Q: I found this vine taking over the back of my garden. The foliage is interesting, but I can’t quite find an ID for it. Is it worth keeping? It is aggressive, but to be fair, most vines are.
A: This is Japanese hops (Humulus japonicus), an invasive species related to European hops (Humulus lupulus). Despite this relation, it is not used to brew beer. As you noted, it has rampant growth and can quickly cover other plants if not removed. It has been in the U.S. for quite some time, and is found in most Maryland counties, plus many states in the eastern U.S. Unfortunately, it seems to have become more rampant in spread during the last couple of decades.
Stems are prickly, and the Minnesota Department of Agriculture notes that an allergy to Japanese hops pollen is not uncommon. The prickles can also nick the skin, causing irritation. Use gloves when trying to pull it out, and avoid mowing it if it’s in bloom or going to seed, as this will spread pollen or seed around. Use either a systemic herbicide to kill the entire plant, preferably before it sets seed, or cut it down to exhaust the roots. The latter approach will require vigilance and persistence, cutting off all foliage as soon as it appears again, since regrowth can be rapid and the foliage needs to be removed promptly so the roots don’t store more energy.
Some references list this as perennial if the climate is mild enough, but in our region and areas to the north, it is more of an annual. Seeds can germinate over a long period, beyond spring and well into the summer. More information can be found in the Maryland Invasive Species Council 2007 article “Asian Vine Climbs into Maryland.”
Q: I need to reduce the height on an arborvitae … might take around 5 feet off the top, because it’s too big. When should I prune?
A: While late winter or early spring is generally a good time to prune conifers, in this case timing won’t matter much, because the amount you want to remove is more than the plant can handle. Instead, it may be better to replace the plant with a smaller-growing option.
Most conifers, including arborvitae, do not regrow foliage when it’s lost due to heavy pruning, deer browsing, or crowding from being planted too close together or too close to a wall or fence. Unlike broadleaf evergreens (boxwood, holly, euonymus, etc.), they don’t have dormant buds along the older stems, lying in wait to grow if the branch or foliage beyond them is removed.
As conifers age, it’s perfectly normal for the innermost branches to become bare, as those older leaves shed over time. They are deliberately jettisoned by the plant because they are being progressively shaded by the outer shell of live growth, so they cost the plant more to keep alive than the meager photosynthesis energy they get back.
Pruning cuts that remove that outer layer of younger growth on the branch tips will result in permanent bare areas. Once the foliage is gone from that inner wood, it will not regrow, even if sunlight now reaches the interior. Reducing the height of a tall arborvitae will stunt the top growth and give it a permanent gap or flat-looking top. If this isn’t really visible from where you typically view the plant, then it’s not necessarily a problem. Otherwise, nothing will give the plant its former shape back.
Fortunately, there are lots of compact and dwarf conifer varieties on the market these days. A couple of conifer types, like yews, will be able to rejuvenate after heavy pruning. Even so, it’s still best practice to select a plant that should fit in a given space in the yard without relying on pruning to make it fit.
I acknowledge that there are many gardeners that inherit poorly-chosen plants in their new yards, but if or when it comes time to replace them, research your options to make sure you won’t run into the same problem down the road. Plants never really stop growing, though older specimens can slow down. Due to a reduced growth rate, dwarf and miniature cultivars will stay much smaller over the same amount of time as their full-size counterparts, even though some dwarf cultivars can also get larger than you’d expect a few decades after planting.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.