Spring is the season to go after thistle in the orchard
By this term, I am referring to how these weeds are introduced and spread into an orchard. Like dandelion, the seeds are light with a fluffy umbrella pappus structure that is easily dispersed by wind. Seeds can be blown from adjacent fields and may become established in specific spots (“clumps”) in the orchard. Once the weeds are established most lateral spread is through the creeping and extension of underground roots. Once in the orchard and allowed to flower their population can “jump” to other spots in the orchard if they are allowed to flower and go to seed.
First or early emergence is an ideal time for application. However, there are some things to consider. How well is the thistle established? If it is part of an existing problem patch established for a couple of years then you are probably going to need multiple years’ applications.
For new problem areas or to begin a new attack on a site, two applications may succeed or at least start the process. Since the maximum yearly rate of Stinger is 2⁄3 pint/A/yr, you should apply 1⁄3 pint now (i.e. late April – early May) and then the second 1⁄3 60 days later or before any regrowth or new growth begins. Some weeds will probably survive although they will appear twisted and stunted but may later grow out. The second later treatment of the 1⁄3 pint should take care of them; assuming they have not been established for several years. If you are spot treating areas of the orchard be sure to treat at least 10 feet before and past where you see any emerged weeds. The residual effects of Stinger should take care of any underground spread from roots.
If you miss the second application, you can expect to see the rebounding of the growth of the thistle. Remember to keep an eye on the 30 day PHI for the second application of Stinger. If you cannot make the second application due to the 30 day PHI, do not allow the plants to flower and release seed. Established patches may need to be treated for several successive years because of the abundance of underground roots. Therefore, it is imperative that you map the areas of the orchard where the weeds are present and return for follow up treatment measures.
Stinger effects on thistle.
Along this vein, you may also want to spray the drive rows in case the thistle roots have moved into the sod. If you are spot treating areas in the orchard be sure to apply the spray in a normal herbicide sprayer as a per acre rate; just shut the sprayer off 10 feet past the patch and start it back up 10 feet before the next patch. One plant can colonize an area 3 to 6 feet in diameter; hence the need to treat outside of the area that you can see young plants. Roots can grow 6 to 15 feet deep into the soil. Although we normally consider Stinger to be primarily a post-emergent material there are some residual benefits from the product being absorbed by the roots.If you have other perennial or broadleaf weeds mixed in with the thistle you might want to add 1 – 2 pt/A of 2,4-D for broader spectrum weed control especially when treating the drive row. Row middle treatments may particularly help control dandelion and reduce the presence of clover. If you are just going to treat the tree rows an alternative may be to add glyphosate or paraquat instead of the 2,4-D. The paraquat, of course, will not control any perennials. If you do not want to use paraquat then either Aim or Venue will work on broadleaf weeds but not grasses; for that you would need Fusilade or Poast, depending upon the crop and bearing status of the trees. In apples we have had good luck mixing Stinger and Treevix in controlling thistle.
As mentioned at the beginning of the article, if the patch has been established for more than 1 or 2 years it will take multiple years to control the weed. Another timing option, if you miss the 60-day treatment, would be to treat in the fall postharvest but before a killing frost occurs. You could also make the single fall application use the full rate of 2⁄3 pt./A and then begin the two applications 60 days apart the following calendar year.
– Robert Crassweller, Penn State University
Photo at top: Application of Stinger on thistle. Photos: Rob Crassweller/Penn State University