Getting rid of thistles

UK - February 11, 2009

A 10% infestation of thistles in a sward, (i.e. 20 plants in a 35 m² area) will reduce grass production by 1.1 tonnes of dry matter per hectare.  At 40% infestation the loss rises to 4.4 tonnes of dry matter per hectare - an incredible waste of a valuable home-grown feed resource.

There are 20 species of thistle growing in the UK.  Of these the two most common and damaging in grassland are creeping and spear thistles.  Over one million hectares of grassland in this country are estimated to be infested with these pernicious weeds.

They most commonly occur in older swards; on soils with low levels of phosphate and nitrogen but high levels of potash. Over and under grazing can encourage thistle colonisation.

As with all weeds, thistles compete strongly with grass for water, light and nutrients, and the grass in infested areas will not be grazed.  IGER research in the 1990's showed that cattle and sheep tend to leave 30 cm around each individual plant.  Their presence in hay and silage lowers feed quality and utilisation.

Creeping thistle is a perennial plant which grows from seed or root sections.  The seed can survive for up to 21 years in soil and is mainly spread by wind.  Mechanical topping reduces seed production, but not root spread.  Topping after the flowers have set will not stop seed maturing.

Roots of one plant can grow to cover up to 5 m² in one year, and up to 80 m² in two years.  The weight of the underground root system can be as much as two tonnes/ha.  These large underground systems allow thistles to withstand adverse conditions, such as drought, much better than grass.  To gain long term control of such large root systems requires treatment with a fully translocated herbicide.

The spear thistle is a biennial plant which grows from seed.  In the first year it can often go unnoticed, since it produces only a  small rosette.  In the second year the plant can grow to over a metre in diameter before flowering.

Spear thistles are totally dependent on seeds for reproduction.  Unlike creeping thistles the plant does not have extensive lateral roots, but does have a very long tap root.

Each flower head produces 200-400 seed heads, and each seed head can product 4,000 to 8,000 seeds, which remain viable for more than three years.

Loss of grass production is not the only good reason for controlling thistles.  On sheep farms they are a danger to lambs, as their spikes can act like dirty hypodermic needles to animals that brush past them - breaking their skin and spreading diseases such as orf within the flock.  In this case, getting rid of thistles also cuts the cost and labour of inoculating lambs against infection.

There is an old saying about thistle control "Cut them in June, they come back soon;  Cut them in July, they will soon die."  Unfortunately this is one 'old wives tale' that is completely misleading.  In fact,controlling them when they have a flower head - with a topper or weedkiller - is often the worst time to try.  If they are topped, seed will continue to mature on the ground.

Lasting control requires herbicide treatment when the thistles are young and actively growing with little stem.  This is known as the rosette stage, when there are four to ten leaves showing.

MCPA has been used in the past but many farmers have found the results disappointing.  This is because it only affects the biomass above ground, with little overall impact on the root systems below.  This means that typically the same fields will need repeat spraying year after year.

Thistlex, from Dow AgroSciences, works differently.  Applied at 1 l/ha, it offers a targeted, cost effective solution which actually tackles the problem underground.

Spraying in May or June is ideal, as long as the thistles have reached rosette stage.

If thistles have developed beyond this stage, it is better to top them and spray the re-growth six to eight weeks later.

Where there is a 10% infestation of thistles, using Thistlex for just one season can reduce the population to a level where only knapsack applications are needed to keep the sward completely clean the following year.

For more information on thistle control contact the Dow Hotline on tel: 0800 689 8899.