Q: I have some Japanese honeysuckle growing among a large stand of forsythia. The honeysuckle covers some of the branches so heavily that the forsythia bloom display in spring is less than it should be. When the forsythias drop all of their leaves for the winter, is it safe to spray the honeysuckle with a herbicide and not damage the forsythias?
A: Since many chemical herbicides can be absorbed by thin bark as well as by leaves, you can’t safely spray the honeysuckle without harming your forsythia. I think your best bet would be to first pull as much honeysuckle off the shrubs as you can.
This goes more smoothly if you have a buddy who can clip as you tug.
Next, crawl under the forsythia and clip the honeysuckle vine where it emerges from the ground. The stems have a distinct reddish color.
Spray each freshly cut vine stump with glyphosate (click for sources). The vine stumps will absorb the chemical and will not sprout again.
Of course you could pull the vine out by its roots, but that’s pretty labor-intensive under a shrub.