Answer: When tomato blooms drop, it is usually due to either poor pollination or is a response to stress. Common stresses include improper or fluctuating soil moisture, extreme temperatures (both hot and cold), hot dry wind, nutrient imbalance, and disease. Since this plant is in a container, I suspect that improper soil moisture combined with hot weather is the culprit. Tomatoes are large plants and require large containers and sometimes daily or even twice-daily watering in order to stay evenly hydrated throughout the hottest days of summer. However, the soil must not remain saturated, so be sure the container has drainage holes. Also, since potting soil becomes depleted of nutrients over time, be sure to fertilizer the plant regularly.
Interestingly, the tomato's wild relatives, native to Peru and from which the domestic tomato was bred, were pollinated by a solitary bee. Since that bee doesn't live here and the plant isn't favored by local pollinators, tomatoes are usually described as "self-pollinating." However, the plants do better when pollinated by insects such as bumblebees. At the very least, the plants must be shaken, either by the wind or by hand, to cause the pollen to drop to the stigma. (Greenhouse tomato growers must provide some means to shake their plants.) By encouraging a variety of pollinators to visit your garden, primarily by keeping insecticide use to a minimum, you may be able to improve pollination. If your tomato is in a very sheltered spot out of the wind, giving it a gentle shake once in a while might help too.
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