Can you share photos of the weed in question? Since there are several species of thistle in our area, the most effective control approach will depend on which species has invaded the planting bed. Canada Thistle, for instance, is very tenacious and a perennial, and will be very difficult to eradicate unless you treat it with a systemic herbicide. Such products are non-selective, though, so any individual Pachysandra stems that are hit with the spray will also die back.

Several songbirds eagerly consume thistle seed (in this case, not feeder seed, but wild plant seed), so they could have passed through the garden (or stopped at a feeder or birdbath) and deposited seeds in that area. Canada Thistle rhizomes, when cut into pieces by a wood chipper, for instance, can readily resprout. (This is the main reason digging them out is rarely successful.)

Preen is used as a pre-emergent, so will only prevent the germination of some weed species seeds; it does not impact existing growth. You would also have to see if the product label mentions that Pachysandra is tolerant of its application, though as a generalization, established perennials aren't likely to be exposed to too much of a pre-emergent chemical's application.

If this is the non-native Pachysandra, there is an opportunity here to replace the entire planting instead, at which point you can try either digging out the thistle or spraying it with less caution. The Japanese Pachysandra, while not listed yet as highly invasive, has escaped into wild areas in several MD counties so far: https://www.marylandbiodiversity.com/view/2208

In that case, depending on site conditions, a wide range of alternatives, including many native species, can be used to control erosion or serve as a decorative groundcover.

Miri

Dear Miri: I greatly appreciate your thorough and thoughtful reply! Thank you. I have taken pictures of these rascals for your viewing pleasure. You are an optimist in referring to this as a possible “opportunity” to replace the entire planting but which will have to wait for the next “lucky” property owner. I was hoping for some die-back during the winter and using Preen to prevent germination in early. I have no idea of the type of Pachysandra this is. Perhaps you can tell from the photos. I have tried very selective spraying which seems to kill the above surface plant but am unsure about the roots. Any further guidance? 

Thanks for your photos.

Unfortunately that does look like invasive Canada Thistle, which is a bear to fight, but you can win over time. Here is our page on it, including links to control methods:

 https://extension.umd.edu/resource/canada-thistle

Years ago I had bulk mulch delivered, which must have been seeded with this pesty plant. If you pull it, it simply forks underground and two plants grow in it's place. Over a long... period of time this can wear out the root energy, but it may not be your best strategy.

Definitely don't allow it to flower and set seed. With the amount that you seem to have, I'd probably forgo the Preen for now (that can have impacts on amphibian life) and focus on treating with herbicides containing the active ingredients glyphosate or triclopyr. They do work but you may need to give it time to see if there is regrowth and then treat again.

** Follow label instructions. These herbicides will kill any plants contacted, so shield desirable ones from application.

Christine