Garden thug: Spotted deadnettle (Lamium maculatum)
Cultivated cousin: L. m. 'Aureum'
Notable as an excellent groundcover for shady situations, spotted deadnettle blankets the ground-and any plants unfortunate enough to get in its way-with dismaying exuberance. It spreads by running along the surface of the soil on stems called stolons, which set down roots that knit together to form an ever-expanding colony about 60 centimetres wide and 30 centimetres tall. Less vigorous than many other cultivars, including 'Beacon Silver', 'Aureum' slowly colours a shady spot with showy golden leaves brushed with a central stripe of white. Lavender-pink flowers bloom in spring and will re-bloom in summer if the plant is sheared back after flowering. While it appreciates moist soil, once it's established, this accommodating deadnettle tolerates periods of drought. Zone 2.
Garden thug: Oriental Limelight Mugwort (Artemisia vulgaris 'Janlim' Oriental Limelight - shown)
Cultivated cousin: A. v. 'Golden Phoenix'
Giving into the attractions of Oriental Limelight soon leads to regret. As irresistible as its golden foliage may appear, beware. This merry mugwort tramples through the garden, taking root wherever it finds a bare patch of soil. Savvy gardeners restrict its growth to containers, where its vivid foliage shines among colourful annuals. A better choice for the flower bed is 'Golden Phoenix', a 30-centimetre-tall mound of golden, lacy foliage that's as attractive as Oriental Limelight but knows its place. As with most Artemisia, this cultivar grows well in dry, poor soils in full sun. Zone 4.
Garden thug: Japanese knotweed (Fallopia japonica)
Cultivated cousin: F. j. var. compacta 'Milk Boy' (syn. F. j. 'Variegata')
An unworthy candidate for any garden, Japanese knotweed is an invasive plant that's storming the North American countryside. The good news for gardeners is that this renegade has produced a more manageable variegated cultivar that spreads slowly to form an attractive patch in the garden. The green leaves of ‘Milk Boy' emerge blushing pink in the spring before turning creamy white. One caveat: once established, it forms a thick, solid clump of up to 1.2 metres wide and 1.8 metres tall that's difficult to remove from the garden. So in small plots, consider growing 'Milk Boy' in a container. Zone 5.
Garden thug: Sundrop (Oenothera fruticosa)
Cultivated cousin: O. 'Cold Crick'
A scallywag that scatters its seed throughout the garden, the common sundrop soon settles into every nook and cranny. For those tired of pulling up progeny, 'Cold Crick' is a sterile form that blooms prolifically without setting seed. Growing to a compact height of 25 centimetres, it produces masses of lemony yellow flowers. Cut it back after flowering to prolong the blooming period. Zone 5.