Information contributed by Dr Ian Popay at Weedwise
How many kinds are there?
- Thistles were already a problem in New Zealand by the 1850s. They were probably introduced accidentally by early farmers in seed mixtures, and are now a nation-wide problem. There are over a dozen species of thistles in New Zealand, but those listed here are the commonest:
- Californian thistle (Cirsium arvense)
- Scotch thistle (Cirsium vulgare)
- Nodding thistle (Carduus nutans)
- Plumeless thistle (Carduus acanthoides)
- Variegated thistle ( Silybum marianum)
- Winged thistle (Carduus tenuiflorus)
- Slender winged thistle (Cardus pycnocephalus)
How do they grow and spread?
- Californian thistles are different from the others - although the aerial parts die down in winter, the plants survive by their extensive underground root system. Because of this all the individuals in a patch of thistles are all exactly the same plant - and that plant is either male or female! It’s very rare for Californian thistle seedlings to grow successfully in pasture.
- All other thistles are annuals or short-lived perennials that grow from seed.
- Annual thistles germinate in autumn or winter and most species flower the following summer.
- Scotch thistles in pasture germinate in autumn or winter but they don’t flower till the second summer. Some nodding thistles also behave this way, but most flower in their first summer.
- Although you see thistledown everywhere in the autumn, most seeds fall off the thistledown quite close to the parent plants, so that the actual seeds don’t move very far.
- Thistle seeds love to germinate in the spaces that open up in pastures over the summer.
- Lax grazing over summer helps pasture grasses recover in the autumn, and hard grazing in autumn can sometimes help control thistle seedlings.
- Be careful: you can buy in seed from hay or silage, and on contractor’s machinery.
What harm do they cause?
- They reduce pasture production by taking the place of grass and clover.
- They reduce the area that stock can graze in a paddock.
- When they are flowering, they reduce stock movement in the paddock and make mustering difficult.
- They can increase scabby mouth and parapox in sheep: these are viral diseases that infect the sheep through punctures on the lips and mouth.
- They lower the market value of wool.
- Try to keep a good continuous pasture sward throughout the year.
- Don’t let pastures become open - new seeds love this.
- Avoid over-grazing in summer - this will open up the pasture to drought.
- Mowing thistles lets stock get to the pasture and may kill some thistles, especially if it’s done before any seed is set.
- Stock will be keener to eat cut or crushed wilted thistles than fresh ones.
- Some people say that cutting Californian thistles in the rain lets water get into the hollow stems and rots them.
- Some people claim that spraying thistles with molasses makes stock eat them.
- The best way of controlling Californian thistles is to mow at a low level just as they start to flower (at Christmas-time), and then to intensively mob stock with sheep at monthly intervals through the summer. Keep it up for a couple of summers if you need to.
- Alternatively, crush Californian thistles to cause wilting and follow up with repeated mob stocking.
- Goats love to eat thistles, but not until the thistles start flowering. Combining thistles with cattle is a great way of controlling thistles and keeping the pastures in good condition.
- Hand grubbing is effective for large individual thistles. To make sure you kill the plant you need to chop the top couple of inches of the tap root off.
- The nodding seed-head weevil (Rhinocyllus conicus), the crown feeding weevil (Trichosirocalus horridus) and the gall-forming fly (Urophora solstitialis) have all been released to attack nodding thistle. The seed head weevil can reduce seed production by 50%, but many seeds still survive.
- A fungal disease based on a fungus (Sclerotinia schlerotiorum) is being developed for Californian thistle, but applying it at the right time is likely to be difficult.
- Early spring (September) is probably the best time to spray nodding thistles.
- Before spraying, graze the pastures to expose the thistles.
- Wait at least 14 days after spraying before grazing again.
- Spot spraying of individual plants can be done with knapsack, hand gun, hand lance or drench gun fitted with an extended lance. Granules are also available. Several herbicides can be used for spot treatment, but many of them damage other pasture species and leave bare ground that is reinvaded by weeds.
- Carpet rollers and weed wipers can be used to apply some non-selective herbicides to flowering thistles without damaging the pasture. Graze hard before treatment. Check with your supplier for the appropriate chemicals to use.
- Spray Californian thistles just before they flower, and then graze heavily at monthly intervals. This crushes or grazes out the newly emerging soft, young plants.