It won’t grow in bogs because the soil is too acidic, but it will flourish in fens and marshy places where there is a reasonable amount of lime in the soil.
Its tall spikes of reddish purple flowers are at their finest in July and are looking spectacular right now. As a fisherman I have cursed them from time to time because they seem to have an uncanny knack for tangling an angler’s tackle, but as an amateur botanist I have nothing but admiration for such an elegant plant.
It has evolved an extraordinary strategy for ensuring that it doesn’t weaken its genetic make-up by self-pollination. There are three different types of flower, each with a completely different arrangement of stamens. Any individual plant only has one of the three flower types and can only be pollinated by one of the other two types, so its vigour is always preserved by cross-pollination.
But plants outside their natural context can cause problems. Purple loosestrife has appeared in parts of the United States and in New Zealand, possibly introduced by foreign ships discharging ballast water. Seeds or root fragments could have been pumped in with ballast water in a European port and pumped out in American and New Zealand ports.
In these new homes, without any natural ecological controls, it became invasive and threatened to choke up important watercourses. The main reason this doesn’t happen in Ireland is that purple loosestrife is kept in check by a number of specialised and very efficient insect predators.
There are known to be two species of beetle, two species of weevil and one species of moth that feed virtually exclusively on purple loosestrife and control its spread. In America, the first thing they tried when it started to become a problem was to control it mechanically, by cutting and removing it. When this didn’t work they tried chemical control, spraying it with herbicides. Not only was this equally unsuccessful, it had some very undesirable environmental repercussions. Using toxic substances in or around water is always problematic.
Then the scientists looked to Europe. They decided the moth with caterpillars that ate purple loosestrife was itself a potential pest, so they left it alone. But they imported the beetles and the weevils and they did an excellent job. It’s one of the classic success stories of biological pest control.
Rather ironically, the name ‘loosestrife’ seems to refer to the plant’s ability to reduce stress, problems and conflict. It was certainly quite popular with ancient herbalists and was also used in the treatment of dysentery and diarrhoea, particularly in children. And rather confusingly the name loosestrife is also applied to some other unrelated Irish plant species which, presumably, were believed to have the same properties for reducing strife.
Yellow loosestrife is a member of the primrose family and totally unrelated to purple loosestrife, though it does grow in the same sort of damp habitats. It is not as common as purple loosestrife in this country but I did find quite a lot of it growing along the banks of the north Shannon recently.
Both purple and yellow loosestrife are such spectacular Irish wildflowers that the horticulturalists have bred several garden varieties. If you want to look for them purple loosestrife is genus Lythrum and the yellow is genus Lysimachia.