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Sandy Feather: Prune butterfly bush in spring

Saturday, August 31, 2002

Q. I have a butterfly bush and was wondering when -- or even if -- I should prune it. Also, should I remove the old blossoms?

A. Butterfly bushes (Buddleia davidii) are best pruned in late spring before they start to put on new growth. They bloom on new wood, which means they set flower buds on the current season's growth. While pruning in spring will delay their bloom time somewhat, they will still put on an exquisite floral display. You can prune them very hard in spring, within 12 to 15 inches of the ground. Depending on the cultivar you are growing, butterfly bushes will still reach 5 to 8 feet in height by fall.

 
 

Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208.

   
 

They do not need to be pruned at all if they have not outgrown their space. However, butterfly bushes are very vigorous plants and easily outgrow their allotted space if they are not pruned.

Removing spent flowers -- a practice known as deadheading -- is not an absolute necessity. However, deadheading will prolong its period of bloom. Also, the plant will look neater without a lot of dead flowers hanging on it. Butterfly bushes have been known to seed themselves. In some climates, they make serious weeds of themselves. I have not noticed them behaving that badly in our climate, but I do see them seed themselves a little. Removing the spent flowers eliminates that potential problem.

There is a group of plants, including butterfly bushes, that should never be pruned in fall. Others are artemisia (Artemisia spp.); blue mist shrub (Caryopteris spp.); St. Johnswort (Hypericum spp.); and Russian sage (Perovskia spp.). They often die when pruned in fall. All bloom on new wood and should be pruned in late spring, before new growth begins.

Q. My zucchini plants started out healthy and vigorous, but one by one (I had four) they started to wilt and die. I could not see any insects feeding on any of the plants. Their leaves did not have any holes in them or any kind of damage I could see. What could cause this, and how can I avoid losing my zucchinis next year?

A. It is likely your zucchinis succumbed to an attack by squash vine borers. They damage the stems and vascular systems of squash plants. They do not feed on the foliage, so you would not have noticed any damage to the leaves. Squash and pumpkins are susceptible; cucumbers and melons are rarely attacked.

Adult squash vine borers are moths. Their bodies are black with reddish-orange markings. They lay their eggs on the vines in mid- to late June. When the larvae hatch, they tunnel into the stem and feed for four to five weeks. Squash vine borer larvae are plump, white and can grow up to 1 1/4 inches. Their tunneling activity is characterized by yellow, sawdust-like excrement (frass) coming from holes in the stems close to the ground. Once they destroy the water-conductive vessels in the stem, the plant dies.

Insecticides applied after the borer is inside the stem are not effective. Insecticide applications should be targeted at the newly hatched larvae before they tunnel into the stem. Sevin (carbaryl), methoxychlor, pyrethrins and rotenone are labeled to control squash vine borers. Thoroughly spray the base and stems of susceptible plants with one of the labeled insecticides.

The timing can be tricky because the larvae bore into the stems within hours of hatching. Usually the adults are laying eggs as the vines begin to run, around the last week of June. Scout for adult activity early in the morning or at twilight. Repeat applications every seven to 10 days for three to five weeks.

Another option is to watch the plants closely. Starting in late June, examine the stems and bases of the plants for signs of borer activity. At the first sign of wilting, slit the affected stem lengthwise with a knife and destroy the borer. Cover the injured portion of the stem with soil, and keep it moist (not waterlogged) to encourage root growth. Squash and pumpkin plants are vigorous and often root, grow and produce fruits if you catch the borer before it does irreparable harm to the plant.

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