If you love buying nuts with their shells, you likely have wondered if you could find a use for all those empty shells. Next time you have a large pile of nut or seed shells, don’t throw them out. Use them in your garden instead.
Nut and seed shells can be easily composted, although being woody means they will break down more slowly, taking 6 to 24 months. You don’t even need a compost pile to start composting nut and seed shells, as you can bury them a few inches into your garden beds using the trench composting method.
How Long Does it Take for Nut and Seed Shells to Break Down?
Nut and seed shells are completely biodegradable but will still take a while to fully decompose. Expect shells to take anywhere from 6 months up to 2 years to break down, less for thinner seed shells like sunflower shells and longer for thicker, harder shells like walnut shells. You can speed up this process by using hot composting [PDF] methods.
You do not need to crush your nut shells before adding them to your compost pile, although doing so will significantly lessen the time it takes to completely break them down. In contrast, composting shellfish shells takes many years to break down if you don’t crush or grind them.
How to Compost Shells of Nuts and Seeds
Treat nut and seed shells the same way you would treat other organic matter. However, one important step before you begin is to rinse off your nut and seed shells to wash away any salt. Salt buildup in your soil can harm both soil life and your plants.
You can also crush your nut and seed shells to increase the surface area so they can break down faster, but this is optional if you’re not in a rush to fully break them down.
Putting Nut and Seed Shells in Your Compost Pile
Like other woody plant materials, nut and seed shells are considered high carbon “brown” matter, as opposed to high nitrogen “green” matter like grass clippings or pulled weeds. In compost piles, it’s good to have a balance between brown and green matter, as both carbon and nitrogen are used by composting bacteria and fungi.
A good ratio is 2:1 or 3:1 green to brown material by volume, and layer them every few inches. For example, you can have 2 to 3 inches of dead leaves and nut shells, then cover them 4 to 6 inches of fresh grass clippings and green weeds, then add another brown layer, and so on.
This is the simplest way to get started with hot composting, and as you go down the rabbit hole, you’ll find that it can get quite complicated to dial in the exact ratios. It’s as much of an art as it is a science. Renowned market gardener Charles Dowding makes all his compost and has written about his approach.
If you are cold composting (i.e. just piling everything up together), you can layer your compost pile or not, but expect your shells to not break down as quickly.
Burying Nut and Seed Shells Directly in Your Garden
An alternative to having a compost pile is doing what your great grandparents likely did in their garden: trench composting.
Trench composting is just another name for burying food scraps and other organic matter a few inches directly into your garden bed. I’m not sure why this fell out of fashion but recently it’s becoming a trend again. This is now one of my favorite ways to compost food scraps.
Burying nut and seed shells is an effective way to compost them and you know that they will break down and release nutrients right your vegetables and flowers need them. Note that they will break down more slowly, just as with cold composting.
There are no rules for trench composting except to bury your shells so that there are at least 3-4 inches of soil above them. This is just to deter curious squirrels who might think they’ve just found another squirrel’s motherlode of nuts.
If you have a lot of food scraps, this could mean digging a trench or hole 8 to 12 inches, but if you are just burying a small amount, you can dig a 5 to 6 inch hole or trench. Spread your shells evenly in the trench and cover completely.
Other Uses of Nut and Seed Shells in the Garden
Using Nut and Seed Shells as a Mulch
Another underrated use of nut and seed shells in the garden is as mulch. If you have a large stack of nut shells, you can use them as a mulch in your garden beds or pots. Not only will they help keep your soil underneath protected from the sun so it stays moist longer, they will also break down and add nutrients back into your soil.
As little as 1 inch of mulch will protect your soil from drying out. For weed suppression, it’s recommended to use at least 3-4 inches of mulch.
Heavier nut shells from walnuts, chestnuts, hazelnuts, or almonds tend to perform better as a mulch, as lighter shells like those of peanuts or sunflower seeds can more easily blow away with a strong wind. It’s much better to just add lighter shells to your compost pile or bury them in the garden.
You do not need to crush nut shells for mulch; you can leave them as large pieces.
Nut and Seed Shells Help Aerate Your Soil
A lesser-known benefit of nut and seed shells is that they add bulk to your soil and can help keep it aerated by reducing soil compaction. In this way, they act similarly to perlite or vermiculite, which are often added to potting mix to
But unlike using perlite or vermiculite, which are volcanic minerals, nut and seed shells will slowly break down, adding nutrients to your soil while still improving soil structure.