The successful cultivation of tomatoes relies heavily on meeting specific thermal requirements throughout the plant’s life cycle. Tomatoes are highly sensitive to temperature fluctuations, meaning their growth and productivity are directly controlled by the environment. Understanding these thermal needs, from early development to mature fruit production, is necessary for maximizing the harvest potential.
Seed Germination Temperatures
Seed germination is entirely dependent on soil temperature, not the surrounding air. A minimum soil temperature of about 50°F (10°C) is necessary for sprouting, but germination will be slow and irregular at this lower limit.
The most successful and rapid germination occurs when the soil temperature is maintained between 75°F and 85°F (24°C and 29°C). Within this ideal range, seedlings often appear in five to seven days, ensuring a uniform start.
Temperatures exceeding 95°F (35°C) can inhibit or prevent germination entirely. Providing consistent warmth to the rooting zone, often using heated mats, is the most effective way to ensure a strong start before transplanting.
Optimal Daily Range for Growth
Once the tomato seedlings are established, the focus shifts to the air temperature necessary for robust vegetative growth, which encompasses the development of leaves and stems. The overall health and size of the plant are best supported by daytime air temperatures ranging from 70°F to 85°F (21°C to 29°C). Within this span, the plant efficiently performs photosynthesis, creating the necessary sugars and starches to fuel its rapid expansion.
The difference between day and night temperatures, known as thermoperiodicity, plays a significant role in plant metabolism. Nighttime temperatures should ideally remain cooler than daytime temperatures, settling between 60°F and 70°F (16°C and 21°C). Cooler nights are beneficial because they slow the rate of respiration, which is the process where the plant burns stored carbohydrates for energy.
By reducing nighttime respiration, the plant conserves more of the sugars produced during the day. This conserved energy is then available to build stronger tissues and support the substantial energy demands of fruit development. If nighttime temperatures remain too high, the plant consumes too much stored energy, resulting in weaker growth and reduced vigor. This balance is necessary for accumulating biomass before the plant begins to flower.
How Temperature Affects Fruit Set
The most temperature-sensitive stage in the tomato lifecycle is the flowering and fruit-setting period. Successful fruit production relies on the viability of the pollen and the receptive state of the stigma. Temperatures outside a narrow optimal range directly interfere with these delicate reproductive processes.
When temperatures consistently exceed 90°F (32°C) during the day, the heat severely damages the pollen grains. This high heat essentially sterilizes the pollen, preventing successful fertilization of the ovules. The plant recognizes this failure and often sheds the unfertilized flowers, a phenomenon commonly called blossom drop, leading to a complete lack of fruit production during heatwaves.
A similar reproductive failure occurs when temperatures drop too low, typically falling below 55°F (13°C) during the night. Low temperatures slow the growth of the pollen tube once it lands on the stigma. The process becomes so slow that the pollen tube often fails to reach the ovary before the flower senesces, resulting in the flower dropping off the plant.
The consistent lack of fruit on otherwise healthy, large plants is often directly attributable to these temperature extremes affecting pollen viability and fertilization success. Maintaining the ideal day/night temperature balance is necessary during this specific window to ensure a successful transition from flower to developing tomato.
Dealing with Temperature Stress and Extremes
Temperatures outside the optimal range cause reproductive failure and physical damage to the plant structure. Prolonged exposure to temperatures above 95°F (35°C) results in generalized heat stress, which manifests as sunscald on developing fruit and a reduction in overall plant vigor.
Mitigating High Heat
To mitigate the intense solar heat, growers often employ shade cloth, which reduces the ambient temperature around the plant and protects the leaves and fruit from direct, damaging sunlight. Deep, consistent watering is another strategy to help the plants regulate their internal temperature through evaporative cooling.
Protecting Against Cold
Conversely, temperatures falling below 50°F (10°C) can induce chilling injury, which slows growth and can lead to water-soaked, necrotic spots on the leaves and stems. The most severe threat is temperatures at or below 32°F (0°C). This causes water inside the plant cells to freeze, leading to cell wall rupture and rapid tissue death (frost). Frost requires immediate protection, such as covering the plants with row covers or blankets to trap residual ground heat. Proactive management is necessary to protect the entire crop.