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To one gardener, an aggressive perennial can be a thug and it becomes a plant that you love to hate.

To another gardener, the same aggressive perennial can be the perfect plant for the garden and you hate to love it.

Salvia uliginosa (bog sage) is suitable for a rain garden. I bought just one plant in 2007 and plopped it front and center. In spring 2008, there were runners going in every direction for about three to four feet from the mother plant.

If this had been an Olympic garden event, these runners would have won gold medals for their speed.

At the same time that I planted the one bog sage, I also planted six tiny seedlings of eupatorium coelestinum Wayside (hardy perennial ageratum) in the rain garden.

By spring 2008, there had been a major population explosion too high for an accurate census count.

I'm sure some gardeners would be cursing and pulling these plants out of the garden. I was happily dancing a jig as I distributed the offspring of these full-sun, moisture-loving, drought-surviving, deer-resistant, rabbit-resistant, BLUE perennials throughout my rain garden.

In defense of the aggressors, there are other good attributes, too.

Bog Sage reaches four feet in height with swaying wands of azure blue flowers that are magnets for bees and butterflies. It is suitable for zones 6-9. It can grow in moist or dry soil. Mine bloomed non-stop from early June and into fall. Right now, in winter, the base foliage is still green.

There's something amazing about the bog sage. Every evening (under the influence of wine) as I took my garden walk, I would tell myself to deadhead the bog sage the next morning.

Every morning (under the influence of coffee), I was amazed that the azure blooms had re-emerged without deadheading. I need to put a slow-motion video on these plants to determine whether they are closing up in the evening, or actually growing new flowers overnight!

Hardy Ageratum is short at around 15" so it works well for the front of the border and is loved by bees and butterflies. It is suitable for zones 5-8.

During the summer, the foliage is very pretty. By mid to late summer through early fall, it blooms profusely, much like the annual variety of ageratum.

The blue color works well with many other colors such as yellow, orange, red, pink or magenta. It's suitable for clay soils, but mine is growing in rich, garden soil.

In the spirit of full disclosure, I have a lot of space that needs fillers and it will take a few years for me to reach my limit on the sage and ageratum.

These two aggressive perennials were very easy to pull (so far). The runners of the bog salvia were so shallow that I just snipped and pulled them off the soil to move to other locations. I moved the ageratum throughout the summer by easily scooping underneath the little seedlings.

If you want to add these two accused thugs into your garden, you'll just have to decide whether to keep them confined within walls of a container or let them wear ankle bracelets and roam your garden.

Photos and story by Freda Cameron