Control

Management and Control

Persistence is the key when attempting to eradicate populations of invasive plants, including giant hogweed. Most reproduction is by seed, though side shoots from the root crowns can also initiate new plants. Established populations will presumably have built up a seed bank (a reserve of long-lived seeds in the soil) along with a "rosette bank" of immature plants. The seeds of giant hogweed are reported to survive for as long as 7 years when stored dry (Morton 1978), though the vast majority germinate within a year (Tiley et al. 1996). Removing any ripe seed from the site, or (better yet) preventing ripe seed from being produced, will greatly improve the effectiveness of any treatment program. Once the patch is eliminated, the site should be monitored at least annually, and any new seedlings should be removed. Dodd et al. (1994) recommend monitoring the site for 7 years, though 4-5 years of treatment are usually enough to eliminate a patch if the plants are not allowed to reseed.

Caffrey (1999) outlines a step-by-step strategy for controlling or even eliminating giant hogweed from a watershed. While designed for the (often quite extensive) infestations in Great Britain and Ireland, his strategy should easily transfer to sites in North America.

Below are some control methods that can be used against giant hogweed. But because giant hogweed is so potentially dangerous to work around, and the patches so hard to eliminate, government agencies dealing with this plant are asking that people contact them before attempting to eradicate it. They may be able to visit the site and help you with eradication free of charge. See "Giant Hogweed Reporting" at the bottom of this page for more information.

WARNING: Skin contact with giant hogweed sap in the presence of sunlight will lead to severe burns and blistering. Burning is a risk even on cloudy days, as some light still passes through the clouds. Protective waterproof clothing must be worn when working around giant hogweed, including gloves, boots and a shield to protect the face (Dodd et al. 1994). Waterproof clothing must be worn to prevent the copious sap from soaking through it and into the skin. And care must be taken to avoid wiping your eyes, face, etc. with your gloves as this can lead to burns as well.

Manual Control

Small patches or individual plants of giant hogweed can be controlled manually. Treatment should be done before the bolting plants set seed. Seedlings can be pulled out, especially in light or moist soils (wear gloves!). Larger plants can be dug up, though this method may be too time and labor-intensive if there are more than a few. Modest-sized plants can presumably be killed by cutting through the taproot about an inch below the crown, using a sharpened, long-handled shovel. Chopping the crown of the root below the base of the stem with a shovel or a mattock (or an ax) will reportedly kill vegetative or flowering plants also (Tiley et al. 1996).

Mowing or cutting alone is usually not recommended as a control method for immature (rosette) plants, as the plants quickly resprout from the taproot. While repeated cutting within several inches of the ground every week or two will eventually weaken these plants, their taproots hold considerable food reserves and frequent cutting for several years is required to kill them (Dodd et al. 1994). If there is any letup in this regime, the plants will quickly recover. Infrequent mowing can make the problem worse, by eliminating competing plants and spreading any seeds that might be present on the site.

If properly timed, cutting can be an effective control method for mature giant hogweed plants. Plants that are cut while bolting but before flowering quickly send up new (though shorter) stalks. If the stalks are cut after the seeds have begun to develop, the food reserves in the detached stalk may be sufficient for the seeds to ripen anyway. But cutting flowering plants near the ground JUST AFTER they have begun to flower but BEFORE seeds have started to form may (because food reserves are depleted and because of physiological changes within the plant) prevent them from reflowering and end their life cycle (Dodd et al. 1994). Even if cutting is properly timed, caution would suggest that the patch be monitored in case bolting does reoccur and a second cut turns out to be necessary.

Giant hogweed is intolerant of anything that severely damages the root crown and attached buds. Thus plowing can be very effective in eliminating giant hogweed from agricultural fields (Dodd et al. 1994).

IF there is little chance of people coming in contact with the plant parts, parts other than seeds can be left on the site to decompose naturally (Dodd et al. 1994). Ripe seed should be bagged and removed from the site, and either burned or disposed of in a way that insures that the seed will not end up somewhere else where it can grow. (And remember to wear gloves when handling the seeds, as they also contain furanocoumarins!)

Chemical Control

Herbicides are most effective against giant hogweed when applied early in the season - plants sprayed just before flowering sometimes survive long enough to produce viable seed. Even plants sprayed early in the season may survive for a few weeks. In the fall, leaf vigor is too low for herbicides to have much effect (Tiley et al. 1996).

