Some logic about spring flowers in the Vail Valley |

Some logic about spring flowers in the Vail Valley

Tom Glass
Tom Glass

It’s looking raggedy outside. The mountains are shedding their winter fur, revealing some bedraggled shrubbery. As a result, you might be tempted to restore order and shear off some of those loosened ends on your shrubs that flap in the breeze.

Hold off, though, on that shrub-pruning project until you read this. Otherwise, you might unintentionally lop off the better part of a shrub: the flowers.

To avoid cutting the show from spring, you’ll need to know a little bit about two types of wood. In the case of woody-ornamental shrubs, depending upon the variety, flowers are either borne on old wood or new wood. Old wood is classified as branches that grew last summer. New wood is classified as branches yet to grow this coming growing season.

Spring-flowering shrubs almost without exception bloom on old wood. That old wood/new wood thing seems confusing to most gardeners ” flummoxes a few landscapers, too.

But, if you think about it, and I’m pretty sure you haven’t until now, there is a natural logic to why the order of events is for spring-flowering shrubs to bloom on last years’ growth.

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In spring, there is not enough surplus energy available within new growth to drive both growth and the process of flowering. New growth being, well, new, those new cells haven’t had a chance to store any extra energy ” all available energy is devoted to growth. Spring days are shorter, temperatures are cooler, and leaves are young and small. The energy leaves produce is limited by the climate and the immaturity of the new growth.

So why do April showers bring May flowers? Flowers appearing on shrubs in spring overwhelmingly result from the use of energy stored over from last year’s growth. And it is further made possible by slow modifications to buds requiring the time and temperatures fall and winter provide for transforming bud tissue into reproductive tissue.

Once we get into the summer months, shrubs often have an abundance of energy. Things can get positively manic for a plant. Sometimes there is enough energy floating around during the long, warm days of summer for many types of shrubs ” occasionally even normally old-growth, early-spring blooming shrubs ” to power both new growth and flowering.

Consider the following evidence of that remarkable set of conditions. Have you ever noticed that many flowering plants pause in growing when they are blooming? That growth pause is the result of available energy being consumed predominately by flowering.

Consider that a happy circumstance, because it causes flowers to properly display on top of the foliage instead of being buried under the foliage of new growth. As an added attraction, when the flush of blooms is spent, plants resume new growth that hides the browning flowers. Kind of cool how that works.

Once in a while, the order of things gets disrupted, and you’ll see new flowers buried under new growth. Not so good. When the bolting of new foliage over the top of fresh flowers occurs in a commercial nursery, it limits the marketability of a crop and can be costly. One cause is conditions favoring both growth and flowering.

To manage the situation, some nurserymen limit the application of fertilizers to plants about to bloom, reducing one of the conditions favoring rapid new growth. (I know. Probably more than you want to know, but it’s amazing the number of bad things that can happen when fertilizer builds up.)

You may be asking, “How do I practically apply this newly acquired knowledge?” People commonly ask when the time is right for trimming flowering shrubs. As a particularly accurate rule of thumb, if your shrubs are spring bloomers, you are best served to wait until they finish blooming. Trim those two to three weeks after they finish blooming. Trim them now, and we’ll all enjoy three shades of green this spring, which is a little short of the potential of things.

Summer-blooming shrubs are more specific in their requirements. You can, however, go ahead and whack that bed-headed potentilla. Just be sure to leave the lilacs alone until later this spring when their browning flower heads become as eye-catching as rusted hubcaps.

Tom Glass writes a weekly garden column for the Vail Daily. E-mail comments or questions about this column to

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