The Mystery of Morphing Plants

August 25th, 2020

   One question I get a lot is about plants that seem to be turning into something else (i.e. some other version of plant, not a lizard or monster or something).

A bigger-branched “reversion” is growing out of this dwarf Alberta spruce.

   A common one is the popular landscape evergreen dwarf Alberta spruce, that dense little pyramidal upright that flanks so many front doors.

   Often times, dwarf Alberta spruce will start growing a branch that looks like it came from a much bigger evergreen – one with longer needles, larger branches, and a much faster growth rate.

   Given a few years to grow, the oddball branch can dominate the bush and almost look like it’s a new tree growing out of the one you planted.

   People mention all kinds of possible explanations. In the 1970s, it would’ve been blamed on TMI. But in reality, this shape-shifting is a case of “reversion.”

   Reversion is an oddity of the plant world that involves genetic plant mutations that are sort of “undoing” themselves and going back to the growth habit of an ancestor.

   In the landscape, it typically happens to plants that came to us from breeders who capitalized on a chance mutation of a plant.

   Dwarf Alberta spruce, for example, is a type of native white spruce that was developed decades ago from a plant that put out unusually small and narrow needles and had a very slow, tight, conical growth habit.

   Mutated branches like this – called “witch’s brooms on conifers – can be removed from parent plants and grafted onto the roots or wood of other plants of the species to create a new plant with the mutated traits. Or a mutated cutting can be stuck in a potting medium and rooted to produce a new baby plant of its own.

   The gardening public sometimes prefers the mutated traits to the original ones. That’s why dwarf Alberta spruce, with its compact, slow-growing habit, became a huge hit despite being highly prone to attack from spider mites.

   Most dwarf trees, shrubs, and evergreens come from people noticing chance mutations, or from breeders using agents to induce genetic changes, such as light, temperature, hormones, chemicals, or radiation.  (Note that this isn’t the same as directly inserting or removing genes to directly create a new organism.)

   Plants with variegated leaves or with altogether different leaf colors often come from mutations. That’s what gave us gold-variegated euonymus, tricolor beech trees, variegated weigelas, and variegated boxwoods, to name a few.

   Now here’s where the mystery comes in.

Some of the leaves of this ‘Kumson’ variegated forsythia, foreground, are reverting back to all-green, background.

   Sometimes the genetic changes of these new, grown-out mutations don’t hold forever. As these plants grow, sometimes the ancestral genes in them begin to express themselves again, usually in the form of a variegated leaf going back to solid green or a dwarf conifer suddenly producing a much bigger shoot.

   Another version is when weather or some other assault kills the grafted part of a plant, allowing new and different growth to emerge from the roots.

   That’s usually the answer when someone asks, “My rose used to be a pink one with nice big flowers, and now it’s growing all gangly with red flowers. What happened?”

   In both versions, the plant is “reverting” back to an earlier state.

   Before a mutated plant goes to market, growers watch for how stable a plant is in keeping the new trait. Many, many curious changes never make it to the garden center because the change isn’t consistent enough or stable enough.

   But even the most stable mutants don’t always stay 100 percent mutant forever. Dwarf Alberta spruce is a good example of a plant with reasonably good stability but not perfection.

   So what are you supposed to do about a plant that’s trying to revert?

To keep this ‘Glow Girl’ spirea growing gold, clip out shoots of reverting green that can be seen on this plant.

   Assuming you like what you bought and prefer it to the reverting look, cut off the reverters ASAP. You may have to police regularly to counteract repeated reversion attempts.

   Why? Because if you don’t, the original form of the plant is likely to become more and more prominent and quickly dominate the mutation you bought.

   In the case of dwarf plants, since the reversion is going back to a bigger, faster-growing trait, the sheer speed of growth is going to win out.

   In the case of variegated plants, those leaves are at a competitive disadvantage because they have less of the pigment chlorophyll, which if you remember from high-school biology class, is what helps plants create energy from the sun.

   The green leaves of a reverted branch have more chlorophyll and therefore will do a better job in mining the sun’s energy, turning it into a stronger and faster grower.

   There’s your biology lesson for the day. Science solves another mystery.