As an avid gardener, I’ve run across a puzzling issue a few times — plants suddenly developing purple leaves. A phosphorus deficiency is usually the root cause of this issue. It occurs when plants can’t access enough phosphorus from the soil due to factors like cold temperatures, high soil pH, or poor drainage.
The purple discoloration happens as plants struggle to produce energy without enough phosphorus. To fix this, you’ll need to address the underlying cause by improving soil conditions. Growers may need to adjust soil pH, improve drainage, or add organic matter to boost phosphorus availability. In some cases, you may need to supplement with phosphorus-rich amendments. In this article, I’ll guide you through identifying and resolving this common gardening challenge.
What Causes Purple Leaves?
Purpling of leaves can have a variety of causes, but the most common one is a deficiency in phosphorus. Phosphorus is a nutrient essential for plant growth that is particularly important for rooting, flowering, and fruiting.
If phosphorus is deficient — or if conditions make it difficult for plants to absorb the phosphorus that’s there — plants will stop or severely limit growth. The older leaves start turning a dull, dark green, which then turns to purple. The purpling begins at the tips or undersides of the leaves and moves its way across the entire leaf.
How To Address Phosphorus Deficiency
In most cases, phosphorus deficiency isn’t actually caused by a lack of phosphorus in the soil but by some other problem that keeps the plants from absorbing nutrients properly. Adding phosphorus won’t help, and high-phosphorus chemical fertilizers are very polluting.
Before adding phosphorus supplements to your soil, do the following:
- Perform a soil test: A soil test will tell you the usable phosphorus levels and other essential nutrients, as well as other conditions that can affect phosphorus absorption. If your soil test does indicate low phosphorus levels, you can gently raise them using rock phosphate, bone meal, superphosphate, manure, or compost.
- Consider the weather: Cold, wet soil can keep plant root systems from absorbing phosphorus properly. Signs of phosphorus deficiency are frequent in early spring if plants are set out a little too early. If cold is the culprit, the plants may recover when things warm up.
- Check soil pH: Highly acidic soil can also affect phosphorus and magnesium uptake. Adjust soil pH if necessary, as phosphorus is most available in slightly acidic to neutral soils (pH 6.0-7.0)
- Look at iron levels: Soils high in iron can bind up phosphorus in a way that makes it difficult for plants to absorb this macronutrient.
- Check moisture levels: Improve soil drainage to prevent waterlogging, which can hinder phosphorus uptake
Other Signs of Phosphorus Deficiency in Plants
Phosphorus deficiency manifests in several ways, with purple leaves being one of the most noticeable symptoms. Here are some additional phosphorus deficiency symptoms you should look out for:
- Stunted growth, weak stems, and delayed root development
- Delayed flowering or fruiting
- Smaller than normal plant leaves
- Overall plant appears dark green before purpling occurs, especially the lower leaves
How I Fixed My Purpling Plants
When I had some plants start to turn purple, I decided that a late spring cold snap was the likely culprit that caused the problem with phosphorus absorption. The flowers were the only plants affected. The nearby vegetables were growing like weeds, and the surrounding grasses were healthy and strong, which makes me think the problem wasn’t in the soil. As further evidence, I had a few extra plants still in their store pots (not in my soil), and they turned a purple color, too.
Rather than rush out for tests and supplements, I decided to adopt a wait-and-see approach, keeping the flowers watered until the weather turned warmer, then continuing with my original feeding plan with a mild, balanced organic fertilizer.
In this case, my plants recovered and had green leaves in a few weeks. However, if the problem got worse or started to spread to my veggies, I would have done a soil test and gone from there.
Final Thoughts on Purple Plant Leaves
Encountering purple leaves in your garden can be alarming, but understanding that it’s often a sign of phosphorus deficiency is the first step toward a solution. This problem happens when plants can’t absorb enough phosphorus, typically due to environmental factors rather than a lack of phosphorus in the soil.
You can fix it by adjusting the pH of your soil, improving your soil’s drainage, adding organic matter, or waiting out the cold as I did. With patience and the right interventions, you can restore your plants to their vibrant, green glory and enjoy a thriving garden once again.
Frequently Asked Questions
Can cold weather cause purple leaves even if phosphorus levels are adequate?
Yes, cold soil temperatures can inhibit phosphorus uptake, leading to purple leaves even when phosphorus is present in the soil.
How long does it take for plants to recover from phosphorus deficiency?
Recovery time varies, but you should see improvement within 1 to 2 weeks after you address the issue. Full recovery may take several weeks.
Are some plants more susceptible to phosphorus deficiency than others?
Yes, fast-growing plants, like tomatoes and corn, are often more susceptible to phosphorus deficiency.
Is it possible to add too much phosphorus to the soil?
Yes, too much phosphorus in the soil can be harmful to your plants. It can potentially lead to nutrient deficiencies, imbalances, and environmental issues.
