Hello, Chris, and thank you for your excellent question to AskExtension/Ask a Master Gardener.

The photo you sent was very helpful. Although it is difficult to definitively diagnose a plant problem from just a photo, I can make a pretty good guess that your hibiscus plant is infested with White Flies.

When I zoomed in on your photo, I saw small dots of damage on the older leaves. They were most likely made by a pest feeding on the sap from the underside of leaves of the plant. Further examination of the newer growth showed small white “dots” that I would guess are white fly eggs or larvae. And on the edge of the right-most mature leaf, at about five o’clock, there is a mature insect (with wings) that looks like a white fly. (I also considered that the pest could be aphids or mealy bugs. Both are pests that are known to attack hibiscus.)

So … what do you do? The good news is that all three pests would be treated in the same way. First, rinse your plants every three days (for two weeks or until pests are gone) with a strong stream of water. Try to hit as much of the stems, leaves and undersides as possible. Use a stream that is strong enough to knock off the pests, but that doesn’t break the stems of your hibiscus. Next (once your plant is fairly dry or on another day) spray your plants with a horticultural oil mixture (such as Neem oil); be sure to spray the underside of leaves. You can buy pre-mixed sprays or mix your own to use in a spray bottle or garden sprayer. Apply every seven days. An important note: apply horticultural oils only in morning, evening or cloudy days to avoid sun scald – kind of like sun burn for plants. With treatment, your hibiscus will likely bounce back quickly and give you beautiful blooms this season.

Preventative measures: White flies tend to return to the same area, so monitor your plant and repeat treatment if pests reappear. The best preventative is a healthy plant, which is less susceptible to pest infestations. Make sure you are amending the soil around your hibiscus yearly with organic materials like compost or compost/manure blends. Water consistently, but of course don’t overwater. Fertilize regularly, but never overfertilize; follow package directions carefully. There is some anecdotal evidence that light sprinklings of worm castings cause hibiscus to uptake materials that give them resistance to pests, but I haven’t been able to verify that with research-based evidence. Another note, in looking at your photo I had to wonder what the white ash-looking ring was around your hibiscus. I also wondered if your mulch was piled too close or too deep around your plant. It is hard to tell from only a photo, but you might want to pull the mulch back from your hibiscus to avoid “smothering” it. Hibiscus tends to like good airflow.

In order to be certain the information I gave you was accurate and current, I consulted several research-based sites. Links to the best of those are below, should you be interested in learning more.

If you have any further questions about this issue, or if something is unclear, please feel free to write back to me. If you respond to this email, it will come directly to me.

Wishing you the best of luck with your hibiscus! They are beautiful plants; I have several in my gardens and I love them!

Resources for you:

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White Flies – Southern California Hibiscus Society https://www.socalhibiscussociety.org/pro-growers-tips/white-flies/

How to Use Neem Oil Spray as an Organic Insecticide https://www.thespruce.com/using-neem-oil-as-an-organic-insecticide-2132579

Hibiscus hybrid – North Carolina State Extension https://plants.ces.ncsu.edu/plants/hibiscus-hybrid/

Hibiscus syriacus – The Ohio State University https://plantfacts.osu.edu/tmi/Plantlist/hi_iacus.html

Hi, Chris!  Thank you for writing back.

Whiteflies would attack phlox but not usually bee balm because the bitter oils (that gives it its distinctive fragrance) repels them.

It certainly could be overspray (also called herbicidal drift) from an herbicide such as Roundup. I think your landscaper, having an opportunity to see the entire plant and its surroundings, is in a better position to make an assessment than I can do from a photo. The curling of the leaves is an indicator of herbicide damage. It is possible that once the plants were weakened, the whiteflies (aphids or other pest) moved in. As noted in my response, they tend to attack weakened plants. so it could be both problems.

A bit about herbicidal drift. Most herbicides can drift in windy conditions or in temperature inversions. Unintended targets of herbicides can also occur with herbicidal runoff related to water flow. This article from The Ohio State University Ohio Line explains herbicidal drift quite well. Although it focuses on grape vines and vineyards, it will help you to understand the concept, if you are interested in learning more.

Regarding your question on  whether or not to start over with a new planting of your hibiscus, depending on what your expectations are for that plant, it might be a good idea. You hibiscus might survive, but it might take a while for it to recover. If you decide to plant a new hibiscus, you might want to replace some of the soil from the site where you are planting to ensure there is not remaining Roundup; different types of Roundup "last" longer.

As to why other plants are not affected, it could be that they were not in the path of the herbicidal drift/runoff or that they are less susceptible to damage. I don't have enough information about the site, the placement of the plants mentioned, the type of roundup used, the conditions on the day of application and if there are other herbicides (such as weed killer on lawns) to answer definitively. You could submit a sample of your plant(s) digitally or physically to the Plant and Pest Diagnostic Clinic at Ohio State. Information on how to do so is here. There is an associated fee of about $20. (We make no profit from recommending.)

Wishing you the best with your hibiscus plant. I hope this information is helpful to you.