Why Can’t You Compost Walnuts?

Why Can't You Compost Walnuts

I’ve always loved how, when raking up the plant litter scattered across your yard, you come across some interesting things. When I was a kid, I would go to my grandma’s house, the one with the 2 towering black walnut trees, and help around the garden. My first job was, you guessed it, raking!

The first time I was shown how to rake, I was told to rake into two areas. The first area was all the compostables, grass, leaves, and garden waste. The other pile was for the things that didn’t do well in a compost pile. These were branches, rocks, and, oddly enough, shelled black walnuts.

Can Walnuts Be Composted?

When I was a kid, I just did what I was told, but when I got older, I really needed to know, can shelled walnuts be composted? As I looked more into it, I started to discover there were some issues with composting certain parts of some plants. At the top of the list of troublemakers is the walnut trees.

Walnut trees are part of a rare group of plants that are allelopathic. This means they use chemicals to change the ecology of the soil near them to be more to their liking. This chemical alteration ensures the soil will support many future generations of walnut trees with little to no competition.

In the case of black walnut trees, clusters of the seeds drop and release a chemical know as juglone. Sensitive plants, especially certain types of fruits and vegetables, are affected by this compound and demonstrate stunted growth, discoloration, and wilting. An easy way to identify that is by looking at the grass nearest a walnut tree.

Once the chemicals sink into the ground, they can prevent germination of some plants, but mainly juglone weakens the growing plant’s root system. It is similar to how we use non-specific herbicides to weaken weeds and give the plants we want to grow a better start.

Small concentrations of walnut husks, leaves, and seeds will not adversely affect most compost pits or future plants. There is no harm in tossing some old walnuts you never finished eating into your compost bin. But if even a 10th of your compostable matter is walnut in origin, you may have problems.

Will Walnuts Decompose?

With the expectation of plastics and nuclear waste, most things do decompose eventually. Walnut is no different. But how will walnuts decompose? The short answer is slowly. Walnut shells are similar to wood and branches and will take around 6 months to break down into spreadable compost.

Walnuts

Walnuts are hard, and depending on how small they are cracked and the type of composting method you use; you should expect to wait half a year or more to see fully broken down walnuts. Smashing them can help speed it up, but realistically this is another reason to avoid composting walnuts.

Taking a look at how long other common compostable materials take to decompose is useful to provide better context. Below is a table with various objects and the time for each to decompose.

MaterialsTime to Decompose
Vegetables5 days to 1 month
Paper2 to 5 months
Cotton T-Shirt6 months
Orange Peels6 months
Walnut Shells6 months to 1 year
Tree Leaves1 year
Wool Socks1 to 5 years
Leather Shoes25 to 40 years

Vegetables

Any food scraps from the garden or table can be considered vegetables. This can include some types of fruit flesh, different food waste, and other perishables. If it’ll start to stink on its own within a few days to a few weeks, it falls into this category.

Paper

Anything from paper plates and cups to tax returns to cardboard boxes could be considered paper waste. This waste breaks down fast if it is chopped up and wet, but if just thrown on top, it can take over a month to disintegrate. Paper, especially cardboard, can be used effectively to sheet mulch areas with stubborn weeds. By the time the weeds are dead, your cardboard is compost!

Cotton T-Shirt

Fabric or cloth that is 100% cotton will break down in around 6 months. Anything from old clothes to rags, you can cut it up and toss it in the compost. A neat trick is to use old cotton clothes to tie up your vegetables so you can compost everything together at the end of the season!

Orange Peels

Fruit and other food scraps that have harder peels and shells can take longer to break down. Citrus especially takes its time as it naturally repels the microbes that break down food. Too much citrus can mess up your composts ecology, and orange peels are not recommended for worms either.

Walnut Shells

Hard shells and seed cases that have the density of wood or bark can take 6 months to break down. While this can be frustrating if you need your compost ready fast, it can work to your advantage as well. Lining walkways and areas where you do not want grass with walnut can discourage the growth of weeds and save you time and money!

Don’t Compost Walnuts

At this point, it is pretty clear that composting a bunch of black walnut shells and litter is a bad idea. To repeat: Don’t Compost Walnuts! Now that we have that established, it raises another issue.

Which plants are the most adversely affected by black walnut’s chemical juglone?

Trees Sensitive to Juglone:

  • Alder – Alnus glutinosa
  • Apple/Crabapple – Malus species
  • White Birch – Betula papyrifera
  • Linden – Tilia americana
  • Hackberry -Celtis species
  • Larch – Larix
  • Magnolia – Magnolia x soulangiana
  • Silver Maple – Acer saccharinum
  • Norway Spruce – Picea abies
  • Pine (most) -Pinus species

Shrubs Sensitive to Juglone:

  • Azalea/Rhododendron – Rhododendron species
  • Chokeberry – Aronia arbutifolia
  • Cotoneaster – Cotoneaster species
  • Hydrangea – Hydrangea species
  • Lilac – Syringa species
  • Viburnum (some) – Viburnum species
  • Yew – Taxus species

Annuals, Perennials, and Bulbs Sensitive to Juglone:

  • Autumn crocus – Colchicum species
  • Columbine – Aquilegia species
  • Daffodil (some) – Narcissus species
  • Flowering Tobacco – Nicotiana species
  • Lily – Lilium species
  • Peony (some) – Paeonia species
  • Petunia – Petunia species

Fruits Sensitive to Juglone:

  • Apple – Malus domestica
  • Blackberry – Rubus species
  • Blueberry – Vaccinium species
  • Grape – Vitus species
  • Pear – Pyrus species

Vegetables Sensitive to Juglone:

  • Asparagus – Asparagus officinalis
  • Cabbage – Brassica oleracea
  • Cucumber – Cucumis sativus
  • Eggplant – Solanum melongena
  • Pepper – Capsicum annuum
  • Potato – Solanum tuberosum
  • Rhubarb – Rheum rhabarbarum
  • Tomato – Lycopersicon esculentum