Effective strategies to control St John's wort

In a previous article, details were provided about suitable herbicides for spot and broadacre spraying. After being involved in a control program on a property with sporadically scattered infestations of wort, I feel it is appropriate to provide more detailed information to ensure control programs are most effective.

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A problem with one-off spot spraying, for example, via a four-wheel bike or via a tractor or ute, is that St John's wort plants are variable in maturity and/or, commonly, some plants are hidden amongst other vegetation. One plant missed this season can be a multitude of plants next year, and seeds are easily spread to form new patches next season. This is a weed that can explode in density, especially in years like the past three with above-average rainfall.

In this particular newly acquired property, which is near some heavily infested private and public land, the aim was to completely kill off existing plants. It looked like previous owners had done a good job in keeping the weed in check, but perhaps it got away a bit in the year of ownership transfer and from nearby infestations.

My control strategy this past summer on this property was to spot spay via a four-wheel bike by up and down travel of paddocks, spotting infested plants and clumps with Grazon Extra at the recommended rate. Each transaction across a paddock was close enough to the previous one to hopefully not miss plants. Slow but hopefully effective.

If necessary, a follow-up inspection and retreatment were carried out 10 days later. A lot of plants had been missed. Treated plants were easy to identify and were well on the way to desiccation. Missed plants were largely later maturing ones that were not flowering at the previous treatment and easily missed in the herbage of largely green summer grass and broadleaf plants. A third inspection and treatment again 10 days further on found a few more plants missed previously.

Another spay issue is that clumps are probably spayed via visible flowering areas. One needs to spay a perimeter around clumps as commonly, later flowering portions of these clumps are shorter and less visible. Perhaps these experiences explain why a lot of properties have had poor success at eliminating St John's wort. Miss an odd plant, and you are back to square one the following year.

Not far from this property is one with outstanding control despite being largely surrounded by a neighbour with a major St John's wort problem and a public area also heavily infested. His success is largely related to through and repeat treatments, including on a public road adjoining the property.

An article on St John's wort in this column last year featured many years of research conducted by Ross Watson, consulting pasture agronomist at Scone. He emphasised that the weed readily spreads via its sticky seed that contributes to spread via adherence to livestock and wildlife. For many landholders, including ourselves, one needs to be on the lookout for this weed every year, especially when plants are visible during flowering.

Key features of St John's wort are bright green plants, 20-80 centimetres tall, bright yellow flowers with five petals, branches and leaves on the stems opposite each other. Good images of the weed can be readily found on the web.

Especially during good seasons, small infestation can develop into a major problem within a few years. Year one, the odd plant of wort. In year two, a few patches here and there, and by year three to five, plants cover extensive areas. The weed is perennial, has deep roots, and seed can be viable for more than one year.

St John's wort can totally crowd out useful pastures, especially in non-arable areas. It can also be toxic to animals and can greatly reduce land value and productivity. Good perennial-based pastures are part of long-term effective control.

Next week: Getting pasture soil nitrogen levels right.