What is Mushroom Compost?

Effective and economical, mushroom compost is simply compost that has already been used to grow mushrooms. Due to the ways that mushrooms impact the soils they're grown in, spent mushroom compost makes an excellent fertilizer or soil additive for all sorts of gardens.

What is Spent Mushroom Compost?

Commercial mushroom growers use varying mixtures of manure and straw to grow the varieties they want. The compost is inoculated with the desired fungus, spread across growing beds and kept cool and moist until harvest. As the fungi grow, they break down the manure and straw mixture. It may take about two months for the mushrooms to develop, depending on the variety.

Is Mushroom Compost Good for Gardens?

Because of the fungi’s work breaking down the manure and straw mixture, the result is a really beautiful, dark compost that works wonders in the garden. Like any compost, it helps to aerate clay or compacted soils and improves any soil’s ability to retain water. Spent mushroom compost also enhances the activity of beneficial soil microbes and contains calcium, which can help beat blossom end rot on tomatoes.

When Not to Use Spent Mushroom Compost

Spent mushroom compost typically has an alkaline pH, which means it’s a good addition to asparagus beds or areas where you’re growing cabbage, cauliflower or broccoli. Avoid using fresh mushroom compost on acid-loving plants, such as camellia, rhododendron, azalea, citrus, blueberries, heath, heather or hollies. It’s also good to avoid using mushroom compost on new seedlings.

Where to Get Spent Mushroom Compost

In some cases, you can buy mushroom compost directly from mushroom growers. Although Pennsylvania is one of the most popular locations for domestic mushroom producers, there are growers scattered around the country. Check with your local extension agent to find out if there are any mushroom growers in your area. Spent mushroom compost is famous among gardeners for its benefits to the landscape, which makes it a very hot commodity.

Producers handle requests for mushroom compost in different ways, but nearly all accommodate gardeners’ requests in some fashion. Some mushroom growers announce days that compost is available on a first-come-first-serve basis. Others may have a sign-up list. Some producers may even have a side operation where they sell mushroom compost. Delivery isn’t usually an option if you get your compost directly from the producer, so you’ll need a truck or some other way to transport your mushroom compost home.

Look for bagged mushroom compost at nurseries, garden centers, co-ops and box stores. Bagged compost is cleaner to transport and also can be drier and less pungent than mushroom compost fresh from the producer.

Also check with your local bulk mulch dealer. These businesses often sell spent mushroom compost in bulk. If you’re creating new planting beds and taking a large delivery of top soil, you can ask these businesses to load their dump trucks with alternating scoops of spent mushroom compost and top soil for a ready-made garden mix that will give plants a solid start.

Make Your Own Mushroom Compost

If there isn’t a mushroom producer in your area and bagged mushroom compost isn’t available at the store, trying making mock mushroom compost at home. Simply mix cow manure and straw. Eventually, naturally occurring fungi and bacteria will arrive on the scene to break down the ingredients. Keep moist and mix regularly until the pile cools down. Apply your mock mushroom compost to the garden or add to other compost products.

If you’re feeling ambitious, consider using your homemade mixture to cultivate mushrooms at home. Growing mushrooms can be tricky, but there are kits and inoculants available to help with the process. After harvesting the mushrooms, you can harvest your very own mushroom compost to use in the garden.