Thorny problems: why won't my wallflower flower?

Incidentally, the mutabile part of the name, meaning variable, refers not to its performance but to the colour change of the flowers, which, as you say, open pale yellow and gradually fade to a mauve-ish pink.

It is an extremely beautiful wallflower, but alas as you and I have found, variable in quite the wrong way.

Mole spurge

Q: Were hoping you can help us. We first noticed a curious seedling when it was about 2in high. It is now some 5ft high and 3ft across with odd green flowers. We have no idea what it is or where it came from, and have been unable to find it in our reference books. Can you shed any light on what it might be, please?

Jill and Derek Watkins, via email

A: The plant in the picture that accompanied your email is called mole plant (Euphorbia lathyris), and is generally regarded as a weed of waste ground. Even if you are enjoying it (from the tone of your email, it seems that initial curiosity has given way to a certain amount of alarm), you should take care around it because the milky sap is a skin irritant. It is a biennial (just leaves in the first year, rapid expansion and flowers in the second, followed by the shedding of masses of seeds and death). If you let it seed around this year, you will get a small forest of seedlings next year that will flower in 2016.

Some people really like this wild plant and encourage it to grow in gardens because it has a certain architectural quality, especially in its leafy first year.

There are those who mistakenly believe that somehow its presence in the garden will deter moles (the in-the-ground type). The truth is that the common names mole plant or mole spurge came about because in former times the irritant sap was used to burn off moles (the on-the-skin kind).

Those nasty caterpillars again

On the July 26 subject of readers nasty little caterpillars (destructive in the foliage of birch and alder trees). Andrew Halstead from the RHS has gently pointed out to me that I have got my bugs and thugs a tad confused. The little black beetles and their offspring on the alder are Agelastica alni, while the loopy caterpillars responsible for the birch leaf devastation are the larvae of geometrid moths. The bit I didnt get wrong was the fact that neither is life-threatening to either tree. My apologies for the error and thanks goes as always to all those observant readers like Andrew, who really do deserve to be called experts.

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Thorny problems: why won't my wallflower flower?