Colorado Master Gardeners: Decorative ground cover or weed?
August 30, 2016
A cultivated plant that has caught my attention in the garden I inherited a few years back is called snow-on-the-mountain, goutweed or bishop's weed. It is a pretty ground cover that provides visual interest for much of the summer but gets rather shabby and thirsty looking by August.
After some research and using David Whiting's weed definitions (plants growing where they are unwanted, visually unattractive plants, plants that pose a health or safety hazard and plants that displace more desirable plants in the garden), I will decide whether the plant is a ground cover to encourage or an weed to remove.
Bishop's weed (Aegopodium podagraria) is part of the carrot family and native to Europe and Asia. It has a long history of edible and medicinal uses, including as an anti-inflammatory, a sedative, a diuretic and a poultice for gout and a number of other ailments.
It was brought to North America as an ornamental plant with the European settlers and is a low-growing plant with green or variegated leaves grouped in threes. The plant blooms mid-summer, sending up tall stalks (up to 3 feet) of compound umbels with small white flowers. CSU Extension lists it as a suitable ground cover for shady large areas as it is low growing, spreads by itself and inhibits weeds. More information is available at extension.colostate.edu/topic-areas/yard-garden/ground-cover-plants-7-400.
The shabby look of my bishop's weed in August may be enough to label this plant a weed, but it may also be a case of right plant/wrong place, as it is growing in a partly sunny and dry location.
My bishop's weed grows into a 12 -nch high mat of attractive, variegated leaves that are bluish-green with creamy white edges, followed by a few tall flowers. Then, it becomes very thirsty looking and lies down unless watered.
It also develops unattractive rust spots on the leaves as the summer progresses, which is a common problem. Gardeners are urged to mow down the ground cover at this point, as it improves the appearance and does not affect the plant's health.
Last fall, I grew tired of looking at the thirsty ground cover and dug up the bishop's weed in the sunnier areas. Bad idea. The plant spreads by way of underground rhizomes, and when digging it up, a root of any size left behind can generate new growth. Once established, the plant is aggressive and invasive and may choke out other plants, shrubs and even trees. In some places, bishop's weed has been labeled one of the "worst" garden weeds in perennial flower gardens.
Needless to say, after my research, I was frightened to survey the damage in my garden. My digging had, in fact, spread the plant, but neighboring plants, shrubs and trees were still OK.
It may not be as invasive in our climate as it is in other areas, but since it is unattractive for part of the summer and may displace other plants over time, I will accept that bishop's weed is truly a weed.
And as it is growing where I am encouraging native plant growth, it needs to be removed.
Unfortunately, it appears that removal is a long process. I will not use chemicals but will instead cut down all growth and cover the area of concern with cardboard and compost, blocking the sun and encouraging worm activity.
Next spring, I may try solarization, another technique that kills weeds. More information can be found at extension.colostate.edu/topic-areas/agriculture/soil-solarization-an-alternative-to-soil-fumigants-0-505.
Finally, I will watch for new growth and dig out roots. Complete removal may take several years but will allow other plants to grow and flourish. Wish me luck.
A longtime Steamboat resident and casual gardener, Vicky Barney is a member of the Master Gardener Class of 2011.