Growing strawberries in a greenhouse allows for an extended season and offers protection from pests and adverse weather. This controlled setting is beneficial for plant health and consistent fruit production, but it challenges the plant’s reproductive cycle. Strawberry flowers are hermaphroditic, meaning each bloom contains both male parts (stamens) and female parts (pistils). For a strawberry to develop into a large, well-formed fruit, pollen must be transferred from the stamen to the central pistil structure. The enclosed greenhouse eliminates natural vectors like wind and foraging insects, requiring the grower to actively manage pollination.
Selecting the Right Pollination Strategy
Growers must choose a pollination method based on the scale of their operation, balancing cost, labor, and expected yield. For small-scale operations or hobbyists, manual pollination is the most practical approach. This technique relies on the grower using a small tool to physically brush the pollen.
Medium-scale operations often use mechanical or air movement strategies. Since strawberry pollen is lightweight, a gentle stream of air from a fan or leaf blower directed at the flowering trusses simulates wind. Alternatively, a small, high-frequency electric pollinator tool can be briefly touched against the flower to vibrate the pollen onto the pistil.
For large commercial greenhouses, biological pollination is the most economically viable strategy. This involves introducing colonies of managed pollinators, such as honeybees or bumblebees. Bumblebees are particularly effective because they remain active in the lower light and cooler temperatures typical of winter greenhouse conditions.
Step-by-Step Manual Pollination Techniques
Manual pollination is highly effective, ensuring a high percentage of successful fruit set for smaller growers. The optimal time is midday, after the greenhouse temperature has warmed and humidity is low, which helps ensure the pollen is dry and easily dispersed. Pollen remains viable for several days after the flower opens, so performing this step on each flower for two or three consecutive days maximizes ovule fertilization.
The required tools are simple, with a small, soft-bristled artist’s brush or a clean makeup brush being ideal choices. Some growers use a small, battery-operated electric toothbrush, which provides gentle vibration to dislodge the pollen without damaging the delicate flower parts. The male stamens are the yellow, anther-tipped stalks in a ring around the center, while the female pistils form the central, cone-shaped structure.
To execute the transfer, gently swirl the tool around the ring of stamens to collect the yellowish pollen dust. Carefully apply the collected pollen to the central pistil cone, ensuring the entire surface is contacted. Complete and uniform pollination is necessary because incomplete transfer results in misshapen or underdeveloped berries. A successfully pollinated flower will show signs within a day or two, with the white petals dropping off and the central structure beginning to swell as the fruit forms.
Managing Greenhouse Conditions for Optimal Pollen Health
The successful transfer of pollen is only half of the process; the greenhouse environment must also support pollen health and viability. Temperature plays a direct role in flower development and pollen quality. Daytime temperatures should be maintained between 16 and 20 degrees Celsius during flowering, while nighttime temperatures should range from 8 to 12 degrees Celsius to support flower bud development and plant respiration.
Humidity control is equally important, as both extremes can be detrimental to the pollination process. High relative humidity, above 80%, causes pollen grains to become heavy and clump together, preventing them from easily dispersing or adhering to the pistil. Conversely, very low humidity, below 50%, can cause the pollen to dry out and become non-viable before fertilization.
Maintaining a relative humidity range of 60% to 75% is optimal for both pollen release and successful germination. Air circulation is an effective tool for managing this humidity, even when manual pollination is used. Gentle air movement from internal fans helps dry the anthers, promoting the release of viable pollen and preventing stagnant, humid air pockets that could foster fungal diseases like Botrytis.