Glyphosate, triclopyr and imazapyr have all been recommended for controlling giant hogweed in Britain (Tiley et al. 1996). The nonselective herbicide glyphosate (Roundup, or Rodeo for wet areas) is considered to be the most effective on giant hogweed (Caffrey 1999, WSDE 2004). Imazapyr is effective against the adult plants and remains in the soil for several months. This has the advantage of preventing germination of remaining hogweed seeds (Tiley et al. 1996), but presumably also limits the recolonization of the site by surrounding vegetation during this time. 2,4-D, TBA, MCPA and dicamba are reported to kill the aboveground parts but are not very effective on the persistent rootstalks (WSDE 2004).

Isolated plants should be spot-treated, to avoid spraying nearby vegetation (Dodd et al. 1994). This allows the nearby vegetation to recolonize the spot vacated by hogweed and to suppress emerging seedlings. Dodd et al. (1994) list several alternatives to spray application, including using a sponge on a long handle to apply herbicide, and cutting bolting plants in the spring and filling the hollow stem with herbicide solution (effective though labor-intensive). Herbicides should be applied on calm days, when no rain is expected for at least 24 hours. Applying herbicide to the undersides of leaves helps keep it from being washed off if it does rain.

Because of the danger associated with working in dense stands of hogweed, and the fact that the large leaves of mature plants tend to protect the seedlings below from the spray, Dodd et al. (1994) recommend cutting or mowing dense patches first (perhaps with a farm tractor and power mower) before treating them with herbicides.

It is important to always follow label directions when using herbicides to control giant hogweed or other invasive plants.

Cultural Control

The best way to keep a natural site from being invaded by giant hogweed is to avoid disturbance to the existing vegetation. If giant hogweed doesn't occur in your area yet, the chances of invasion in the near future are of course very small.

Because of its ability to quickly resprout, fire will presumably have little or no direct effect on giant hogweed populations. Fire may be useful in some habitats as a pretreatment, though, as it is for wild parsnip (Hoffman and Kearns 1997). Like wild parsnip, which has a very similar life history, giant hogweed would presumably quickly reemerge after burning, making it easily visible on the blackened ground. Plants could then be dug, pulled, or spot-sprayed, before most competing plants reemerge. Restoring natural conditions including periodic burning (typically every 2-3 years) to prairies and other fire-dependent habitats may also favor the growth of native species, helping them to outcompete giant hogweed. As always fire should only be used in fire-adapted habitats, or in fire-tolerant disturbed areas such as pastures.

Biological Control

Cattle, pigs, sheep and goats all can eat giant hogweed with no apparent adverse effects (Tiley et al 1996), and the plant has even been grown for silage in Russia (Khodyrev et al. 1980). Pigs may potentially be the most effective because they also dig for and eat the roots (Tiley et al 1996). An enclosed pasture formerly dominated by hogweed was completely freed of the plant after being subject to seven years of grazing by sheep (Vogt Andersen and Calov 1996). Germination tests indicated that no viable seeds were left in the soil of this pasture after this time. Plant diversity within the pasture was significantly reduced, though. Grazing by cattle and sheep may be ineffective in spring and early summer, when fresh grasses are available.

Giant hogweed is an acceptable host for the parsnip webworm Depressaria pastinacella (Duponchel 1838). This European insect, really not a worm but a moth caterpillar, was accidentally introduced to North America over 100 years ago. Its primary host over much of its European and North American range is a relative of giant hogweed, wild parsnip (Pastinaca sativa). It hatches in early spring, and begins feeding on the growing shoot tips and then the newly forming seeds. As it grows it spins a web that helps protect it against predators. It is able to feed on giant hogweed (and wild parsnip) because it can break down linear furanocoumarins, which are highly toxic to most other insects.

Other insects, including the caterpillar Depressaria daucella (Denis and Schiffermüller 1775) and the weevil Liophloeus tessulatus (Müller 1776) feed on giant hogweed in Europe (Tiley et al 1996). There seems to be no evidence that these insects have significantly limited the abundance and spread of giant hogweed there, though. The Centre for Agriculture and Biosciences International in Switzerland is currently searching for biological control organisms for giant hogweed (CABI, no date).

Giant Hogweed Reporting

Giant hogweed is a federally-listed noxious weed, meaning that its sale and possession are illegal in the US. Giant hogweed is a major threat to wild lands and a significant health threat to humans and some animals as well. If you spot giant hogweed on the landscape, please inform your state Department of Agriculture (or GLIFWC):

In Michigan:
Michigan Department of Agriculture
Pesticide and Plant Pest Management Division
PO Box 30017
Lansing, MI 48909
(517) 373-1087

In Wisconsin:
Wisconsin Department of Agriculture, Trade, and Consumer Protection
Plant Pest Survey and Control
2811 Agriculture Drive
PO Box 8911
Madison, WI 53718-8911
800-462-2803