Beating back fast-growing mint is tough

Q . I have an herb bed that is being overrun by mint. What was one small plant now occupies a third of the bed. My husband has threatened to kill it with an herbicide; I want to beat it back without harming the butterfly bush, azaleas and herbs it has surrounded. Can this be done?

A. Mint is adept at spreading via long rhizomes that lie just under the soil surface. It might even find its way around or under obstacles such as bricks placed in its path. It is best in a pot.

There are no herbicides that are selective for mint. Your only option is to treat the portions of the plant that you can with a non-selective herbicide, such as Roundup. With luck, the herbicide will be drawn down to the mint's root system without harming the desired plants. You might need to combine hand weeding with herbicide treatment to eradicate the mint. Fortunately, the rhizomes tend to be just under the soil surface, so you won't need to dig deep to get rid of them.

Q. I have a pink dogwood that has not flowered well in recent years. It used to be filled with blossoms. I usually work in some manure around the tree each spring and add a fertilizer after flowering. I also added some superphosphate to stimulate blooming. The tree does tend to get black spot on the leaves and mildew in summer, but it always comes back the next spring looking healthy. How might I get it to flower again?

A. Dogwoods do not need fertilizers if they are growing in average soil with a natural mulch of fallen leaves. The presence of lots of nutrients suppresses the production of flower buds in favor of vegetative growth. Other factors might reduce the flowering display, notably excessive shade or improper pruning, but I suspect your problem is linked to fertilizing.

You mention powdery mildew and what sounds like septoria leaf spot. Unlike the dreaded dogwood anthracnose, this leaf spot appears in summer rather than spring and is never fatal. However, if the leaves are seriously compromised, there might be less carbohydrate available when the flower buds are set in the fall, resulting in a sparse bloom the next spring.

Powdery mildew has become more serious on dogwoods than dogwood anthracnose in many locations. It exacerbates drought impact because the mildew's fungal threads draw more water out of the tree than the leaves alone. Water stress might be diminishing the number of flowers, particularly in times of drought, when powdery mildew is usually most prevalent. Powdery mildew is worse on rapidly growing trees, so easing off the fertilizer might help keep it under control and promote flowering next spring.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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