Pollinating in Greenhouses
Greenhouses are often verdant oases of lush, flowering plants of all kinds. They are often a treat for the senses when visited during colder and darker months. How though, do plants in greenhouses get pollinated? Chris Bond explains.
There are several ways greenhouse plants get pollinated — both by using natural and mechanical methods. In some cases, of course, plants that don’t require pollination are grown or are propagated through other means, but for those that rely on old-fashioned plant reproduction, some very non-traditional methods have been developed over the years.
Not all greenhouse crops need to be pollinated. Two crops common to greenhouse production, however, rely on it to be successful: cucumbers and tomatoes. Cucumbers and their relatives (squashes, pumpkins, gourds, and other cucurbits) produce male and female flowers. Pollination of cucurbits can only occur when pollen from the male flower makes contact with the female flower. Plants such as tomatoes, have what’s known as a “perfect” flower, meaning both male and female elements are housed within the same structure, but even as such, the pollen sometimes needs help hitting its target.
Just like in the great outdoors, some greenhouse plants are pollinated byinsects traveling from blossom to blossom. Sometimes these beneficial insects find their way inside through open ventilation, but many growers intentionally release them to improve pollination in their greenhouse crops. Bees are often the answer.
Honeybees are great pollinators, but few growers keep an active hive inside their greenhouse. Many, however, do raise them nearby and they will find their way inside during the months they are active. Growers with no interest in taking care of an active honeybee colony usually find eager, local beekeepers in need of an additional location to place one of their hives. The grower gets the benefit of pollination and the beekeeper gets additional honey. Honeybees, though excellent outdoor pollinators are not always the best choice for greenhouse pollination — this is where bumblebees come to the rescue.
Bumblebees are generally considered superior to honeybees in greenhouse environments for several reasons. Both are relatively docile (if you have ever been stung it was likely by a wasp or hornet, not a bee), but bumblebees boast better vision than honeybees which is useful in an artificial environment where instinct alone may wholly not serve them. They are also more adaptable to wider temperature ranges and environments than honeybees tend to be. They are more successful at pollination than honeybees because they have a longer proboscis (insect lingo for tongue) which collects pollen and nectar from deeper-throated flowers. Their wing vibration, being a bigger insect than honeybees, also fans and spreads pollen further than honeybees.
Honeybees and bumblebees are not the only types of bees being pressed into the service of greenhouse growers. Some are choosing to use smaller bees like Mason bees or Leafcutter bees. These two different species will do the same thing but are active at different times of the season. For early pollination, Mason bees can be considered, and for mid-late season pollination, Leafcutter bees will do the job. These smaller bees can crawl right inside of even tiny flowers that some of the other types of bees cannot get into. As the name implies, though, Leafcutter bees will “cut” holes out of leaves to use in their nests. If leaf crops are raised in conjunction with crops needing to be pollinated, Leafcutter bees may not be the right choice in that instance.
Other beneficial insects released or nurtured in greenhouse environments can help with pollination as well. Many predatory and parasitic insects that growers may rely on for biological pest control will often inadvertently aid in pollination. Ladybugs, green lacewings, and a whole slew of other flying and crawling “good guys” scour plant surfaces and undersides for their next meal or haven for their next generation of progeny. As they travel, they trample through pollen and nectar in blossoms. Pollen sticks to their legs and wings, they deposit some elsewhere as they go about their business. Of course, these insects should not be considered for their pollination contribution alone, but they do provide a value-added service.
Environmental and Mechanical Pollination
Setting a conducive environment for pollination is critical, especially if there are no natural pollinators. This does not necessarily involve mood music and dim lighting, although attention to the right temperature and humidity levels, and even the time of day, is highly suggested if pollinating by hand.
If the temperatures are not correct, the plants cannot properly form their blossoms and release or be receptive to pollen. Temperatures for optimum pollination vary between plant species, but generally, nighttime temperatures should not fall below 55˚F and daytime temperatures should not exceed 85-90˚F. Humidity levels need to be controlled, or at least monitored, to know when to pollinate. If the humidity levels are too high, the pollen will likely stick too much to the flower and be too difficult to disperse. Humidity levels should be between 50-80 percent, with most growers attempting to seek a humidity level of about 70 percent. Once the temperature and humidity levels are within optimum range, pollination will be most successful between mid-late morning and early to mid-afternoon. This is when most blossoms are fully open and most accessible for pollination.
Aiding in Mother Nature’s processes is nothing new and neither is manipulation of pollination. Some techniques used today have been done for hundreds of years or more. In some cases, only the tools have changed, but the method hasn’t. Pollination of flowers can be achieved by various hand tools. Some of these are common household items and others are more specialized. An artist’s paintbrush has been used for hand-pollination for a very long time. The grower essentially swabs the pollen from the male flower and then swabs the same brush across the female flower. This is repeated across each flower that’s desired to be pollinated. Once is not often enough with this method. Some flowers require pollination dozens of times to bear fruit. Many growers will perform this process for the entire duration of a plant’s flowering cycle. Toothbrushes are also used. Recently, electric or battery-powered vibrating toothbrushes have been used. While this method may mimic the vibration of a pollinator’s wings, it should be done with the utmost care as the vibrating brushes can also damage the flower’s reproductive capability if done too vigorously. Cotton ear swabs can also be used.
There are many devices on the market for the professional and hobbyist alike to perform hand pollination. They range in complexity and cost from modest and simple to expensive and elaborate.
Most greenhouses have some sort of air movement system in place even if it is nothing more than a few fans. Fans help excess water to evaporate quicker and lower the incidence of diseases, but those same fans can help with pollen dispersal. While it is more of a saturation bombing than a strategic one, fans will help released pollen find some of its target. Strategically placing fans at flower level during times of pollination helps increase that ratio of off- to on-target hits. An occasional or even steady breeze will not just aid in plant pollination, studies have shown that greenhouse-reared plants are often stronger when they are exposed to some amount of swaying or exposure to moving air. Some high-tech greenhouses have vibrating tables or mechanical brushes that gently and occasionally pass over or by each plant. While systems such as these are generally cost-effective for the average to even larger grower, when there are millions of plants to be pollinated, there are only so many viable days to get the job done and mechanization can help.
Whether pollinating by hand, using bees, or releasing beneficial insects to do the job, many growers have many methods to help them achieve maximum yields on their greenhouse crops.
Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional
Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.