Is spurge harmful to other plants?

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Q: I have a lovely clump of spurge growing in my flower bed; I’ve contained the offshoots with edging and pulling out new shoots as they appear. This year, the plants on either side (Pernettya mucronata and Rhododendron ‘Carmen’) are drying up..I suspect the spurge might be responsible. Does it exude a toxic substance which could be affecting its neighbours? Help!

Members of the spurge family (Euphorbiaceae) have sap that contains latex, but I’ve not heard of it causing harm to plants nearby. At the Northwest Flower & Garden Show in 2007, I attended a terrific (and hilarious) talk by Timothy Walker, who curates the National Euphorbia Collection at the Oxford Botanic Garden in England. He also gardens with this genus at his home garden (and is the author of the Wisley Handbook “Euphorbias.”) He told us what great garden plants spurges make, so I don’t think that your spurge could be killing nearby plants.

Perhaps these plants are not compatible in terms of their soil needs. Spurge likes hot sun and well-drained soil, while pernettya (which is now in the genus Gaultheria) and rhododendrons are both in the heath family and have very fine roots that dry out easily. Both like plenty of organic matter and supplemental water in summer. Maybe they are simply too dry. Please check out the root zone to see how dry it is, and soak plants deeply with a slow drip to see if they respond.