Annual growers come to mind when discussing forcing bloom for seasonal sales. But perennial growers can benefit too. Customers tend to purchase flowering plants when they are in bloom. Many are looking for flowers to plant during spring, as annuals in their glory are available at every garden center. Those perennials which aren’t spring bloomers can get overlooked.
This isn’t to say that forcing perennials is for everyone. It’s not without controversy, as selling a customer a perennial plant – by definition, one that is going to bloom again for years in their garden, assuming they properly care for it – that is blooming this May but isn’t going to bloom in the same timeframe next season or any season thereafter isn’t necessarily the best way to educate customers (or to win repeat sales). Plus, you may not be growing enough of any one cultivar to make it worthwhile to manipulate the environment and force bloom, even if you desire to do so.
Using greenhouses to force bloom might work for those growing on a wholesale basis. But if you’re growing perennials to stock your nursery or for sale to local garden centers, cultivating customers who understand the seasonal effect of a well-designed perennial garden may be your best strategy. Forcing untimely bloom may not be in your best interest.
How to Force Bloom
An understanding of plant photoperiod is required to alter bloom time. Photoperiods are either short-day, long-day or day neutral. Short-day indicates that the plant blooms when daylight hours fall below some critical threshold. Long-day plants, in contrast, bloom as day length increases above a threshold. Day neutral bloomers don’t alter their bloom time in response to daylight hours. Perennials come in all three types.
To complicate things a bit further, not all plants in short-day or long-day categories are obligate bloomers. The ones that are really won’t bloom without the essential trigger of critical daylight hours. Others – known as facultative – will still bloom if they don’t reach that critical number of daylight hours, but their bloom will be lessened.
But it’s not really the day length that matters – it’s the hours of nighttime. If growers shorten nighttime hours by adding light at a critical time – interrupting the night – they can either inhibit bloom (with short-day plants) or stimulate early bloom (in long-day plants).
Just how long and how often you need to alter nighttime light patterns varies. For some plants, requirements to stimulate flower initiation differ from those needed to actually develop that flower. It ain’t easy blooming out of season.
As an important aside, it seems many landscape perennials and shrubs are blooming out of season as the climate changes – not just blooming early or late, but reblooming at odd times due to the abnormal climate patterns which are happening regularly these days.
Vernalization is the requirement that some plants have for a period of cold temperatures in order to bloom. It’s a part of the reason why flowers are blooming earlier or later than normal on their own. For greenhouse growers, it’s important to provide the needed cold period prior to forcing a perennial to bloom. Cool greenhouse temperatures of 42º – 45º F induce the best response. As temperatures rise, the time needed to complete vernalization increases.
Light is also a factor in vernalization. High intensity light exposure can reduce the time needed for vernalization. The combination of light and temperature affects blooming too. At differing temperatures, different light intensity and different lengths of time of exposure are needed to force bloom.
Detailed Words of Wisdom
The American Floral Endowment’s Grow Pro Webinar Series recently featured Dr. John Erwin, University of Maryland, who spoke on the topic of forcing flowering in perennials. Erwin shared detailed information, including some species and cultivar requirements.
“If the plant is immature, and you need it to bloom, it’s not going to work,” Erwin said. Immature plants are those which don’t have enough nodes to be sexually mature for their species. Only mature plants can flower. Increased light intensity can sometimes decrease the juvenile stage of development and cause perennials to reach maturity more rapidly.
Perennials typically need between four and eight weeks of the appropriate photoperiod, Erwin said, before blooming will occur.
Nighttime interruption using infrared light of 10 foot-candles is required for long-day perennials to induce bloom. The interruption should occur between 10 p.m. and 2 a.m., as the plants need some darkness first in order to trigger a response to the light interruption. If temperatures are below 63º, increased light duration or intensity is needed.
For short-day plants, black cloth covering from 8 p.m. – 8 a.m. will simulate extended nighttime hours and promote flowering. If temperatures are above 74º at midnight, an increase in time spent under cover to 15 hours is needed.
Stem elongation is another factor in plant development, and it happens primarily at the end of the night and the beginning of daybreak. Dropping the temperature for several hours before the nighttime period ends (to about 55º) and holding it there for a few hours of daylight can reduce stem elongation by about one-third.
Using plant growth regulators is another means of keeping plants bulky and branched. PGRs which increase branching can inhibit flowering, and growers should learn about the effects of these products prior to use.
In order to get bloom at the desired time, growers have to start manipulating the plant environment several weeks in advance. Bulking, cooling and forcing stages of plant development will require different temperatures and hours of daylight. And each species – or even different cultivars – will have their own particular requirements to force bloom.
“Do all your pinching and bulking before you do your cooling,” Erwin said. “If you pinch off your shoots afterwards, you’ve just pinched off the flowers you spent a lot of time making and threw them away.”
Additionally, the plant has to have entered maturity first. “If you don’t have that leaf number unfolded before you do the cooling,” the plant isn’t at maturity and won’t bloom anyway.
Root temperature research has shown that growing media temperature can be elevated well above room temperature. As the media temperatures goes up, root loss occurs, and plants get smaller as a result. Growing on weed guard material increases the media temperatures. Growing on gravel cools media and enhances root growth.
There is a specific gene in perennials that causes flowering after cooling. CRISPR gene editing has been studied to alter the flowering of plants, with researchers editing genes to “turn on” the flowering gene all the time.
Whether you’re interested in forcing perennial bloom or not, understanding the mechanisms of plant growth and flowering and knowing what environmental factors influence these is beneficial, particularly as our environment is changing and growers have to find ways to adapt. Bloom time, whether in the landscape or the greenhouse, is no longer a sure thing.