Vegetables grow well in containers with enough room, right potting mix

Question: I grow my tomato plants and pepper plants on my deck with southern exposure in 18- to 24-inch pots. I use potting soil, and they do very well. Would it be possible to grow them in mushroom manure rather than potting soil?

Answer: Tomatoes and peppers are just two of many vegetables that perform well in containers. As long as the plants are grown in the right-sized pot and filled the right kind of growing medium, all sorts of vegetables can produce excellent yields when grown in containers.

Typically, I suggest that each tomato plant be grown in a pot that holds a minimum volume of 5 gallons. The same goes for other large vegetables, such as zucchini, pumpkins and melons. For peppers and eggplants, a 3- to 5-gallon container is ideal.

For slightly smaller vegetables, such as cabbage, broccoli, and cauliflower, I recommend a soil volume of 2 to 3 gallons per plant. And for crops that can be grown in groups, such as radish, lettuce, chard and other greens, I suggest using a container that holds from 2 to 3 gallons of potting soil.

Larger containers generally mean less watering, better root growth and higher yields.

After you select the right size container, your question regarding what to fill it with comes into play. While mushroom manure (also called mushroom soil or mushroom compost) may seem like an inexpensive, widely available option, it isn't a good choice for container gardening.

Here's why: Mushroom manure is composted organic matter that was once used to grow a crop of mushrooms. It's composed of horse bedding, wheat straw, hay, manure, ground corn cobs and other products. After the mushrooms are harvested, this substrate is then steam pasteurized and, ideally, composted for a few weeks before sale to the consumer. The finished product contains between 25 and 50 percent organic matter, but it generally contains low amounts of plant nutrients.

Mushroom manure can be fairly high in soluble salts, and those salts can build up to excessive levels in containers and burn young vegetable plants. And, the pH of mushroom soil generally ranges between 7.0 and 8.0. That's far too alkaline for most vegetable plants that prefer a pH of about 6.5, though this measurement varies greatly, according to what products were originally used to make the mushroom manure.

The potential soluble salt and pH issues you could face when using mushroom manure are less of an issue when it's applied as an amendment to garden soil or turf grass, where the product is distributed over a large area and the salts can be diluted by irrigation.

I recommend mushroom manure only be used as a soil amendment, not as a container-growing medium. And, if you do use it in the garden as a soil amendment, I suggest you avoid using it around acid-loving plants such as azaleas, rhododendrons and other evergreens, and around young seedlings that may be susceptible to possible salt damage.

Another way to avoid potential issues with mushroom manure is to either mix it with compost before application to the garden, or use it only every three or four years.

As for your vegetable containers, I recommend using a 50-50 blend of quality potting soil and finished compost. The compost adds nutrients and beneficial soil microbes to the sterile potting soil, and it increases the potting soil's water-holding capacity, leading to reduced irrigation needs throughout the summer.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 503 Martindale St., 3rd Floor, Pittsburgh, PA 15212.