The appearance of purple coloration on a tomato plant, particularly on the stems and the undersides of the leaves, is a common signal of stress. While the sight of a purple plant can be alarming to a gardener, it is a diagnosable and frequently reversible condition. This distinct color change is the plant’s physical response to an internal imbalance, and understanding the root cause is the first step toward ensuring a healthy harvest.
Identifying the Primary Cause: Phosphorus Deficiency
The most frequent reason a tomato plant turns purple is a lack of available phosphorus (P), a primary macronutrient. Phosphorus is essential for energy transfer, robust root growth, and cell division within the plant. When a tomato plant cannot acquire sufficient phosphorus, its growth becomes severely stunted.
The lack of P prevents the plant from efficiently utilizing sugars produced during photosynthesis. These excess sugars are then channeled into the production of a purple pigment called anthocyanin. This purple or reddish-violet hue typically appears first on the veins and undersides of the older, lower leaves and along the stems.
Environmental and Genetic Factors
A phosphorus deficiency does not always mean the soil lacks the nutrient altogether. Often, the phosphorus is present but is chemically “locked out,” meaning the plant cannot absorb it due to environmental conditions. The most common environmental inhibitor is cold soil temperature, which is a major concern for early spring plantings.
Tomato plants, being warm-season crops, require a soil temperature of at least 60°F (16°C) for healthy root function, with an ideal range being 65°F to 70°F. When the soil is cold, the plant’s metabolism slows down, severely limiting the ability of its roots to absorb phosphorus, even if the nutrient is abundant in the soil solution.
Soil pH also plays a significant role in making phosphorus inaccessible, a condition known as nutrient lockout. Phosphorus is most available when the soil pH is slightly acidic to neutral, ideally between 6.0 and 7.0. In highly acidic soil (below 5.5), phosphorus binds with elements like iron and aluminum, forming insoluble compounds. Conversely, in alkaline soil (above 7.5), phosphorus precipitates with calcium, making it equally unavailable for root uptake.
The purple color may also be a natural trait of the variety being grown. Certain heirloom varieties, such as Black Krim and Cherokee Purple, naturally exhibit dark purple coloration on their stems or leaves, especially in the early stages of growth. This genetic characteristic is not a sign of deficiency and will persist even when the plant is perfectly healthy. If the plant is growing vigorously and only the stems or new growth are purple, genetics are the likely explanation.
Solutions and Prevention
For a plant showing purple discoloration due to cold stress, the first and most effective action is to warm the soil. This can be accomplished by using black plastic mulch, which absorbs solar radiation and can raise the soil temperature by several degrees, or by waiting until the soil consistently registers above 60°F before transplanting. Providing immediate access to readily available phosphorus can also offer a quick fix.
A fast-acting remedy is a foliar application of a water-soluble fertilizer high in phosphorus, such as a 0-52-34 blend, which is absorbed directly through the leaves and bypasses the compromised root system. For a longer-term solution, incorporate an organic source like bone meal into the soil before planting, mixing one to two tablespoons into each planting hole. Bone meal provides a slow-release source of phosphorus and calcium, but it is most effective in slightly acidic to neutral soil.
If soil pH is suspected to be the issue, a soil test is necessary to determine the exact level and what amendment is required. For soil that is too acidic, applying lime will slowly raise the pH over time, enhancing phosphorus availability. If the soil is too alkaline, elemental sulfur can be applied to gradually lower the pH. This process works best in warm, moist soil and can take several months to a year to fully take effect.