Q: We have a butterfly and bee garden in our yard with varieties of Echinacea and Rudbeckia. This fall I noticed several of the seed heads had odd-looking growths. I removed the seed heads from all plants affected, and threw in the garbage. Can you identify what type of growth this is and if it is good or bad for the plants? — Suzanne McTiver, Moorhead.

A: These deformities on coneflowers (Echinacea) seem increasingly common, based on my mailbag. Two causes of similar-looking disorders are insectlike mites and the disease called aster yellows. Both are considered serious situations.

Mites appear to be the disorder’s cause in your case. Coneflower rosette mite is an eriophyid mite that makes its home inside developing flower buds, causing the central cone of the flowers to develop tufts, knots, bumps and unusual growths sprouting from the top of the central cone. The flower petals surrounding the cone often look normal. Cut and dispose of flower heads immediately as the symptoms begin, to reduce mite populations. Applications of neem oil or horticultural oil applied to flower buds about 10 days before they’re estimated to open, and again immediately upon opening, can help.

Similar symptoms are caused by aster yellows disease, except the plant’s foliage usually shows mottled yellowing and flower petals are often affected. The bacterialike phytoplasm that causes the disease is transmitted by leafhoppers. Once plants are infected, there is no cure and affected plants should quickly be rogued out, roots and all, to contain the spread. Control leafhoppers with permethrin, Sevin, neem oil or insecticidal soap.

Q: I just read your leaf-mulching article. Does it matter the type of tree leaf you are mulching? We have many black walnut trees and have been told they make the soil a challenge for certain plantings. I know it is from the root system but didn’t know if leaves affect things also. — Jon Vik, Rochester, Minn.

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A: Thanks for bringing up a good point. Leaf types vary in nutrient and fiber content, but most are great additions to the yard and garden except one: black walnut.

Black walnut trees produce a chemical called juglone that adversely affects the growth of some plants, like tomatoes, but not other plant types. The roots of black walnut trees exude juglone into the soil, probably as the tree’s natural attempt to control its territory from invaders. Other parts of the tree also contain the chemical, and there are contradictory recommendations about whether black walnut leaves can be used safely in compost or as a mulch, as juglone does decompose over time.

However, Purdue University advises against using black walnut leaves or sawdust as soil additives, mulch or compost. Black walnut leaves don’t seem to adversely affect lawngrass, as our common grasses Kentucky bluegrass and fescue seem to grow fine in the vicinity of black walnut. Mulching walnut leaves into the lawn seems to be a workable option.

ARCHIVE: Read more of Don Kinzler's Fielding Questions columns

Q: Should rhubarb leaves and stems be pulled after frost, or left in place during the winter? I never know if I’m supposed to remove them, or just leave them alone. Thanks. — J. Jenson, Fargo.

A: Rhubarb plants are best cleaned up in the fall, after several hard freezes have made the plants totally limp. Gently pull the leaf stalks out of the plant’s crown, clean up leaf debris from the plant and dispose in the garbage. Rhubarb certainly won’t die if old stalks are left in place, but practicing good fall sanitation helps prevent future leaf and stem diseases that survive winter on rhubarb plant debris.

A similar question is often asked about asparagus. The tops of asparagus should be left intact over winter, as they help catch snow, providing extra insulation to asparagus, which has been known to winterkill in severe temperatures. Remove tops in early spring before the new spears emerge.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at kinzlerd@casscountynd.gov or call 701-241-5707. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.