The tomatillo is a distinct species prized for its tart flavor in Mexican cuisine. Gardeners often observe the papery husk, known as the calyx, swelling to full size, only to find the fruit inside remains small or undeveloped. This signals that the plant’s processes encountered an obstacle, preventing the fruit from reaching its potential size.
The Critical Role of Cross-Pollination
Tomatillos (Physalis ixocarpa) are generally self-incompatible, meaning a single plant cannot effectively pollinate itself to produce viable fruit. Unlike the self-pollinating tomato, the tomatillo flower requires pollen from a genetically distinct plant to fertilize its ovules and initiate proper fruit development. If only one specimen is planted, the resulting fruit will often be tiny and poorly formed, even if the husk appears mature.
For successful fertilization, at least two tomatillo plants are necessary, and planting three or four can significantly increase overall yield. Pollen transfer is primarily accomplished by insects, particularly bees. If insect activity is low due to poor weather or a sheltered environment, manual intervention may be needed. Gardeners can use a small brush or cotton swab to gently collect pollen from one flower and transfer it to the open flower of a second plant.
Inadequate or partial pollination is a direct cause of small fruit because the developing fruit contains fewer seeds. These seeds are the source of growth hormones that signal the fruit to swell. The papery husk develops normally because its growth is vegetative, independent of successful fertilization.
Nutrient Ratios That Stunt Fruit Growth
The chemical composition of the soil influences whether the plant dedicates energy to foliage or fruit development. A common imbalance leading to small fruit is an excess of nitrogen (N). While nitrogen promotes lush leaf and stem growth, high levels inhibit the plant’s reproductive cycle. This results in a large, bushy plant with few or small fruits.
To support optimal fruit size, the plant requires adequate levels of phosphorus (P) and potassium (K) during the flowering and fruiting stages. Phosphorus is integral for energy transfer and flower development, while potassium plays a fundamental role in fruit swelling. Applying a fertilizer with a higher ratio of P and K relative to N, such as a 5-10-10 formula, is beneficial once the plant begins to flower.
The most accurate method for managing soil nutrients is performing a professional soil test before planting. This analysis determines if any specific nutrient is deficient or present in excess, allowing for targeted application of amendments. Using a soil test prevents the unnecessary addition of nitrogen. If a test indicates sufficient phosphorus, a low- or no-phosphorus fertilizer should be chosen to maintain balance.
Water Management and Heat Stress
The consistent availability of water is directly linked to the plant’s ability to swell its fruit to maximum size. Tomatillos thrive when they receive a consistent supply of moisture, typically around one to one-and-a-half inches of water per week. Irregular watering, such as allowing the soil to dry out completely between brief, shallow soakings, can cause significant stress. This inconsistent moisture delivery prevents the fruit cells from fully expanding, leading to stunted or small final sizes.
Maintaining consistent soil moisture is particularly important once the fruits begin to set. While the plants are somewhat drought-tolerant, water stress forces the plant to conserve resources, often at the expense of fruit growth. Conversely, overwatering or waterlogged soil can suffocate the root system, inhibiting the uptake of essential nutrients needed for fruit development.
Beyond moisture, extreme temperatures can halt the reproductive process, contributing to small fruit. Tomatillos, like their nightshade relatives, perform best when daytime temperatures are in the 80 to 90 degrees Fahrenheit range. When daytime temperatures consistently exceed 90 degrees Fahrenheit, the plant experiences heat stress. This stress can cause flowers to abort or can lead to the production of non-viable pollen, resulting in poor fertilization and subsequently smaller fruit size.
Overcrowding and Light Deprivation
Tomatillo plants are sprawling, vigorous growers that require substantial space to produce the energy needed for large fruit. They need a minimum of six to eight hours of direct sunlight each day to maximize photosynthesis. The sugars produced during this process are the energy source that fuels the fruit’s final expansion.
When plants are placed too closely together, they begin to compete intensely for light, moisture, and available nutrients. This overcrowding results in self-shading, where the large leaves of one plant block sunlight from reaching the leaves of its neighbor. Reduced light exposure lowers the plant’s overall photosynthetic output, directly limiting the amount of energy available to fill the fruit, causing them to remain small.
To prevent this issue, plants should be spaced at least two to three feet apart in all directions. Providing support through staking or caging is highly recommended, as it manages the plant’s naturally sprawling habit. This support keeps the canopy open, ensures better air circulation, and maximizes light penetration to all parts of the plant, promoting uniform fruit sizing.