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Garden Q&A: How to grow St. Johnswort for summer color and treat rhododendron leaves

Q: What can I use as a summer-blooming shrub, especially if this part of the garden is sunny and somewhat dry? I also sometimes have deer problems.

A: I think St. Johnsworts (hypericum) are underused, and several species are native here in Maryland, though those might be harder to source. Some of the commonly-grown forms are nonnative hybrids, though well-behaved ecologically. (The only locally invasive species, hypericum perforatum, is fortunately not likely to be sold at a nursery.)

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St. Johnsworts bloom anywhere between June and September, prefer direct sun, generally tolerate drought well and are distasteful to deer. Blooms are nearly always an intense yellow, and some species or cultivars have colorful summer or autumn foliage. A few cultivars have berrylike seeds that ripen by fall and make good bouquet accents. I love the bark on native hypericum densiflorum – peeling with a smooth underlayer that’s a rich, warm-toned cinnamon-brown that’s especially showy during dormancy.

You’ll find St. Johnsworts sold as both perennials and shrubs, because some species stay low, sprawl like a ground cover and have stems that aren’t very woody, occasionally dying back in winter as other perennials do. Other species have woody stems and grow to about three or four feet tall and wide.

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Flowers are loaded with pollen, but no nectar, so butterflies will probably detour, while bees and flower flies (predators we like to keep in the garden) will visit. Don’t deadhead developing seed capsules if you want to support gray hairstreak butterfly caterpillars, which can use hypericum as a host plant (among a huge variety of other plants).

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Q: I think bugs are chewing my rhododendron leaves. I thought the only pest they get is lace bug, but they don’t chew, right? Are they going to be a problem? So far, the damage looks minor.

A: This type of damage is caused by black vine weevil [BVW], a nonnative species widespread throughout our continent. Indeed, it seems to be a more sporadic pest of rhododendron (at least in our area), perhaps because they can consume a large variety of plants, unlike the lace bugs which stick to fewer related species. Plus, even though they possess wings, adult BVW are flightless, so that limits how far any one population can spread unless they hitch a ride on plants being sold or traded.

Beetles have chewing mouthparts while lace bugs have piercing-sucking mouthparts, so yes, their damage is different: missing leaf tissue versus leaf tissue with many tiny pale dots, like the chlorophyll was sucked out, respectively.

The wing covers on these beetles are fused together, and they tend to drop when startled, so that makes monitoring for their presence a bit easier since they almost literally fall into your lap when you jostle a plant to see who’s home. If you see notches chewed out of rhododendron leaf edges, check the plant again at night if at first you don’t see anything. The weevils tend to feed after dark and hide under debris on the ground during the day.

Minor leaf damage isn’t serious, but BVW grubs might be more damaging as they chew into roots and potentially plant crowns as they mature. If they strip bark off a main stem, significant dieback can occur.

Weigh any considerations for the use of insecticides (for either life stage, though applications during adult feeding will help interrupt reproduction) against potential harm to pollinators and other beneficial organisms. Fortunately, by the time adults are present, rhododendrons should have finished blooming, but this may not be the case for other host plants used by this species. A few more details about this beetle can be found on our Weevils on Trees and Shrubs page.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.


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