Joan Boram saw Carolina lupine in a mail-order plant catalog and sent away for it.

In her Ferndale, Mich., garden, she loves its striking yellow flowers.

What Boram didn't expect is that the plant's seeds scatter and grow so freely that she now has Carolina lupine everywhere.

Call it plant spam.

Like computer spam, plant spam pops up where it's not invited. And it can be tough, though not impossible, to exclude.

But why talk about it now, when many gardens are asleep for the winter? Because this is the season of new plant catalogs, packed with vegetative eye-candy in jaw-dropping, knee-knocking, heart-stopping color.

Leafing through plant catalogs in winter is like watching infomercials at 3 a.m. It's dark. You're vulnerable. You can't help yourself. You pick up the phone and call.

When common sense returns, useless exercise devices or flimsy kitchen knives may be sent back for refunds. But plant spam? In the garden, it may take several seasons to realize a particular plant is invasive.

Of course, plant spam doesn't only come by mail.

Veteran gardener Judy Cornellier speaks for herself and peers:

"We didn't buy invasives in catalogs. We got them at plant exchanges or friends gave them to us," says Cornellier, who works at Telly's Greenhouse in Troy, Mich.

Cornellier urges gardeners to know in advance what kind of plant and habit they're dealing with -- good advice, yet not always easy to follow.

Still, that approach worked last summer for Marilyn Barber of Detroit.

Barber wanted to grow spearmint and peppermint but learned they could get out of hand. She took a class that discussed control of invasive plants and decided to corral the spearmint in a pot and the peppermint in a raised box.

"I just kept them separated from the other herbs," Barber says.

Sometimes one person's passion in another's plant spam.

Take Brazilian verbena. One gardener -- OK, it was me -- planted seeds of the tall, see-through annual several years ago. It thrived and, as is its wont, self-seeded. The following year, more plants showed up in not only my garden but also my neighbor's meticulous beds.

Turned out she was not as enamored of the verbena as I was.

To her, it was plant spam.

In her garden, Boram allows many uninvited volunteers of her Carolina lupine (Thermopsis caroliniana) plants to stay where they grow. The good news: Birds enjoy those seeds. The bad: That's how the seeds move into new territory.

Still, Boram, a freelance writer, considers another plant in her garden a bigger problem. The 7-foot-tall coneflower, Rudbeckia nitida herbstsonne, requires a firm hand.

"It's kind of like having a dog -- you have to show them who's boss," Boram says with a laugh.

For people who enjoy trying new plants, she adds, it can be difficult to determine before buying whether a plant will grow too well.

One reference Boram uses is the letters section of Fine Gardening magazine, where readers comment on plants that can be invasive.

And if you know the plant's botanical or common name, you can always Google it.

A few questions -- and the right answers -- may be the best firewall for plant spam.