Mulch: The Great Saver of Water and Fertilizer

There are a lot of misconceptions about mulch and even more that is misunderstood about it. In almost every landscape and horticultural case I can think of, mulch is better than no mulch. If you are designing a Japanese Garden, organic mulch may not be the in the scope of the design, but stone or gravel mulch may very well be in the design. Mulch has a lot of advantages, including stabilizing  soil moisture, erosion and soil temperature. Decomposing mulch adds organic acids to the soil beneath it, slows weed incursions, retards nematode development, and stabilizes fertilizer delivery to the root system. Over time, decomposing mulch can create a very workable organic soil, especially important in very poor mineral soil areas or in very rocky areas. In South Florida, we have a combination of extremely rocky areas and extremely sandy areas. In both cases, mulch can help a lot in making the plant root zones a better habitat for plants to live in. 

Cypress mulch

Eucalyptus / Australian Pine mulch

'Volcano'  mulching
a serious threat to tree and palm trunks

Mulch can come in many forms, the most common being tree bark of one size or another. In many states, cypress is the most common, whereas eucalyptus is the most common type in the western states. One of the biggest misconceptions is that mulching should be done every year or two. Organic mulches do decompose, sometimes rather rapidly, necessitating re-application every 6-12 months. Many discount-price mulches are made of shipping pallets or chipped wood from assorted tree species, both of which decompose quickly to leave behind a gummy residue. At Pinecrest Gardens, we use chipped Australian Pine Tree mulch ( Casuarina species), and the results have been very good. The mulch is cheaper than Cypress mulch, lasts twice as long, and does a great job of preventing weed growth . We buy it in bulk lots from a vendor who specializes in bulk mulch. We also have serious ethical concerns about using Cypress trees as mulch since they grow so slowly and are not cultivated as a timber crop. Australian Pine is a weed species and we find it very "green" to use a weed tree species as a mulch. Other organic mulch types include grass clippings, hay, pine needles and even old newspapers. These all decompose, adding organic material to the soil, but less than bark or wood products do. They should be used as one component of a soil-enriching program. They are all useful, but pay more attention to them than bark or wood mulches.      

There are inorganic mulches as well, including crushed tires, crushed glass, and gravel. These are decorative and should be more properly referred to as decorative additions. None of these hold water in the soil, but they do help with weed control. Crushed glass is usually sold in a semi-polished chunk form that is safe to handle. In some applications it can look like flowing water in a pondless water feature, or can make a xeriphyte garden look more colorful. Crushed rubber tires are often used in children's playground areas to great effect, and are very good at repelling weed growth. The only downside is that this product gets rather hot in direct sunlight. This heat can translate to the rootzone, to a deleterious effect in summer, but a good effect in winter. Gravel is also effective in repelling weeds, and is often seen in Japanese garden designs, as well as cactus / succulent gardens.   

recycled glass mulch

recycled tire mulch
courtesy of NuPlay
One of the larger problems I see is called "volcano mulching", where mulch is piled up against a tree or palm trunk. This is almost always damaging to the trunk, and can be fatal. Mulch should be pulled away from the trunk by several inches. Some trees and palms can tolerate mulch against the trunks, but it is rare. People often ask how deep mulch should be applied to the rootzone, to which I usually suggest a depth of 8-10 inches, starting several inches away from the trunk, and continuing out to the edge of the tree or palm canopy. This usually brings about some level of surprise from a customer, who often believed that mulch only need be applied an inch or two deep. One good method for determining how much mulch is required is to place an unopened bag of mulch on the ground. The bag shows how much mulch will be needed to cover the "footprint" on the ground. In other words, a bag of mulch does not cover much real estate, and once people understand this, they know the true quantity of mulch a garden requires. In our garden areas at Pinecrest Gardens, we use over 600 cubic yards of mulch per year, and could use double that amount in the future.

In most gardens, mulching is a good idea and should be pursued with the same importance as the selection of the plant, irrigation, and fertilizing. Choose a method that complements your garden, your budget, and one which produces an improved soil quality. Your plants will benefit, and you'll have fewer weeds to pull.

Craig Morell
Pinecrest Gardens