can bluebonnets choke out purple knapweed
I have heard a rumor that blue bonnets will choke out purple knapweed. I am wondering if this is true. Purple knapweed is an invasive plant by me. Also, what planting zones can blue bonnets survive in?
Thank you for your question. There are several knapweed species that have been introduced to the U.S. that are considered invasive (for example, spotted knapweed (Centaurea stoebe L. subsp.micranthos (Gugler) Hayek), Russian knapweed (Rhaponticum repens (L.) Hidalgo) and diffuse knapweed (Centaurea diffusa Lam.)). However, I could find no reference to a plant called purple knapweed, so I'm assuming it's a local name. I was also unable to find any reference to bluebonnets (I assumed you were referring to Texas bluebonnets (Lupinus texensis)) being an effective species in forcing out knapweed.
Spotted and diffuse knapweed have allelopathic properties which means they secrete chemicals that can prevent other plants from growing. Knapweed species are also prolific seed producers and create a seed bank capable of producing plants up to 15 years following treatment.
Treatment for these species can vary from state to state. I suggest, if you haven't already done so, that you take a sample of the knapweed species you're dealing with to your local Extension office to have it identified, and they should be able to give you recommendations for treatment protocols approved for your area.
As far as the hardiness zone for Texas bluebonnets is concerned, the Lady Bird Johnson Wildflower Center shows the distribution of this species in Texas, Oklahoma, Louisiana and Florida.
Hope this provides you with some useful information, and thank you for using Ask an Expert.