The Unusual Thing People In Hawaii Add To Their Gardens — And It's Brilliant

Hawaii is unique among the 50 states: It's a cluster of volcanically-formed islands, in the middle of the ocean, 2,500 miles from the California coast. As a result, there is much that is unique about gardening in Hawaii, from incredibly bountiful harvests to year-round bloom times. Another example: macadamia nut shells used in the garden. Although macadamia nuts are not native to the islands, they've been an emblematic agricultural crop since their introduction in the late 1800s. For a culture that deeply values sustainability, macadamia nuts offer more than just edible seeds and plant-based protein: Their shells are also utilized to help enrich the soil and retain water, and provide attractive ground cover everywhere from home gardens to resort landscaping.

Of course, anyone can use macadamia nuts as mulch, compost, or ground cover, but they're particularly popular (and most affordable) in regions that grow them, such as Australia, Southern California, and Hawaii. To that end, although it may seem like macadamia nuts are an unusual thing for people in Hawaii to add to their gardens, it is actually a brilliant way to source local, natural fertilizer and mulch. Though down from a production peak in the 1990s, Hawaii produces about 42 million pounds of macadamia nuts annually. Since the nuts are largely sold already shelled, the tough husks are shredded or chopped for use as compost or mulch, or sold whole for ground cover, rather than being thrown away.

There are several ways to use macadamia nut husks in your garden. One of the most common is to compost them for use as a natural fertilizer and soil enhancement. Macadamia nut shells are particularly helpful for adding calcium and potassium to the soil. Shells can also be tilled into the soil (or added as a layer in container gardens and potted plants), aerating it and improving drainage and structure.

Another benefit for gardeners, both in Hawaii and anywhere, is using macadamia husks as mulch. After all, one of the main purposes of mulching gardens is to help the soil retain water and macadamia husks do that extremely well. In fact, they may do it too well depending on the crops you are growing — a thin layer is usually best. So, macadamia nuts may not be the best type of mulch for your garden if you have plants with shallow roots or that are particularly susceptible to root rot. Also whole, unground shells are incredibly tough and take a long time to break down, much like hazelnut shells. However, this same characteristic makes them an ideal option as an attractive ground cover in place of bark.

Because they are naturally hard with sharp edges, macadamia nut shells may also serve as an effective barrier against crawling and slithering pests like slugs. This essentially works the same way as using gravel to keep snakes out of your garden. Unfortunately, the sharp husks and any remnant flesh from the nut can be a hidden danger for dogs. This is because the nuts, which are so healthy for humans, are toxic for dogs and the sharp shells can cause intestinal issues. Thus, if you have pets, it might be worth choosing a different material for mulch.