I’ve been working to remove invasive plants on my property, and some I can dig out successfully, but some species like Japanese knotweed, paulonia or bamboo just can’t be removed that way. Is there advice on how to balance the drawbacks of using herbicides vs allowing the invasives to persist?
Also, if the only solution for a plant is herbicide, what are best practices?
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Use it before the plant goes to seed. You’d be surprised how many city agencies don’t know that one.
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See related previous topic: https://forum.inaturalist.org/t/effectiveness-and-ethics-of-using-herbicide-on-invasive-species/28467.
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Herbicide application timings and rates vary by species. While using herbicide the label is the law, and laws may vary based on where you live, but this is my personal knowledge and solely my opinion as a conservation professional.
For Japanese knotweed, we use a foliar application of a 5% solution of 53.8% aquatic approved Glyphosate (Aquaneat, Rodeo) with an aquatic approved surfactant (CideKick II, Cygnet Plus) applied while the plant is flowering. I would also recommend you purchase a tracker dye for your mix so you can ensure you have sufficient coverage of the leaf surface (there are lots of these, if you just look up herbicide tracker dye you’ll find what you’re looking for. Your mix ratio would be ~6oz glyphosate + ~2oz surfactant + ~2oz tracker dye per gallon of water. If the plants are too tall by the time they flower, you can cut them earlier in the season and allow them to regrow to make them more manageable. You should achieve control after about 3 years with diligence.
Alternatively, you could clip and drip or do stem injection if you’re concerned about the broadness of foliar applications. I don’t use this method, so I won’t provide specific rates for it but you can find recommendations online.
For the Paulonia, cut it low to the ground leaving a stump approximately 4 inches high. Immediately after cutting apply a 45/45/10 solution of glyphosate, surfactant and water to the cut surface. You only need to cover the cut surface, you don’t want it running off the stump at all. If you have a lot of woody invasives on your property, I would recommend investing in a Buckthorn Blaster we use them extensively and they’re fantastic for minimizing herbicide use.
As for the bamboo, I don’t deal with that so I don’t have any personal recommendations to make unfortunately.
If you want to avoid herbicides entirely, you could repeatedly cut the area or you could cover the plants with a heavy mat after cutting them to the ground while they are dormant. Though knotweed and bamboo are particularly tenacious and depending on the size of the population you’re dealing with you’re going to need a pretty big mat to deal with all the runners the plant will put out in response.
Your local university extension office may also have additional educational resources available for assisting you with understanding best management practice for various species and the laws in your area.
To summarize, the best practice for using herbicide to control invasive plants is to understand the species phenology, application timing, application technique and pesticide choice. Other’s have said it elsewhere, but the amount of herbicide you would use to control invasive species on your property is infinitely small compared to how much is used in agriculture every day but it’s still worth doing it right not only for the environmental aspect but also so you get the best possible results.
Hope this helps, if you have questions feel free to send me a message or reply to this comment.
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For herbaceous invasives like Taro, poison-ivy, and non-native Euphorbia’s, I do an initial foliar treatment with concentrated (30%) vinegar as a defoliant. The vinegar is usually available in hardware stores either in the gardening or cleaner sections (with soaps and bleaches). As a defoliant, it may not entirely kill perennial herbs with a deep rootstock, but with a few repeat treatments, they usually end up using up all their stored energy and die out. ***IF you choose to use 30% vinegar, although it is “non-toxic”, treat it with caution as you would any herbicide. It can irritate eyes, skin, and lungs.
I should mention that the reason I’m comfortable using the vinegar in my area and in my habitats is because we are on limestone-derived soils which are somewhat alkaline (basic). So any of the acidic vinegar “spilled” into the environment is almost immediately neutralized by the soil and rock, rendering it harmless to other plants, animals, and waterways.
Woody invasives are particularly tough for such decision making. I have eliminated such species as Ligustrum’s and Chinese tallow by hand cutting or girdling, then spraying any resprouts with the vinegar. For larger ligustrums and tallows, I’ve cleared dirt from around the base of the tree and top roots, scored those with a hatchet, and repeatedly sprayed/soaked those wounds with vinegar to kill the root system. These methods take time, repeated effort, and patience.
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I have the same issue, but with creeping thistles 
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