Here’s how to get your shrubs to rebloom

Why settle for a shrub that blooms for just a few weeks in spring when you can get one that blooms both spring and fall — and maybe a bit in between?

That’s been the aim of plant breeders in the last decade or so as they develop and roll out shrub varieties with extraordinarily long and repeat bloom times.

The result has been reblooming options in such plant types as azaleas, lilacs, hydrangeas, weigelas, spireas, and ninebarks.

But as with once-a-year bloomers, sometimes the rebloomers don’t always deliver their encore performances either.

What to do?

Proven Winners horticulturist Kristina Howley says that since most rebloomers rebloom on buds formed during the current season, the key is to keep plants producing healthy new in-season growth.

Most important, she says, is keeping plants “appropriately watered according to their needs, typically not soggy or desert dry.”

The stunting stress of a hot, dry summer can abort new flower buds and/or kill ones that formed but haven’t yet opened (not to mention kill whole branches themselves).

Howley recommends two or three inches of wood chips or shredded hardwood mulch over the soil surface to help retain soil moisture and keep the summer sun from baking the soil surface.

A second job that can help is “deadheading,” which is the practice of removing spent flowers.

Howley says that snipping off spent flowers saves the plant from using energy on seed production. That energy can then be redirected into producing new flower buds.

Remove only the flower stems, though. Pruning the branches will remove the wood where the new flower buds form, especially if you make your cuts while or after the buds formed but before they’ve opened.

If pruning a rebloomer is necessary, it’s best to do it immediately after a round of blooming has finished.

A third possible hurdle to rebloom (or first bloom, for that matter) is out-of-whack soil nutrition.

Flowering plants need adequate nutrients to produce flower buds. If the soil is lacking them or has an array in which an excess of one nutrient interferes with the uptake of another, plants can fail to flower.

Good flowering is a sign that the nutrition is adequate, so automatic fertilizing isn’t always needed.

However, if flowering is poor or not happening, a soil test is a good first corrective step. DIY, mail-in soil-test kits are available for $10 from county Extension offices, most garden centers, and directly from Penn State University’s soil testing laboratory.

Soil-test results will tell you which nutrients your soil needs and in what amounts.

Getting woody plants to rebloom is no quick or guaranteed enterprise for breeders.

One difficulty is getting that second bloom to amount to much. Most of the rebloomers on the market so far rebloom less spectacularly than the debut show, or they’ll rebloom only sporadically.

A second difficulty is developing varieties that are genetically stable enough to hold the reblooming trait in the long term — and do so in varying climate zones and varying year-to-year weather.

The best road to rebloom is starting with plants or species that sometimes naturally flower in the fall.

Many gardeners have seen this in their own yard when a fall warm spell follows a cold period.

A forsythia or rhododendron or weigela, for example, will strangely send out a smattering or so of flowers in November, prompting worried calls to Extension offices about whether something is going woefully wrong in the landscape.

In fact, the usual explanation is that the spring- or early-summer-blooming plant has been tricked by the weather into “thinking” it’s already gone through winter.

Plants prone to premature blooming are particularly useful for breeders looking to capitalize on varieties they can market as rebloomers.

A reblooming jackpot occurs when a woody plant manages to muster two decent flower sets in the same year — an early one on last year’s old wood and a later one on that year’s new wood.

Breeders also can capitalize on sterile flowers to develop a rebloomer. By failing to produce viable new seed to keep the species going, plants often “try again” by producing repeat flower sets.

Sterile plants also have the side benefit of no unwanted seeding, which is useful for ensuring that a new introduction doesn’t turn into an invasive plant.