Your gardening questions answered: What is my mystery plant?

Q: I have an unknown plant in my garden. I’m not sure if it came in on the wind or bird feeder. I first noticed it two years ago. It never flowers, it self-seeds and it grows to about 110cm. Please advise any details on its name and care. Hopefully it is not some invasive weed. DD, Co Dublin

A: Your mystery plant is a member of the euphorbia genus known as the caper spurge, the mole plant, or more properly as Euphorbia lathyris. A non-native annual or biennial species often described as a garden escape or a weed, it’s technically not classed as invasive in this country, but is certainly known for its tendency to self-seed about the place, quite often appearing on recently disturbed or cultivated ground as if out of nowhere.

There are any number of ways that it arrived in your garden, from the seed hitching a lift (Trojan horse-style), in the compost or soil of another plant purchased or given to you, a bird dropping it, or it being brought in unwittingly on the muddy soles of someone’s shoes.

Its “flowers” are the lime-green, triangular bracts as seen in your photograph, which are followed by plump green seed capsules which were sometimes used as a substitute for capers, hence the plant’s common name. As is true of all parts of this plant, these are highly toxic and can only be made edible by steeping them for a long time in salty water and vinegar, and even then, great caution is advised. Its other common name, the mole plant, comes from the belief that its poisonous roots deterred moles from digging anywhere near the plant. Like all members of the euphorbia genus, its stems contain a sticky, milky white liquid known as latex that’s a painful skin irritant and can badly hurt, burn and even permanently damage your eyes if you happen to get it on your hands and then inadvertently rub your face. For this reason, it should only be handled with great care and needs to be kept out of reach of young children.

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Although it’s in many ways a handsome, statuesque plant, caper spurge is likely to become more abundant in your garden over time unless controlled, especially as the seed pods “explode” once they’re ripe, a clever seed dispersal mechanism that allows the plant to fling them far and wide. To eradicate it entirely, you’ll need to keep pulling up any young seedlings that you can find (wear gloves and a long-sleeved top to protect your hands against the latex and take care not to touch your face). Alternatively, if you like it but just don’t want it to make itself too much at home in your garden (or others nearby), then keep culling the vast majority of young seedlings before they flower and set seed.