The line between what is defined as an invasive weed and what is not can be fine, but some rules apply in the state of Washington. English ivy, for instance, is a no-no.

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It seems like every time I write about one plant or another these days, I get e-mail from readers warning that it’s a menace. I’m sometimes scolded for even mentioning a buddleia, verbena or ranunculus, although among the hundreds of species and cultivars of these plants, few or none are truly invasive.

Yet for every plant denounced, I also hear from a gardener or two struggling to nurture along this same weak darling. Is it just that one gardener’s gem is another’s nuisance? For every gardener who deplores Japanese anemones, Ranunculus ficaria ‘Brazen Hussy,’ bronze fennel or love-in-a-mist, there are 10 more who love it when plants spread themselves about so generously.

I’ll confess that I may be the only gardener on the planet who failed with English ivy. Many years ago I blanketed a difficult hillside with ivy starts. Moses, our German Shepherd, amused himself with the endlessly repeated game of running to the top of the hill, dropping his tennis ball, then charging back down the face of the slope to retrieve it. Even ivy couldn’t withstand such an onslaught.

Such embarrassing tales aside, we gardeners need to figure out how to be responsible without ruining the excitement of trying new plants. We need to learn which plants really do pose a problem, and which are merely annoyances in certain situations.

I think the defining question is this: Which plants are management issues within our garden, seeding about too freely, or spreading more vigorously than we’d prefer, and which are a danger to our parks and woodlands? If growing a plant has repercussions for our neighbors and surrounding natural areas, then it’s time to ban it.

I turned to Dr. Sarah Reichard, a University of Washington conservation biologist who is an enthusiastic gardener as well as an international expert on invasive plants. “Just because a plant self-sows, doesn’t mean it’ll be invasive,” says Reichard. “And just because it doesn’t self-sow doesn’t mean it won’t be invasive — it depends on soil and irrigation.”

Reichard is researching how to determine ahead of time if a newly introduced plant will end up becoming problematic. One warning sign is a rapid juvenile period, meaning that if a plant grows quickly, flowers and fruits when it’s young, it might well be too efficient at reproducing itself. Plants that regenerate rapidly from root fragments (beware the horseradish) and ones with deep tap roots might well cause problems in wildland areas if they did escape from gardens. For example, popular and pretty Himalayan honeysuckle (Leycesteria formosa) is invading natural areas in New Zealand, spread about by birds. Reichard is suspicious of Verbena bonariensis, too, which is taking over grasslands in Australia, although she hasn’t yet found it populating natural areas around here, despite how many gardens feature its violet blooms.

Just to complicate it further, many invaders have a “lag phase” that can last up to a decade. Sometimes pollinators haven’t shown up yet, or it can take a while for birds to recognize fruit on a plant that’s new to them.

Ecologist Richard Mack of Washington State University explains that gardens can be staging areas for future problems. We irrigate and fertilize our plants so birds begin to notice and recognize them as food sources. So in a way, good gardeners are nurturing the invasives of the future, which is why we all need to pay attention.

OK, bottom line, which plants should we never allow into our gardens? “English ivy and any of the knotweeds. All knotweed cultivars are quarantined,” says Reichard (see www.nwcb.wa.govto learn which plants are outlawed and/or quarantined in our state).

Valerie Easton is a Seattle freelance writer and author of “A Pattern Garden.”