How to Protect Your Home from Japanese Knotweed

by Claude LaVallée, Updated August 2025

Japanese Knotweed (JKW) has invaded Pittsburgh, including Polish Hill, and it’s more than a nuisance; it’s an extremely aggressive plant that can cause extensive ecological degradation and expensive infrastructural damage to our houses.

While conventional weed control methods counter-intuitively lead to further propagation, numerous peer-reviewed field trials have shown that one method, popularly known as the flower-to-frost (FF) window, stands out as the most effective and sustainable.

Japanese Knotweed: A Pretty, but Aggressive Invasive

Japanese Knotweed (JKW), a perennial native to Japan’s volcanic areas, was introduced to North America in the 1800s for its beauty and assumed ability to stabilize slopes. However, its expansive lateral root system—rhizome—has instead been found to promote erosion. Additionally, without its native co-evolved insect and fungal controls, JKW’s fast and dense growth takes over, replacing biodiversity with monoculture. JKW propagates mostly via its rhizome, but seed hybridization is being studied as a potential emerging means of dispersal.

Property Damage

JKW, having evolved in volcanic rock, can also damage infrastructure. Its tenacious roots can exploit tiny weaknesses in foundation walls and other hard infrastructure, prying its way through and eventually destabilizing structures.

Once it has penetrated a wall, damage control becomes very challenging, because cutting JKW only stimulates it to grow more. Prevention is key.

How To Avoid Making the Problem Worse

Do not cut, mow, pull, dig up, tarp, or especially, compost live JKW (dry stalks are harmless). Above-ground injury or barriers only temporarily deprive the plant’s rhizome of photosynthetic nutrients, prompting it to spread laterally up to 50’ or more underground and push up new shoots in a bid for survival. Digging it up is also very risky; each leftover live piece the size of a fingernail can become a new plant, thus leading to further infestation instead of eradication. Also be aware that bits of JKW are frequently, though inadvertently, tracked via excavation vehicles, landscaping equipment, and shared pots, plants, and potting soil.

Comparing the Effectiveness and Sustainability of JKW Control Methods 

Of all JKW control methods field-tested to date —biocontrol agents, root exudates, excavation, covering, electrical treatment, and physical management methods such as mowing and cutting—the most sustainable is also the most effective and simplest: a low concentration of glyphosate-based foliar spray. It uses the least materials, has the lowest environmental impacts, and has the lowest economic costs. This method is popularly known as the flower-to-frost window method, in reference to the late summer to early fall period when it is applied.

How the Flower-to-frost (FF) Window Method Works

The flower-to-frost window method works like a Trojan horse: the glyphosate sneaks into the JKW’s power center—its rhizome—and disables it.

During the plant’s period of peak nutrient flow from leaf to rhizome (in preparation for overwintering), a weak (2-4%) solution of glyphosate “quietly hitches a ride” with the nutrients down to the rhizome—the plant’s nutrient storage center. There, the glyphosate accumulates, disabling the plant at its core. This process of translocation, from leaf several feet down to rhizome, takes about two weeks.

Note that healthy and intact plant material is required for successful translocation. So, a higher concentration of glyphosate would sabotage this process, because it would destroy the plant material before translocation could be completed. Similarly, if the plant is subject to frost less than two weeks after treatment, translocation will be cut short before the glyphosate can reach the rhizome. So a minimum of two weeks of healthy plant material after foliar application is essential for effective treatment.

Flower-to-frost (FF) Treatment Instructions

In late summer (around when JKW flowers) to early fall (up to two weeks before first frost):

  • Mix a 2-4% solution of glyphosate (glyphosate must be the only active ingredient).

  • You can add a bit of blue dye indicator to help you see where you’ve already treated, but make sure the glyphosate still comprises 2-4% of the total solution (contact me for a cheat sheet of the math).

  • Using a spray applicator, cover but don’t soak, the leaves. If you want to protect nearby desirable garden plants, use a sponge to selectively “paint” JKW leaves.

  • Watch for leaves to yellow within about two weeks.

  • If, after two weeks, some leaves are still green, they might have been missed. A second treatment may be applied as long as it’s still at least two weeks before first frost.

  • Monitor annually for continued dormancy, and repeat FF treatment if/when necessary.

Important Safety Notes: when working with glyphosate, wear protective gear; treat in temps no higher than 85ºF; treat on a non-rainy and non-windy day; keep pets and children away; spray at end of day/dusk when pollinators are less active. Glyphosate solution dries within 30-60 minutes, remains active on live tissue for only an hour or two after application, and does not bind to the soil. If treating by a stream or river, use aquatic glyphosate to protect marine life.

Research is ongoing to find bio-control methods, as well as to study post-treatment habitat restoration.

For sources and more detailed instructions on glyphosate application, contact Claude LaVallée (claudelavallee@me.com)

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Main Sources:

Hocking, S., Enhancing Japanese knotweed control and longterm habitat restoration post-treatment (thesis paper held from publishing until Nov. 9th, 2026)

Kathy Thomas, New study counts the environmental cost of managing knotweed, Swansea University Press Office, Friday 17 March 2023

Hocking, S., Toop, T., Jones, D. et al. Assessing the relative impacts and economic costs of Japanese knotweed management methods. Sci Rep 13, 3872 (2023). https://doi.org/10.1038/s41598-023-30366-9

Jones et al. Please don’t mow the Japanese knotweed! 2020.

Jones et al. Sustainable Control of Invasive Japanese Knotweed, Outlooks on Pest Management, Oct. 2019.

Jones et al. Optimising physiochemical control of invasive Japanese knotweed, 2018.

Kennell, Wade, Bacon. Japanese knotweed (Fallopia japonica): an analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension, PeerJ Plant Biology, July 25, 2018.

Special thanks to Hannah Hudson, moderator of the Worldwide Japanese Knotweed Support Group (Private Fb Group).