Goutweed

Two varieties of goutweed exist, variegated and non-variegated. The leaves of the non-variegated variety are solid green throughout, while the leaves of the variegated variety are outlined in white/cream-coloured margins with a blue-green base. In some instances, variegated goutweed can revert to the wild type (solid green leaves). The upper leaves of goutweed are compound and ternate (divided into 3 groups of 3 leaflets each). Individual leaflets are oval, and their margins are coarsely to finely serrated. Oftentimes, leaflets are not fully detached at the base, resulting in a mitten-like appearance. The lower leaves are doubly compound and twice ternate (9 leaflets per leaf). Most of the leaves are basal (connected to the lowest section of its stem), except the petioles (leaf stalk) which are connected straight to the rhizome. Larger plants may have fewer leaves and an alternate leaf arrangement along the stem.

Flowering stems are deep green, vertical standing, hairless, hollow, and 0.4 m to 1 m tall. The white flowers are organized in flat-topped umbels (umbrella-shaped clusters of short-stalked flowers) 6 – 12 cm in diameter. Each umbel is divided again into 10 – 20 divisions called umbellets, and each umbellet can harbour 15 – 25 rays of individual flowers. The individual flowers have 5 white petals and measure 3 mm in diameter. These flowers typically bloom from June to August, occasionally as early as May in specific regions. Plants in shaded areas with limited sunlight may produce fewer flowers. Each flower develops a small, brown, oblong, flattened seed known as a schizocarp, measuring 3-4 mm in length.

Common look-a-like species for goutweed include wild carrot, Canada honewort, smooth sweet cicely, hair sweet cicely, and golden alexanders.

Outside of its native range, goutweed is a highly aggressive species capable of overrunning closed-canopy forests, displacing habitats, and reducing native biodiversity. The nonvariegated variety of goutweed is believed to be more problematic than the variegated type as the solid green leaves have a higher photosynthetic rate and allow the plant to spread more expansively. Goutweed is a habitat generalist, meaning it can thrive in a wide variety of soil and light conditions and is highly adaptable to many different habitats. It is highly shade-tolerant and can even invade closed-canopy forests. It can take over fields, trails, tree lines, pastures, and disturbed sites where it produces leaves earlier than many native species, giving it an advantage. Goutweed can also reduce the survival rate of native tree and shrub seedlings by increasing the humidity and reducing sunlight penetration near the herbaceous layer of the forest floor. In addition, goutweed’s flowers attract pollinators, altering native plant-pollinator interactions, and possibly decreasing the pollination of native species.

In its invasive range, goutweed can be encountered in Canada’s Atlantic and central areas, with established populations in Prince Edward Island, New Brunswick, and Ontario. In the United States, goutweed is present from Maine down to South Carolina and extends westward to Minnesota and Missouri. Additionally, it can be found in the Pacific Northwest spanning from Montana to Washington and Oregon.

Goutweed is an extremely resilient plant, and once established, can take multiple years to effectively eradicate from an area. Goutweed spreads rapidly through a network of rhizomes, allowing it to extend up to 70 cm per year from its parent plant. The rhizomes easily break, giving rise to new plants from the fragmented pieces. These fragments can survive in the soil for over four years! Prevention is the most practical and cost-effective defence against goutweed establishment. Avoiding the planting of goutweed, especially near natural areas and preventing its spread before it becomes locally established will reduce its impact on biodiversity, the economy, and society. 

When dealing with established populations, various control methods have shown effectiveness, including manual, mechanical, cultural, and chemical approaches. When managing a small area, hand-pulling stands out as the simplest and most efficient method, especially when the soil is loose and moist. To prevent setbacks, focus on small sections at a time and ensure thorough removal of all rhizomes. Neglecting to remove root fragments can promote reproduction and worsen the spread. When managing larger sites, manual pulling and digging may be less efficient, as it becomes challenging to thoroughly eradicate all existing rhizomes. 

Consistent mowing near the ground can limit the spread of goutweed, but it might not effectively manage extensive patches unless combined with another approach. Mowing and cutting does, however, help remove goutweed leaves, hindering photosynthesis and reducing its growth. The optimal time for mowing is early in the year (April – May) when the plant has grown mature leaves but not yet gone to seed. While goutweed typically reproduces through rhizomes, it’s best to avoid cutting it late in the season to prevent seed dispersal.

Tarping is another method that has been demonstrated to effectively prevent goutweed photosynthesis. Tarping is the most practical during the early spring when the plant has fully developed leaves and can help control the plant by depleting its carbohydrate reserves. Tarping can be done with any solid/opaque material such as landscape fabric, black plastic tarps, sheeting or cardboard. 

All control measures should be carried out cautiously to minimize soil disruption and prevent damage to neighbouring native vegetation. Managing goutweed can be challenging, and it is essential to regularly monitor and control sites over multiple years for successful eradication.

For a more in-depth guide to managing goutweed, visit Ontario Invasive Plant Council – Goutweed Best Management Practices