Should I cut back or replace my wisteria to fight this pest?
I believe both of my wisteria are infested with what is probably snow scale (see white specks on bark). It started about 3 years ago on one plant and now it is on both & getting worse. At first I thought it was just what mature bark looked like. When a new branch grows, it does not have the white specks right away, then it gets covered in it.) I have attached photos. Now I believe there may be multiple types of pests (in addition to the white snow scale covering all the bark, weepy brown scale on bark & aphids under leaves) because the plants are so weakened. I trim them in summer to remove new runners that are growing in the wrong place and in January as recommended for healthy blooms. The first blooming in the Spring is usually the strongest, then the plant gets weaker and the leaves start to spot, yellow, & drop. Year to year, both plants are increasingly weaker than the year before. I had not wanted to cut them back drastically because I’m training them over a pergola and each new growth looks so promising. But I am ready to take extreme measures now that I see how it has gotten worse every year. I have read about maybe using Neem oil to smother Scale. Will that work given the little crevices in the twisting bark? Should I cut both of the wisteria to the ground & restart with new growth? Should I just get new ones? How do I prevent this from happening again? Please take a look & tell me what you think the problems are and how I should best treat them so I can get my pergola covered with beautiful wisteria like I had planned. Thank you so much for your time!
Kent County Maryland
We viewed the photos and the vines look stressed. There are several types of wisteria including Asian and American wisteria. It looks like it may be a type of Japanese or Chinese wisteria, invasive types. Take a look at our page on wisteria for more information, when the plant blooms, growth habits, etc. https://extension.umd.edu/hgic/topics/chinese-and-japanese-wisteria
We see scale, a sucking insect on the trunks/branches in the photos. There are several types of scale and we cannot tell what type it is. It is possible it may be white prunicola scale, a hard scale. No matter the type, we recommend that you spray with a dormant rate of horticultural oil now. You will have to get good coverage. This should only be applied if air temperatures are to remain above freezing for at least 24 hours after the spray application. Monitor temperatures and Follow all label directions. https://extension.umd.edu/hgic/topics/armored-hard-scale-trees-and-shrubs
We notice in the right photo, assassin bug nymphs. They are beneficial predators that feed on many pest insects. They do not need to be controlled.
The vines are most likely stressed due to being planted in containers, poor growing conditions, hot, dry growing conditions, etc. Also, the strong root system will eventually grow through the bottom of the containers. This spring we recommend that you remove the vines from the containers and plant in the ground. Keep watered during dry periods and make sure mulch is no thicker than several inches.
Also, here is a link from Fine Gardening on pruning. https://www.finegardening.com/article/pruning-and-training-wisteria
If the vines continue to decline, you may want to take a look at the above wisteria link for what to plant instead.
Thank you so much, Marian! I will take your advice. I forgot to mention that Three years ago, we cut the bottoms completely off the containers and dug oversize holes in the ground and enriched the soil under the plants with Leaf-Gro. So, the plants only look like they are in containers; underneath, the roots are free to spread. However, the ground itself may be a container of sorts, because We had to dig the holes through packed gravel used as a driveway for over 50 years, so the soil surrounding the holes is gravelly and poor. I bought the American variety that is slow growing with compact blooms. I sure hope that’s what it is! Thanks again for your help! I’ll give it a try for another year.
Congratulations on getting the American species. The flowers look great, actually.
Keep them well watered in dry periods. (Last summer-fall we had a record-breaking one.)
Growing in the poor gravelly soil will always be a stressor for them. So they may need a little extra TLC. Adding organics to the soil, by top dressing with a composted product at least once a year should help, too.
You'll probably want to spray with a horticultural oil and follow tips for white prunicola scale on our website. (Given above.) You can take a stiff (not metal!) brush to the scale before you spray with oil to help the oil get onto the organism hiding under the scale "shell." But don't scratch through the actual bark, of course.