The combination of their sharp spines on their leaves and being unpalatable means they will reduce the grazing area on your pasture. Animals will not travel near them so they will seek out other areas to go. This can lead to overgrazing and increased traffic to other areas.
Once they have fully established in a field it is very hard to get rid of them. It will take many years as well as the combination of different methods to really get them under control.
What are pasture thistles?
When we are talking about pasture thistles we are not talking about just 1 particular plant but rather a whole group of them. This “tribe” of plants are closely related to the sunflower family.
Typically they are a biennial or a short-lived perennial plant. They reach maturity in their second year at which time they produce seeds.
They start off by forming a low rosette of spiny leaves that can be up to 1ft across. It is during this stage of growth when it is easier to get rid of them. Herbicides are only effective during this stage, after that you become limited on your options.
In the 2nd year it will become anywhere between 2-8ft tall forming occasional lateral branches. The leaves will alternate and are up to 9in long by 3in wide being green in color on the top and a whitish color underneath.
This plant will produce 1 flower per stem that can range from pink to purple (rarely white) in color. It is in this flower that the seeds are grown. Though the flower is scentless many species of butterflies like them as a food source.
The seeds can live for a long time underground, some types up to several years. This can make it hard to know if you will have an infestation or not.
Though there are any different varieties of thistle some of the most common that you will come across here in Texas are:
- Bull Thistle
- Distaff Thistle- I go into more detail about this type in another article here.
- Texas Thistle
Impact of Pasture Thistles
As mentioned earlier if left uncontrolled thick stands can overtake an area reducing grazing. Their spiny leaves deter movement for both wildlife and livestock. I mean you wouldn’t like to travel through an area where you are always being stuck with needles would you?
Because of this it will make your animals go to other areas of the pasture. This can cause overgrazing in those areas which will cause an increase in soil disturbance. Soil disturbance is a requirement for the seed to germinate and begin to grow.
A single thistle plant can produce over 4,000 seeds that will be spread easily by the wind. As you can imagine this makes it easy for it to spread over a large area.
Like most weeds they will compete with and choke out more desirable pasture grasses. So you can see that getting these things under control early is very important.
When to control
Courtesy of foragefax.tamu.edu
Best time to control pasture thistle is when it is in it’s early, or rosette, growing stage. When it is in this stage all of the options (mechanical, chemical, and manual) are available to you.
Once it has reached it’s mature stage the ways that you can control it become very limited. Using herbicide is no longer an option at this stage. Also any control method you use you will need to make sure that the seeds in the flower are not able to spread anywhere.
If it is already established it will take several years to get it under control. Which is why it is better to get it taken care of before it can take hold.
The best time for control of pasture thistle is during it’s growing stage from late winter to spring. This occurs typically between November to April.
Methods of control
Though there are different types they all have similar control methods:
- Manual:
- This is the least efficient method
- It consists of hand pulling, hoeing, or cutting each individual plant
- One thing to remember is the root needs to be severed 2 to 4 inches below the soil surface
- Possible to use this method during the plant’s mature stage but will need to clip the flower off and store it in a plastic bag to prevent the seeds from spreading
- Mechanical
- This can be done by things like mowing, tilling, and disking
- Method is useful before the plant seeds otherwise you will just cause it to spread
- If tilling is used plants should be cultivated before they reach a height of 3in
- Mowing is best when used combined with other strategies since it may not entirely eliminate flowering and seed production
- Herbicide
- As mentioned earlier this method is only viable when the plant is in it’s rosette stage
- Some herbicide options that you can use:
- Weedmaster
- 2,4-D
- GrazonNext
- Remedy
- PastureGard
- CimarronMax