The cultivated strawberry, Fragaria × ananassa, is a widely grown hybrid plant appreciated globally for its flavor and texture. This species originated in Europe in the 1750s from a cross between Fragaria virginiana (North America) and Fragaria chiloensis (South America). Pollination is the biological process that allows a strawberry flower to develop into fruit by transferring pollen from the male part (anther) to the receptive female part (stigma). Understanding the source of this pollen is key to maximizing a harvest.
Strawberry Pollination: Self-Fertility and Necessity
Most commercial strawberry varieties are self-fertile, meaning a single flower contains both male and female reproductive parts and can produce fruit using its own pollen. This self-pollination ensures that fruit will set even without the presence of another plant. However, relying solely on pollen transfer within a single flower often results in incomplete pollination, which affects fruit quality.
The strawberry is an aggregate accessory fruit; the fleshy, edible part develops from the receptacle tissue after successful fertilization of the ovules. Each flower has hundreds of individual ovules, which appear as achenes (tiny “seeds”) on the fruit’s exterior. For the fruit to develop into a large, uniform, and well-shaped berry, a high percentage of these ovules must be fertilized.
Self-pollination frequently fails to deliver pollen to all receptive stigmas, resulting in fewer fertilized ovules. When only some ovules are fertilized, the fruit is often smaller and misshapen because the unfertilized areas do not fully expand. Therefore, while cross-pollination (the transfer of pollen between different flowers) is not strictly necessary for fruit production, it is highly beneficial for improving the size, uniformity, and yield of the harvest.
Agents of Cross-Pollination
The transfer of pollen between strawberry flowers is primarily facilitated by insects, a process known as entomophily. Strawberry pollen is relatively heavy and sticky, meaning wind plays a minor role in transfer, unlike in grasses or corn. Therefore, pollinator activity is required to effectively move the pollen from the anthers to the numerous stigmas across the flower.
Bees, including domesticated honeybees and various native species, are the most effective agents for achieving complete pollination. Their movements within and between flowers ensure that pollen is adequately distributed to fertilize the high number of ovules. Studies suggest a single strawberry flower may need between 16 to 25 bee visits to achieve maximum pollination and fruit size.
Gardeners can encourage insect activity by planting pollinator-friendly flowers near their strawberry patches. Increasing the density of pollinators is directly linked to higher yields and a lower incidence of deformed fruit. Without sufficient pollinator activity, even self-fertile varieties will produce lower-quality berries.
Cross-Pollination: Effect on Fruit Versus Seed
A common concern is that cross-pollination between different strawberry varieties will ruin the flavor or appearance of the current season’s fruit. The physical, fleshy part of the strawberry that is eaten is the enlarged receptacle tissue, which is entirely the mother plant’s tissue. Therefore, if a June-bearing variety is pollinated by an everbearing variety, the resulting berry will still possess the taste and characteristics of the June-bearing mother plant.
Cross-pollination only affects the genetic material contained within the achenes, the true fruits on the exterior. These achenes contain the fertilized seed, which results from the genetic combination of the mother plant and the pollen donor. Planting these seeds will produce a new, hybrid strawberry plant that is genetically distinct from either parent, a phenomenon utilized by breeders to develop new cultivars.
While the mother plant determines the fruit’s identity, the pollen source can influence the fruit’s physical expression, a phenomenon known as xenia. Research shows that cross-pollinated fruit may be slightly darker, less acidic, and possess a higher sugar-to-acid ratio than fruit resulting from self-pollination. Gardeners can safely plant different varieties near each other for improved fruit quality, but they should avoid saving and planting the achenes if they wish to maintain varietal purity.