In last weeks column I reported that I had been horrified with the veritable witches’ brew of six or seven of the most invasive weeds that a gardener can imagine in two new large perennial beds that had been filled with soil last fall. Instead of a beautiful loamy planting bed I was astonished to observe a cocktail of burdock, sow thistle, Canada thistle, Japanese Knotweed, quack grass, Japanese lantern and colt’s foot.
I was pleased to receive a comment on the article – writing a column can be a lonely pastime at times – it is very nice to get feedback. I am also touched by people in the community that mention that they enjoyed my column when they run into me in town (and I apologize if I don’t always remember names).
The comment, in part, was that:
“All bulk soil is subject to contamination by windblown weed seeds….By the time one takes delivery of bulk soil it contains a witches brew of the seeds of dandelion, plantain, ragweed, thistle, you name it, it’s got it. An experienced gardener should not be surprised at the discovery of large weeds growing from soil left last fall in an open perennial bed.”
I stand by my view that I cannot fathom where a landscaper would have been able to find the soil that I observed. I certainly agree that weed seeds will blow in and I would not be surprised to see annual weeds such as Lambs Quarters and pigweed and ragweed and seedlings of perennial dandelion and perhaps even Colt’s Foot. But I cannot concoct a scenario where all of these thugs that I observed would be found growing together in new soil. Many of the most invasive plants spread primarily by underground roots or rhizomes and I believe that such a healthy stand of invasive plants could not have been established by seed alone. Not all of the plants I observed spread readily by seed, for example Japanese Knotweed seeds are too heavy to be spread by wind.
My advice to exercise caution when ordering soil remains – know your supplier and their reputation.
When I’m leading workshops with beginning vegetable gardeners I always stress the importance of knowing what types of weeds you are digging out when you are preparing your planting beds. For example, dandelions have a long tap root – if you are able to get the whole root the plant will not come back – however any piece of root left in the ground will regenerate. Perennial grasses are difficult to eliminate as they have long horizontal roots that may stretch half a metre or more. These are best removed with a spading fork. A rototiller will break those roots into little pieces, every one of which will send up a new plant.
The following photo shows what this looks like. The whole area was rototilled and growing beds were prepared but the roots of the perennial grasses had not been eliminated. Now they are back with a vengeance.