Garden thug: Myrtle spurge (Euphorbia myrsinites)
Cultivated cousin: E. griffithii 'Fireglow'
A horticultural philanderer, myrtle spurge sows its seeds with wild abandon. Instead of continually weeding out its progeny, plant the prim and proper 'Fireglow'. This cultivar forms a well-behaved, shrub-like clump that looks striking in a border filled with hot-coloured perennials. In late spring, 'Fireglow' is aflame with orange-red, petal-like bracts that encircle tiny, bright yellow flower clusters atop handsome, reddish brown stems. A sun lover that appreciates evenly moist soil, 'Fireglow' grows to a height of 75 centimetres. Zone 4.
Garden thug: Yellow archangel (Lamium galeobdolon syn. Lamiastrum galeobdolon)
Cultivated cousin: L. g. 'Hermann's Pride' (shown)
A galloping groundcover, yellow archangel spreads where no other plant dares to grow-neither dry shade nor poor soils slow its pace. Less aggressive is 'Hermann's Pride'. Unlike its rampant relative, this version forms a restrained clump of verdant foliage brushed with pewter, providing a handsome background for the buttery yellow flowers that bloom in late spring. Freshen the foliage by shearing it back to the lower leaves soon after flowering. A mature plant will grow to 30 centimetres tall and 45 centimetres wide. Zone 3.
Garden thug: Obedient plant (Physostegia virginiana)
Cultivated cousin: P. v. 'Miss Manners'
Named for the compliant manner in which its flower heads remain in place when bent or moved, obedient plant does anything but obey the rules of good garden behaviour. Time and again, it aggressively breaches its boundaries. Thank heaven for little 'Miss Manners', a plant that lives up to its name. Its tidy 90-by-60-centimetre clumps of blossoms in late summer have white, snapdragon-like flower spikes that tower above stems cloaked in glossy, dark green leaves. Plant 'Miss Manners' in full sun or partial shade. Zone 2.
Garden thug: Goldenrod (Solidago)
Cultivated cousin: S. rugosa 'Fireworks'
Nomads of roadside fields and ditches, goldenrods can seem too wild and weedy for our cultivated plots. Yet many are not only well behaved, but also strikingly beautiful. 'Fireworks' is a goldenrod without the wanderlust of some of its genus. It forms a tidy mound 1.2 metres tall and 75 centimetres wide, and sets the garden aglow with an explosion of arching stems that sparkle with golden flowers. It thrives in poor soils, stands tough through hot, humid summers and prefers full sun but will tolerate partial shade. Zone 4.