If you’ve noticed your tomato plants turning purple, it’s a sure sign they’re trying to tell you something. This unexpected color change can be surprising, but it’s a common issue with specific, fixable causes.
Purple leaves or stems on tomatoes aren’t a normal part of growth. The color usually points to a nutrient or environmental problem. Don’t panic, though. With a little detective work, you can figure out the cause and get your plants back to a healthy green.
Tomato Plants Turning Purple
The purple hue you see is often due to a pigment called anthocyanin. Plants produce this pigment as a response to stress. Think of it as a sunburn or a cry for help. The key is to identify what’s triggering that stress response in your garden.
Primary Cause: Phosphorus Deficiency
The most common reason for purple leaves, especially on the undersides and stems, is a lack of phosphorus. Phosphorus is vital for energy transfer and root development in plants. When it’s scarce, sugars build up and trigger anthocyanin production, creating that purple color.
How can you tell if it’s phosphorus? Look for these signs:
- Purple coloring starts on the older, lower leaves first.
- The veins on the leaf underside may turn a deep purple.
- Leaf stems and the main plant stalk can also show purple streaks.
- Overall plant growth may be stunted or seem sluggish.
Why is My Tomato Plant Lacking Phosphorus?
A deficiency doesn’t always mean your soil has no phosphorus. Several factors can lock it up, making it unavailable to your plants roots.
- Cold Soil Temperatures: This is a huge factor in early spring. Tomato roots absorb phosphorus poorly in cool soil, even if it’s present. The plant acts deficient until the soil warms up.
- Soil pH Imbalance: Phosphorus is best absorbed in soil with a pH between 6.0 and 7.0. If your soil is too acidic (low pH) or too alkaline (high pH), the phosphorus gets “locked” and the plant can’t use it.
- Genuinely Low Soil Phosphorus: Sometimes, the soil is simply depleted and needs amendment.
Other Potential Causes for Purple Coloring
While phosphorus is the usual suspect, other issues can cause similar symptoms. It’s good to rule these out.
Potassium Deficiency
A lack of potassium can sometimes cause purpling, but it’s more often associated with yellowing and browning leaf edges. If purpling is accompanied by crispy, scorched-looking leaf margins, consider potassium.
Varietal Characteristics
Some tomato varieties naturally have purplish stems or leaves. This is especially true for certain heirloom or “blue” tomato varieties, like ‘Indigo Rose’. If the plant is otherwise vigorous and healthy, it might just be its genetics. Check your seed packet or plant tag.
Environmental Stress
Intense sunlight on young plants, combined with cool nights, can cause a temporary purple tint. This often happens to recently transplanted seedlings. They usually grow out of it as they acclimate and the weather stabilizes.
Step-by-Step: How to Fix Purple Tomato Plants
Here is a clear action plan to diagnose and correct the problem.
- Check the Weather and Soil Temperature: If it’s early in the season and nights are cool, this is the likeliest cause. Be patient. Protect plants with cloches or row covers at night to keep them warmer. The purple should fade as soil temperatures consistently stay above 60°F (15°C).
- Test Your Soil pH: Get a simple home test kit or send a sample to your local extension service. This is a critical step. Knowing your pH guides all your other actions.
- Amend the Soil Based on Results:
- If pH is too low (acidic): Add garden lime to raise it.
- If pH is too high (alkaline): Add elemental sulfur or a soil acidifier to lower it.
- If pH is correct but phosphorus is low: Add a phosphorus-rich fertilizer or amendment. Bone meal is a good organic option. Use a balanced, water-soluble fertilizer with a higher middle number (e.g., 10-20-10) for a quick fix.
- Apply Fertilizer Correctly: When feeding, water the soil first, then apply fertilizer to damp soil. This prevents root burn and helps nutrients dissolve. Always follow package instructions—more is not better.
- Ensure Even Watering: Inconsistent watering stresses plants and hampers nutrient uptake. Aim for deep, regular watering that keeps the soil consistently moist but not soggy. Mulching helps tremendously with this.
Prevention for Future Seasons
The best cure is always prevention. A little prep work can stop purple leaves before they start.
- Warm the Soil Before Planting: Use black plastic mulch for a few weeks before transplanting. It absorbs heat and warms the soil underneath.
- Amend Soil in Advance: Mix in plenty of well-rotted compost or a balanced, slow-release fertilizer into your planting beds each season. This builds long-term soil health.
- Choose Resilient Varieties: If you garden in a region with cool springs, seek out tomato varieties known for better cold tolerance.
- Harden Off Seedlings Properly: Gradually expose indoor-started plants to outdoor conditions over 7-10 days. This reduces transplant shock and environmental stress.
When to Worry and When Not To
A little purpling on seedlings or lower leaves often corrects itself. Focus on the new growth at the top of the plant. If that new growth is coming in green and healthy, your fixes are working. The old purple leaves may not fully recover, but that’s okay.
However, if the purpling is spreading rapidly to all new growth, or if the plant is severely stunted and wilted, you need to act fast. Review the steps above and consider if a disease could be involved, though purple coloring is rarely a primary disease symptom.
Frequently Asked Questions (FAQ)
Can overwatering cause tomato plants to turn purple?
Not directly, but overwatering damages roots and leads to nutrient deficiencies. Soggy, cold soil is a double-whammy that can induce phosphorus deficiency and purpling.
Will the purple leaves ever turn green again?
Leaves that have turned a deep purple are unlikely to revert to a full green. The good news is, once you correct the problem, the new leaves that grow in should be a normal, healthy color.
Is it safe to eat tomatoes from a plant that had purple leaves?
Yes, it is completely safe. The fruit is not affected by the nutrient deficiency causing the leaf color. Just make sure the tomatoes themselves are ripe and show no signs of rot.
My tomato plant stems are purple but leaves are green. What’s wrong?
Purple stems alone can be a mild sign of phosphorus deficiency or a varietal trait. If the plant is growing well and the leaves look fine, it’s probably not a major issue. Monitor it and ensure it has good nutrition.
What’s the fastest way to add phosphorus to soil?
A water-soluble fertilizer high in phosphorus (the middle number on the bag) will deliver nutrients most quickly to the plant’s roots. Organic options like bone meal or fish emulsion work well but act a bit slower as they need soil microbes to break them down.
Seeing your tomato plants turning purple can be a momentary shock, but now you know it’s a solvable puzzle. Start by checking the soil temperature and pH—these are the most common culprits. With the adjustments outlined here, your plants will likely bounce back with strong, green growth and go on to produce a wonderful harvest for you.