Floating Pennywort: Part 1: why is it so invasive?

Earlier the year I attended an excellent workshop about Floating Pennywort hosted by Cam Valley Forum & Cambridge Conservation Forum. Below are some of the facts I gathered about this menace which is plaguing the Cam, Ouse and various Lodes in the area. Part 2 will be about the control methods currently in use and what may be available in the future.

Why is it such a pest?

All down to biology ….

  1. The plant has an amazing ability to reproduce itself, not by making seeds but by shedding parts of its stems which contain growth points called nodes – these nodes have the ability to start growing roots and shoots all over again. It is estimated that a square metre mat of the plant contains approximately 2,300 nodes, all of which are capable of regenerating new plants. The broken off plant parts can float pwort node
You can see the roots starting to grow at the nodes along the stem.

Photo credit: GB NNSS

2.It was bred by the aquatic nursery trade to be a vigorous spreader – which they successfully achieved. It can easily out-compete most native plant species.

3. It is not fussy about nutrient requirement and has no limiting nutrient level. Whatever nutrient it can find it tends to scoop up and high levels of toxic elements such as mercury have been found in plants in the Netherlands

4. It is frost tolerant and needs a good hard spell of freezing weather to stop growth. Frost will kill the top growth but the growth underwater may survive.

5. It doesn’t move (translocate) herbicides well to its leaves and stems and to really kill it off the apical (end) bud needs to be sprayed. A plant will survive a spray if the apical bud is under the water or is “hiding” on a non sprayed bank side. It can take up to three weeks for glyphosate (the herbicide used in control) to take effect.

6. In the south of the country it has started to set seeds; this could be due to warmer weather or more worryingly, species adaptation.

Next week: how it spreads and how it is currently being controlled in Cambridgeshire

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