Ok, I admit it. I’m an organic mulch addict. But I don’t have any plans to change. And why should I? In Haiku, where the climate and rainfall create the potential for really great gardening, the biggest obstacle to luxurious plant growth is the lack of organic “humus” in our soil (i.e. too much red clay). To resolve this problem, I’ve been adding organic mulch to all my flowerbeds. And given the number of inquires regarding the “stuff that I’ve been putting on my garden” over the last few years, I thought it fitting to share what I’ve learned about organic mulch and how it has helped my garden to produce healthier plants, better fruit and more flowers.
Hawaii is blessed not only by great year-round weather, but also by year-round growth of all-things-green. The result is a copious amount of green waste much of which is processed at green waste facilities and turned into compost products which you can buy by the bag or truck load. Composting is the process of breaking down organic waste like grass clippings and tree branches. The final product of all composting activity is dirt or “humus” (which more closely resembles expensive “potting soil” than backyard “dirt”). So what is mulch? Organic mulch is basically any type of organic ground covering material (an example of inorganic mulch would be shredded tires). Organic mulch can range from large wood chips to grass clippings. But the best kind of mulch for your garden is the type that produces a fine grade of “humus”. In many areas of the country, significant quantities of this mulch can only be found for sale at the local home center. But in high-priced Hawaii, it’s free!
The type of free mulch we have on the windward side comes mostly from Hawaiian Earth Products in Kailua. It is available there and at Ho’omaluhia Botanical Gardens. From my many visits to Hawaiian Earth Products, I’ve noticed that most of the organic materials being composted are tree trimmings. Thus the local-kine mulch looks like shredded tree bark. At Hawaiian Earth Products they call this product “fresh mulch” or “fresh grindz”. Basically this material represents the end of the primary processing at that facility. The branches and leaves are shredded and then a natural process breaks down the organic material over several months. With more processing and the addition of other materials (like sand, fertilizer and manure), Hawaiian Earth Products creates “compost” which is sells. But as a service to the local community, it sets aside some material as “mulch”.
The benefits of adding mulch to your garden are numerous. From the US Department of Agriculture:
Mulching can be one of the most beneficial things you can do for your soil and your plants… Mulches are a labor saving device for the gardener. A layer of mulch will help prevent the germination of many weed seeds, reducing the need for cultivation or the use of herbicides. Mulches also help moderate the soil temperature and retain moisture during dry weather, reducing the need for watering. Mulches help improve the soil by adding organic matter as they decompose. They also encourage the growth of worms and other beneficial soil organisms that can help improve soil structure and the availability of nutrients for plants… Mulches also can be used to enhance the look of your garden. Many bark mulches provide uniformly rich brown color that contrasts with the plants. The mulch helps keep plants clean by reducing the splash of soil onto leaves during rainstorms, and helps infiltration of the rainfall into the garden.
And an excerpt from an excellent article from UH Manoa’s College of Tropical Agriculture:
Most plant roots grow in the top 3 feet of the soil, and 80 percent or more of their fine, nutrient-absorbing roots are within the top 6 inches of the soil. Mulch protects these roots from drying and temperature extremes, and it enhances the soil conditions that improve plant growth and health. Mulch helps reduce competition from grass and weeds around the base of landscape plants, resulting in improved growth, especially during establishment of new plantings. Grasses and weeds compete for water and nutrients, and some release chemicals that injure other plants. Research indicates that allowing turf to grow over tree root zones reduces tree growth… Mulches improve the quality of the soil in the root zone by improving soil structure, especially by increasing porosity… Mulch acts as a vapor barrier to prevent rapid evaporation of water from the soil… Mulch also contains plant nutrients and provides food for beneficial soil microorganisms.
I’d like to share a couple examples of the transformation I’ve seen in my garden from the use of mulch. I was digging holes to plant fruit trees in various locations around the yard. Some areas had already been cleared and mulched several years ago, while other areas were still hip deep in weeds. After digging the holes in the weedy area, I noticed that the soil color was brownish to a couple inches of depth, but below that the soil was solid red clay. But in the area which I had previously covered the ground with mulch, the top 3 inches of soil was dark brown continuing to a lighter brown for over a foot in depth, below which was red clay. The difference was quite dramatic. The mulched area had much better soil. I was easier to dig, and had a much higher organic content than the non-mulched area. Today the size difference in the trees is also apparent: the trees in the previously mulched area are larger with more vibrant foliage.
In another example, a night blooming jasmine (Cestrum nocturnum) that I had planted in a non-mulched area grew slowly with sparse leaves. I propagated several cuttings from that bush and put them in a previously heavily mulched area. The new bushes are now densely covered with leaves and produce 20 times the number of blooms as the original bush. While neither of these examples is very scientific, the results are hard to ignore.
One secret to mulch’s success seems to be the dramatic rise in the worm population. Several years ago, I put some black plastic sheeting on a “weed garden” and then put several cart-loads of mulch on top to keep the plastic in place. Many months later, when I went to remove the plastic, the mulch has mostly been transformed into humus, and in the mulch were HUNDREDS of fat healthy worms (not the small sluggish worms found in clay soil). Apparently the worms view mulch the way we view the Taste of Honolulu. The population explosion of worms resulting from the use of mulch has numerous benefits including aeration, fertilization and general “organic mixing”. In the above fruit tree example, I believe the worms were one of the main reasons why the “brown zone” extended deeper into the ground.
So with all the benefits of free mulch, what are the detractors? Although the mulch is free, you need to transport it to your yard. Once spread around, you will see an immediate improvement in the look of your garden (as mulch makes a nice background) but the benefits to your plants and trees will take several months. And by that time, most of the mulch will be gone (turned into humus by worms and other organisms). The result is you will need more mulch. But given the benefits, the twice-yearly addition of organic mulch is certainly worth consideration.
One way to wean your garden off the biannual addition of mulch is to plant ground cover plants. There are many species which grow well here from mondo grass (Ophiopogon japonicus etc.) to oyster plant (Tradescantia spathacea etc.). Once these ground covers take over, the weeds will have a difficult time competing. Another way is to use a “decorative mulch” which is typically larger wood chips which take longer to breakdown. There are also inorganic forms of mulch like shredded tires and gravel. But I would recommend topping off with the organic mulch as needed until the plants and bushes in your garden become so dense that there is no space for weeds to get a foothold.
Having a garden which is literally exploding with beautiful tropicals is the final payoff; and given the availability and the price of mulch, it’s a hard not to be an organic mulch addict!
More articles on mulch and composting can be found at:
http://www.ciwmb.ca.gov/Organics/CompostMulch/CompostIs.htm
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