Thistles are growing vigorously in grassland this summer, causing problems in fields that are grazed or shut up for silage, according to agronomists.

The two most common species are creeping thistle - a perennial growing from seed or root sections in the soil - and spear thistle, a biennial that goes unnoticed in its first year, but can grow more than a metre wide before flowering in year two competing with grass for space, said David Roberts of Dow AgroSciences.

“Thistles are tough and easily adapt to the prevailing conditions – be it wet or dry,” he said.

“They need to be actively managed because they won’t go away and will only get worse.

“Many farmers top thistles – but to be honest this is a waste of time, fuel and labour. Some plants will be missed, and those that are chopped will soon re-grow, coming back even stronger.

“Spraying with a modern herbicide, which has been specifically formulated to work against them, is the only way to gain long term control.”