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Berries of invasive species toxic to humans, how to cut back lilacs and more

Don Kinzler says this reader-submitted photo shows common buckthorn, an invasive species that's toxic to humans. Submitted photo

Q: What is this bush with the black berries that's growing right next to some chokecherry trees? — Judy and Tim Hansen, Sabin, Minn.

A: Thanks for the chance to discuss one of the most invasive, yet unrecognized plants in the Upper Midwest. It's common buckthorn, Rhamnus cathartica, which becomes a small tree or large shrub, growing 10 to 24 feet high. When I was with the Extension Service, this was the plant most commonly submitted for identification because it pops up in unexpected places as birds deposit the seed after eating the berries.

Identifying features include the egg-shaped, waxy dark green leaves with prominent veins, white dot-like lenticels on twigs and buds termed sub-opposite, meaning the buds are located opposite each other along the twigs, with one bud slightly below the other. The dark black berries are sometimes tragically mistaken for chokecherries.

Although the berries are highly prized by birds, they are toxic to mammals, including humans. The Canadian Poisonous Plants Information System indicates "Buckthorn poisoning symptoms are usually mild and are limited to abdominal pain, vomiting and diarrhea. Ingesting 20 berries or more can have more serious consequences such as gastrointestinal symptoms, fluid depletion, kidney damage, muscular convulsions, hemorrhage, difficulty in breathing and collapse."

Because common buckthorn spreads rampantly by bird-dropped seeds, it's become a severely invasive species, overtaking many wooded areas and shelterbelts as it crowds out other plant species. Many states placed it on their list of noxious, invasive plants, and control is difficult.

Q: My mom has had tremendous success with a beautiful white dipladenia this summer. She would like to overwinter it, but is unsure how. Any help would be appreciated. — Sara Tjosaas.

A: We've enjoyed wintering our dipladenia and its close relative, mandevilla, indoors for several years. Bring them indoors before temperatures drop much below 45 degrees, first washing plants with a gentle stream of water to remove any insects, and spray with insecticidal soap if desired. We add systemic houseplant insecticide granules to the soil because dipladenia, mandevilla and hibiscus are prone to attacks by barely visible spider mites and aphids, whose populations can balloon by midwinter.

Dipladenia and mandevilla need direct sunshine during the short days of winter, such as in front of a south-facing patio door or large window. Grow indoors as you would other house plants, watering thoroughly when dry, but letting the top several inches of soil dry out before watering again.

In March, prune plants back severely by at least half to remove winter's spindly growth and repot into fresh soil if desired. Begin fertilizing with a water-soluble type every two weeks. Move plants outdoors in mid- to late May, transitioning gradually to outdoor sun and wind.

Q: I have a stand of very mature lilacs that I want to cut back to promote new growth. When would be the best time to do this and how close to the ground should I cut them? — Tim Bergien, Detroit Lakes, Minn.

A: I've observed older lilacs being rejuvenated beautifully for many, many years, giving new life to old woody lilacs whose structure isn't nice anymore. The best season for rejuvenation is early spring before they leaf out, so usually late March or early April.

Prune all the lilac branches down to 6 inches above soil level. Don't hesitate, and go only halfway, or you're still left with old, woody lilacs, only shorter. Tools to use include a chainsaw, a hand pruning saw or large loppers. No need to treat pruning cuts with any products, as the cut surfaces heal fine. Lilacs require full sun to develop healthy growth and good flowering and usually won't bloom the year of rejuvenation.