Why You Should Never Mulch Pea Plants with Pea Straw!

Mulching the garden is great way to protect the soil and keep it healthy. A good layer of mulch around 5-10cm (2-4″) thick on top of the soil surface prevents evaporation of soil moisture, saving water, and also keeps plant roots cool in summer. It also creates a cool, dark environment at the soil surface which supports soil organisms, which also slowly break down the mulch, increasing the organic matter content in the soil and releasing nutrients into the soil for plants to use. With time, mulches break down and need to be replaced to maintain their effectiveness.

In nature, there are many organisms responsible for breaking down plant materials to return their nutrients back onto the soil. Fungi are the primary decomposers of dry and woody plant materials in soil. They play a crucial role in breaking down the complex organic compounds found in wood and other tough plant materials by secreting enzymes that convert them into simpler organic compounds that can then be absorbed and used by other organisms within the soil ecosystem, including plants. This is an important nutrient cycling and recycling process in natural ecosystems.

Also, it’s important to point out that different groups of fungi can specialise in breaking down different types of plant materials based on their composition and structure. This phenomenon is referred to as substrate specificity and is closely tied to the enzymes that the fungi secrete. Materials in plants such as cellulose, lignin, hemicellulose, and various organic compounds, require specific enzymes to be broken down effectively. Fungi have evolved to produce a variety of enzymes that can target these different components.

What Does Peas Straw Mulch Contain that Affects Pea Plants?

Pea plants that were mulched with pea straw developed fungal diseases causing leaf spots

Pea plants are typically annual, they grow, flower, and produce pea seeds in less than a year, then die down and dry out. If we gather the dry plants and chop them up, we have pea straw mulch that we can use to mulch the garden. Commercially purchased peas straw mulch is just made from dry pea plants, the ornamental non-edible variety with multicoloured flowers, and packaged into a bale, it’s no different.

If we think about it, dry dead pea plants will be naturally broken down by fungi that are likely specialised for peas, in addition to other generalist fungi that may break down various other plants as well. Fungi spread by fine spores that travel through the air and are carried by the wind. It’s inevitable that a bale of pea straw mulch will contain spores of the fungi that will begin the decomposition process. If there is sufficient moisture in the pea straw mulch, it may be possible to observe the emerging pea-specific fungi growing.

Placing peas straw mulch, which is likely carrying pea-specific fungi, around healthy, growing pea plants will spread the fungi to them, which will inevitably lead to fungal diseases that may appear as dark spots on the leaves, and cause the leaves to wither and drop, weakening the plant, causing it to decline and die off.

The simple solution to avoid this problem is to not use pea straw around or near pea plants. Just use any of the other mulches instead, such as straw, lucerne or sugar cane mulch. That said, it’s perfectly safe to use pea straw mulch around the rest of the garden, or to put finished pea plants into the compost, there’s no issue there!

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