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Garden Q&A: What’s damaging the roses and do social media hacks work?

Q: I see some neighborhood roses being chewed-up by something, while others just look sickly and deformed. Are these different stages of the same problem? Can I save my own roses if they start to show these symptoms?

A: These are different issues and the plants have different prognoses — one will be fine while one may be doomed. I’m assuming the chewing is manifesting as scattered holes on multiple leaves, since entirely missing or torn-off foliage would be due to deer, which love to browse roses.

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Usually, the primary culprit for chewing holes in rose foliage (especially before summer) is an insect larva called roseslug sawfly. Despite its name, it’s neither a slug nor a fly; sawflies belong to the same insect family as wasps. They behave very differently, though: they can’t sting, don’t create nests, and don’t bother people.

The green larvae blend-in very well on the undersides of the leaves and look nearly identical to caterpillars (a term which applies only to butterfly and moth larvae).

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Maryland has three species of roseslug, and their life cycles differ a bit. Fortunately, the same management approach applies to all: pick off those you find, and if damage is becoming extensive despite your removal efforts, treat the foliage with either horticultural oil or the organic pesticide ingredient spinosad.

Re-treatment will be needed, so follow the product label directions and do not spray blooms to avoid harming pollinators. If you shun pesticides, just let the rose live with its swiss-cheese status, as it will probably be fine unless it’s weakened by defoliation every year.

Deformed foliage, on the other hand, is caused by either herbicide injury, temporary cold damage on tender growth, or a virus. Plants will grow out of cold damage on their own. Herbicide applications can drift onto nearby plants if the weather is breezy, or they could seep into shrub roots when certain chemicals are applied to the soil.

Some plants can grow out of herbicide damage while others are permanently injured, depending on the chemical and the dose.

All rose varieties are vulnerable to rose rosette disease, a virus transmitted by plant-feeding mites. Contaminated sap can also spread the virus if you happen to trim an infected plant before healthy plants. Mites carrying the virus blow from host to host easily, and the widespread invasive multiflora rose growing in natural areas serves as a reservoir for both this virus and the mites. (Fortunately, the virus kills the weedy roses too.)

There is no cure for rose rosette disease: mite-killing pesticides won’t prevent infection, and the virus becomes systemic in all plant tissues, so you can’t just trim off the symptoms. Infected plants must be discarded. You can replant roses if you want to gamble they won’t be recolonized by mites, or just plant a different flowering shrub.

Q: I see tons of gardening “hacks” on social media videos and blogs. How many really work?

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A: As you might expect, not many, and several are downright ludicrous. We’ll be here all week if I try to debunk a list of particulars, but suffice to say, several are not based on sound gardening science, and some are just downright weird. The rest are generally just interesting uses of space or repurposed materials, and I don’t take issue with those.

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I’m not saying you can’t experiment, get creative, or tweak plant cultivation methods to suit your budget or other constraints, but try to use science as the foundation for your choices because you’re more likely to be successful. While it’s true we don’t have research studies performed on every minutia of horticulture, reputable sources for gardening information should at least base their recommendations on the well-studied science of horticulture.

University Extension programs, arboreta and botanic gardens, and other science-based organizations update published gardening resources as ongoing scientific research demonstrates how we can improve the way we care for plants, reduce our negative environmental impacts, and understand the impacts of accidental species introductions.

Home remedies used as a substitute for synthetic pesticides is a particular pet peeve of mine, especially when their popularity is based on the assumption of greater safety to people, plants, or the environment, which is not consistently true. Plus, using any registered pesticide in a manner inconsistent with its label is not just ill-advised, it’s also illegal. Save your time, money, and supplies for other experiments and go with the products tested (and retested) with scientific scrutiny. Besides, if a plant ailment is mild enough that you’re willing to try an unproven concoction, you probably don’t need to treat the plant at all, or can avoid using a pesticide entirely when managing the problem.

When encountering an appealing “hack,” ask yourself: does the presenter explain why this should work, and why it’s recommended in place of the typical method used? Does it really save me time or money or produce a significant improvement in results? Is the source of information credible or reliable – have they presented other ideas that have been consistently successful? Can you find a reference to the technique in a science-based resource?

Hacks that actually work have usually just stumbled-upon a method already proven to aid plant growth, such as adding organic matter to the soil or using darkness and moisture to encourage rooting or germination. Can you root stem cuttings in a potato? Maybe, but I’d rather not waste the potato when soil and rooting hormone works just fine. Are Epsom salts a garden cure-all? Probably not, unless your soil tests deficient in magnesium.

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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