freezing and thawing in winter. To prevent diseases from overwintering under a leaf mulch, clean the garden in autumn after first frosts have blackened tender vegetation. Leave ground bare until hard frosts arrive; then apply two inches of chopped leaves around but not over your perennials.

In spring you may find that the mulch has pretty much become integrated into the garden soil; if not, remove it as tulips begin to bloom.