Asked May 11, 2021, 3:20 PM EDT
Harford County Maryland
While Sweetspire (Itea virginica) is fairly trouble-free, they can contract one or more types of "leaf spot" in the right conditions. Leaf spot is a collective term for an array of fungi or bacteria that cause foliage spotting symptoms, not all of which are identified in horticulture literature¬†as to the exact organism responsible.¬†Despite the eyesore they can cause, plant health is not usually significantly impacted.¬†Often, conducive conditions for such infections¬†are cool and damp weather, or any other time of year when¬†leaves are frequently wet, such as from a sprinkler used to irrigate in summer.
We do not recommend fungicide treatment, not only because it won't likely impact any bacterial pathogens, but also because once symptoms of leaf spot manifest, controls are not effective. (Fungicides are preventative, not curative.) Instead, you can wait to enjoy the flowers that are about to open, then experiment by cutting back symptomatic growth. Plants that are otherwise healthy, well-established, and vigorous should rebound well from even a drastic pruning, though it may take a few weeks or more to regain their former height.
You can also add mulch¬†(or replace it with¬†fresh, if some is already there) over the surrounding soil¬†to help suppress future spore-splashing from the soil surface onto the leaves. A layer only a couple of inches thick is sufficient, and keep the mulch off of the plant crown (where the stems emerge from the soil) so it gets good air circulation. Affected leaves do not resolve their symptoms even if disease progression halts, but if the infection stops spreading, you'll see subsequent young growth stay spot-free later this season.
Thank you, Miri. So helpful and very reassuring. My May calendar is set.¬†Pruning and mulching in front will be stage two after the thorough weeding currently underway. That said, I am herbicide adverse and so I do¬†all weeding by hand. Unfortunately my front beds are rife with thistle that I have been at war with for going on four years. I suspect an infected batch of mulch unwittingly spread by past owner. Do you have any advice for supressing thistle and other relentless invaders--like Star of Bethlehem and Wisteria which run rampant in my side yard--other than constantly, perennially digging them up? I have read about concocting vinegar and salt solutions for target spraying but¬†fear that it¬†would damage the soil.¬†
Some organic herbicides (horticultural-strength vinegar, for example) can be just as hazardous to human and environmental health than synthetic herbicides, so while they are a viable alternative for persons wishing to stay as organic as possible, they may not work as effectively (as they are not as targeted in action) or will still require repeat application to control tenacious weeds. We do not recommend home remedies, due to variation in dosage and ingredients that have been untested with regards to unintentional, harmful side effects on the environment. This is a somewhat lengthy article on the subject, focusing mainly on insecticides, but still applies to remedies such as household vinegar, salt, etc.:¬†https://extension.psu.edu/are-home-remedies-a-good-solution-penn-state-extension-says-no¬† You are correct that sodium salt is not an ingredient that should be added to soil.
If you wish to avoid all herbicides, the only recourse is to exhaust the weed's root supply of energy reserves by starving it of light. (As starving it of water is less practical and may harm nearby roots.) You could pull or dig weeds up to get roots out, but this not only increases the odds of re-colonization by new weeds (as the soil is being disturbed), but also can result in leaving behind pieces of root or¬†rhizome that will re-sprout anyway. Canada Thistle and Star-of-Bethlehem both easily detach roots/rhizomes/bulbs when pulled, so these two in particular will be, as you point out, relentless and harder to remove in this way. Wisteria's growth is so extensive that it's unlikely it's entire root system could be removed without lots of digging.
Sans herbicide or pulling, repeated cutting or smothering with a thick mulch or wood-chip layer are your best options. Landscape fabric and cardboard are sometime suggested as smothering tools, but among a bed of desirable plants, these two materials are less practical; similarly, neither of these should stay in place permanently, while mulch/chips can.
Repeated cutting-down, while tedious, is the least disruptive to surrounding plants and habitat. You have to be vigilant and remove any regrowth that you see, as soon as you see it, to ensure the roots don't have time to rebuild some starch stores from photosynthesis¬†that will continue to fuel their return.
If you decide to try herbicides, systemic options are the most practical, as they will require the fewest applications and are more guaranteed to kill roots as the chemical moves through the plant's tissues. They will be non-selective, though, so protect nearby desirable plants from overspray by either shielding them with a barrier (scrap cardboard, for instance) or by painting on the application with a disposable foam brush instead of spraying. Check the product label regarding use, as instructions may vary from product to product. Some formulations may advise that for killing "brush" (shrubs, woody vines, weedy tree saplings), you can coat cut ends with herbicide immediately¬†after pruning/sawing off top growth¬†so less chemical is needed to treat the plant.
Ok, so interesting. Acutally, what you're saying is great¬†because it means that an easier option--topping off the weeds as opposed to digging them out--is really a good way to go because photosynthesis is denied. Excellent, got it,¬†pull off the top whenever I see one and just keep mulching. Very helpful. Ok, last question about invasives. One that I know will return with a vengeance in late summer: stiltgrass. I had basically just surrendered to thinking that there is nothing that I can do to stop it after learning from a neighbor that it is a standard feature throughout the neighborhood. But I have a tiny corner of standing woodland garden with a patch of¬†beautiful mayapples and I am concerned about it heading that way. Is there any way that I could create a buffer or firewall that it couldn't jump? Like maybe just a thick border of mulch along that section?
It seems that this yard poses¬†a cornacopia of challenges. Just when I identify and resolve myself to coping with one, another springs to my attention. So I'm very grateful for your advice and help. Thanks again.
Japanese Stiltgrass is challenging to eradicate; while the plants are annual and die each winter, the seed bank can persist for some time, and new seeds are transported back into areas on the fur/feet of wild animals - namely deer. If you can exclude deer, control efforts may be more effective. Here is more information on this weed:¬†https://extension.umd.edu/resource/japanese-stiltgrass
Mulch whose surface dries relatively quickly after wetting (watering or rainfall) stands the best chance at discouraging weed germination. While all mulches block light from seeds already in the soil, and help prevent seeds landing on the surface from reaching soil in which to root, those that are more broken-down can harbor tougher¬†seedlings in the mulch itself. For example, double-shredded hardwood stays moist for longer than the large-chunk grade of pine bark. Wood chips are probably in-between these two,¬†or at least¬†faster-drying than shredded mulches. "Gravel," like marble chips or seashell/oystershell chips, while also quick-drying, can be difficult to work with overtop soil as they tend to settle into the earth over time.
Barrier materials are best used for plants which primarily spread vegetatively (running stems or roots, as opposed to seed). Bamboo and mint are examples where a soil-level barrier can help contain them. For species spreading primarily by seed, several factors - wind, water, animals - carry the seeds far enough into the landscape that most physical barriers will be impractical or ineffective. In this case, though, limiting deer foot traffic via some form of fencing¬†has been observed to be a good way to hinder recolonization by Stiltgrass (if the local seed bank is dealt with concurrently).