What to do now
Mildest step first.
- Cut flower spikes as they appear in late summer, and bag them instead of composting.
- Pull or hoe seedlings in spring while they are still small.
- If you want seed, let one or two spikes ripen inside a paper bag.
- Keep perilla well away from pasture, hay fields and creek banks.
Not sure this is it? Check other korean perilla (kkaennip) problems
Why it spreads
Korean perilla is an annual, so each plant dies at frost; the spread comes from seed. As nights lengthen in late summer, plants stop making new leaves and send up flower spikes. Mississippi State University Extension reports that flowering usually begins in late July to August, that one plant can produce 1,000 to 1,500 seeds, and that stored seed still germinated above 60 percent after more than four years. Clemson Extension notes the seed can stay viable in the soil for several years. Let a few plants ripen seed and you may find hundreds of seedlings in beds, paths and lawn the next spring.
Why it matters beyond your garden
Perilla has escaped gardens across much of the East. NC State Extension lists it as invasive in North Carolina, Virginia, West Virginia, Tennessee and Kentucky, and Clemson notes that the South Carolina Invasive Pest Council lists it as an invasive exotic. It also matters for farm animals. Clemson and Mississippi State both warn that perilla can be toxic to cattle, horses and other grazing animals, and Mississippi State notes it is most toxic while flowering and setting seed. If you garden near pasture or hay ground, treat seed control as essential.
Cut the flower spikes
The fix is timing. Clemson recommends snipping off the flowers of perilla grown for food so it cannot set seed, and NC State says pinching flowers before they go to seed slows the spread. From midsummer on, check plants weekly and cut each spike back to a pair of leaves as soon as it shows. Put cut spikes in the trash rather than the compost, since a home compost pile may not get hot enough to kill the seed. Steady picking does not stop flowering, but it gets you most of the season's leaves before plants turn.
Seedlings next spring
Volunteer seedlings come up once the soil warms; Mississippi State sees them from mid-May, at soil temperatures of 68 to 77°F. They have square stems, paired leaves and the familiar smell when crushed. Pull them or slice them off at the soil surface with a sharp hoe while they are small, and keep only the few you want to grow on. Because seed in the soil can last several years, expect to weed seedlings for a few springs after the last plant went to seed.
If you want seed
Some gardeners want seed, either to replant a favorite strain or for deulkkae, the seed pressed for perilla oil and ground into powder. Let one or two healthy plants flower in a spot you watch closely. When a spike starts to turn brown, slip a paper bag over it, cut it off, and let it finish drying in the bag so the seed falls there and not in the bed. Store dry seed in a sealed jar in a cool place, and cut every other spike in the garden before it browns.
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- How do I save seeds from my garden plants so they'll grow true next year?Open-pollinated and heirloom varieties come true from saved seed; hybrids do not — and after selecting and fully drying seed, cool, dark, dry storage is one of the biggest factors in how long it stays viable.
- When and how should I harvest herbs for the best flavor?Harvest herbs before they flower — leaf essential oil concentration peaks right before flowering, and leaf production tends to decline once the plant shifts energy to seed production.
- How do I keep garlic chives (buchu) coming back thick every year?Once plants are established, cut the whole clump 1–2 inches above the soil about every 3–4 weeks and feed lightly after each cut. Snip flower stalks before seed forms, and lift and divide clumps about every 3 years.