I've received a large number of queries about my directive last week not to add nightshade and mustard family plants to compost. The topic garnered more e-mail responses than any recent topic I have covered. So that might be a good subject to focus on this week. All the concerns you expressed notwithstanding, it was great to hear that so many of you are composting.
Q Since I grow tomatoes and peppers in my city garden, I cheerfully throw the residues, at this time of the year, into my compost heap. No untoward effect appears to have ensued, but your column says I have been making a great mistake. Why exactly do they harm the compost? -- Ben King
A Here is my reasoning. There has always been concern about spreading disease through composted materials. All cruciferous plants are susceptible to viruses, including cabbage, broccoli, cauliflower, Brussels sprouts, kale, mustard, turnips and kohlrabi. Therefore, I do not use them as compost matter.
I also eschew plants with phytophthera and annual garden plants that are highly susceptible to blights and wilts. Some experts even go so far as to wipe their pruners with bleach after every cut for fear of spreading viruses and funguses through tools. Lubricate pruners with light oil when done to prevent corrosion.
Diseased debris that is not fully composted can spread funguses. Fungus-susceptible tomato, potato, pepper, eggplant greens and other plants in the nightshade family, including tobacco, Jerusalem cherry, and annual flowers nierembergia and petunia, should be discarded. The Rodale Organic Gardening Basics Handbook on Vegetables, Vol. 3, recommends the same if you see any sign of disease on your tomato plants.
For another opinion, I went to plant pathologist Ethel Dutky, who directs the Plant Diagnostic Laboratory for the University of Maryland Cooperative Extension Service, and asked, "Do you think advice to not compost mustards and nightshades is correct?"
Her answer was no.
Dutky said that most funguses are "wimps" and are controlled during composting. Even when certain funguses form more resistant coats and survive slightly higher temperatures, they are destroyed by the temperatures produced when decomposing.
"Proper composting is the key," she said. "As long as you break it down into crumbly, fine-textured compost where you can't recognize the 'parent material,' the diseases will not survive. This is true for all garden vegetables."
She added: "It doesn't take a perfect mix to create compost. As long as the material has enough moisture and air while decaying, it will reach the temperatures at which virtually all funguses are destroyed, at least 100 degrees Fahrenheit."
If you have the perfect ratio of woody to high nitrogen materials (manure and leafy greens), about 30 to 1 by volume, and mix it regularly, it will reach temperatures from 130 to 140 degrees Fahrenheit, decay fast and be disease free, she said.
At the same time, Dutky advises that if you have materials that harbor diseases in your pile and they do not fully compost, you do take a chance of spreading the problem. In those situations, you might have been smart to leave out the diseased nightshades and cabbages.
She offered other suggestions to avoid having compost that carries disease spores: Ensure that the material is composted fully into small, crumbly particles; grow only disease-resistant plant strains; rotate your crops; plant in containers using soil-less mixes that have never had plants growing in them; and/or make two piles, one of only disease-free material and the other of infected debris. Leave the questionable compost pile to decay and condition the area where it stands.
Disease-resistant strains of most flowers and vegetables can be found. The more resistance the better. Most plants will only be resistant to one or two of these diseases if they are only susceptible to those. The following notations can be found on seed packets or plant labels at garden centers and designate a plant is resistant: Look for A (alternaria), ANTH (anthracnose), ALS (angular leaf spot), DM (downy mildew), F1 or F2 (fusarium wilt), PM (powdery mildew), T or TMV (tobacco mosaic virus) and V (verticillium wilt).
Crop rotation is a way to combat funguses in the garden. Very generally, the theory is to plant a tomato one year, feed the soil heavily with fresh compost and fish or kelp products the following spring, and then plant cabbage. The next year, plant cucurbits such as cucumbers, pumpkins, gourds, melons and squash. The fourth season, install parsley family plants such as celery, coriander, cumin, parsnip, parsley and carrots in that location. Then go back to tomatoes the following year.
To achieve this rotation and be able to plant tomatoes annually, you must have room to move the plants to other parts of your garden through the years.
You won't have diseases carrying over from last year if you plant a container with a soil-less potting mix that has never been used. Make sure that your pot has holes in the bottom, and you will automatically have good drainage. Fertilize every other watering with a water-soluble fertilizer such as Peter's Plant Food. If you plant this way, you can completely control the environment and will always have disease-free flowers and vegetables if you change the planting medium every year.
Did you mean only the plants mentioned such as broccoli and peppers, etc., should not be put in the compost pile or also the scraps of the vegetables that were cut up for cooking? -- Hilde Cousins
All uncooked scraps that have no animal products in them are fine for composting. They are probably disease free and fine as long as the composting process is complete.
I've always been told to have more nitrogen than carbon in the compost mix and to layer and mix them thoroughly to avoid the bad smells. Recommendations have ranged from at least 50-50 to 25-75 with nitrogen predominating. You seem to reverse this order. In fact, if I read you correctly, you recommend a 25 to 1 mixture with carbon predominating. Why? -- Peter Schwartz
Those are the correct ratios for the most efficient composting, more carbon to nitrogen by volume (not weight). But as I stated above, all organic material will decompose if left long enough for the organisms to break it down.
Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. His e-mail address is email@example.com; his Web page is www.gardenlerner.com.