Tomatillos, often called the Mexican husk tomato, are a staple ingredient in Mexican cuisine, notably used to create salsa verde. While they belong to the same nightshade family as tomatoes, their reproductive biology is fundamentally different. Tomatillos generally do not self-pollinate and instead rely on cross-pollination to successfully produce fruit.
The Necessity of Cross-Pollination
The inability of a tomatillo plant to fertilize itself is due to a genetic mechanism known as self-incompatibility. This biological safeguard prevents a plant from setting fruit with its own pollen, even though the flowers contain both male and female reproductive parts. For a successful fruit to develop, the flower must receive pollen from a separate, genetically distinct tomatillo plant.
This self-incompatibility is gametophytic, meaning the genetic makeup of the pollen grain itself dictates whether fertilization can occur. If a pollen grain lands on the stigma of a flower from the same plant, a biochemical reaction prevents the pollen tube from growing down the style to reach the ovule. This mechanism ensures genetic diversity, requiring gardeners to plant a minimum of two separate tomatillo plants.
Encouraging Natural Pollinators
In a natural setting, the primary vectors responsible for cross-pollination are insects, most notably bees. The tomatillo flower structure attracts these visitors, who brush against the pollen-covered anthers before moving to the next plant. Bumblebees and honeybees are highly efficient at this task, though flies and butterflies also contribute to pollen transfer.
To maximize the efficiency of these natural helpers, gardeners can employ several strategies. Planting a diverse array of pollinator-friendly flowers nearby increases insect traffic, boosting the chances of effective pollen transfer. It is also important to avoid using broad-spectrum pesticides, which can harm the beneficial insect populations responsible for pollination.
Adequate spacing between the sprawling tomatillo plants is helpful, as it improves air circulation and allows for easier insect access to the flowers. While insects are the main mechanism, slight air movement can play a secondary role by shaking pollen loose. When natural pollination is robust, the result is a high yield of fruit encased in the papery husk.
Techniques for Manual Pollination
When natural pollination is insufficient due to poor weather, a low local insect population, or if plants are grown in a protected environment like a greenhouse, manual cross-pollination becomes a viable solution. Though the flowers are “perfect,” containing both male stamens and a female pistil, the pollen must still be moved between two separate plants.
The best time for manual transfer is in the early morning, typically between 8 a.m. and noon, when the flowers are fully open and the pollen is viable. A small, soft-bristled artist’s brush or a cotton swab are the best tools. Begin by gently swirling the brush or swab inside an open flower on the first plant to collect the fine, yellow pollen grains.
Immediately after collection, the pollen-dusted tool must be transferred to an open flower on the second, genetically distinct plant. The collected pollen should be carefully dabbed directly onto the central female stigma of the second flower. This process must be repeated, moving back and forth between the two plants and using a fresh flower from the first plant for each transfer, to ensure successful fruit set.