Q: No matter what variety of phlox I plant, it reverts to a garish shade of purple-pink after a few years. I also have some heuchera ‘Palace Purple’ that is now quite green. How can I keep my plants true to their original colors? — Marjorie Taylor, Litchfield, Conn.
A: Your phlox are not reverting; they are gradually being overgrown or upstaged by their more vigorous offspring. Regardless of the flower colors they have been bred to display, tall garden phlox varieties often produce seedlings that turn out to bloom an undesirable pink. If these giant upstarts are allowed to grow up, they can eventually obscure the original cultivar you planted and ruin your color scheme. The remedy is to deadhead that original plant: Cut off its spent flower heads immediately after the last bloom drops, before seed has a chance to mature and scatter. “Losing” the parent cultivar can occur with many perennials that are able to seed prolifically. In some climates, for example, hardy asters grow freely, and their seedlings are not only unremarkable shades of pink and purple but also very vigorous. Even a richly colored dwarf aster variety such as ‘Purple Dome’ can disappear from view when its much larger offspring are allowed to tower over it.
Heuchera rarely self-sows as profusely as phlox and asters, though if such seedlings do grow, many will be green. It’s likely that your first purchase of ‘Palace Purple’ included greenish plants inadvertently propagated by a commercial grower who raised them from seed rather than from cuttings or divisions. Today the seed quality is very good and produces fewer than 10 percent nonpurple plants, but that wasn’t always the case — especially about 10 years ago, when ‘Palace Purple’ was a rare novelty and growers hustled to satisfy the sudden demand. The next time you shop for heuchera, look for even darker purple cultivars (such as spectacular ‘Plum Puddin’ and ‘Bressingham Bronze’) that are grown by tissue culture for consistent coloration. Of course, it won’t hurt to deadhead these, too.
A final note: True reversion, whereby a plant naturally mutates and changes its color, is rare. Hosta is one of the better-known perennials in which this process can occur. When hosta does revert, dark-green shoots may outgrow the yellow or variegated shoots a particular cultivar has been hybridized to produce. If you notice this happening, simply cut out any solid-green growth at the roots and add it to your compost pile.