Why Are My Cabbage Leaves Turning Purple?

While growing cabbages is an excellent way to enjoy fresh, home-grown vegetables, you may occasionally encounter some problems with your plants. One of the most common issues you may come across is when the leaves of your cabbage plants start to turn purple. This can be problematic and confusing, especially if you don’t know what causes it.

Your cabbage leaves turn purple due to a nitrogen and phosphorus deficiency, extremely cold weather, and insufficient or excessive watering. Additionally, transplanting the seedlings into an open ground stresses the plants, causing the leaves to turn purple.

Proper care and attention are essential for the healthy and vigorous growth of cabbage plants. Keep reading to learn more about the causes of purple leaves on cabbages and how to fix the issue.

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Common Causes of Purple Leaves on Cabbage Plants

Although cabbage leaves turning purple can be alarming, it’s usually not a reason to worry. In most cases, the problem can be easily fixed with simple changes in how you care for your cabbage plants. The following are some of the most common reasons for purple cabbage leaves.

1. Nitrogen and Phosphorus Deficiency

Nutrients like nitrogen and phosphorus play a vital role in the growth and development of cabbage plants. When there’s not enough of these nutrients in the soil, the leaves of your cabbages are bound to turn purple. Here’s why:

Nitrogen is a major component of chlorophyll, the pigment that gives plants their green color. It’s also a critical component of amino acids, the building blocks of proteins. A nitrogen deficiency in the soil will cause the leaves of your cabbage plants to turn yellow or pale green first and then progress to purple or red.

Phosphorus is essential for cell division and the development of a plant’s root system. It also helps with the production of chlorophyll. A phosphorus deficiency manifests as purplish or reddish leaves, stunted growth, and slowed maturity.

You can amend the soil with organic compost or manure to correct a nitrogen or phosphorus deficiency. Organic compost is an excellent source of nitrogen and other essential nutrients for plant growth.

You can also use a balanced fertilizer that contains nitrogen and phosphorus. However, follow the instructions on the package, as too much fertilizer can damage your plants. Do a soil test to determine the nutrient levels in your soil and adjust accordingly.

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2. Extreme Cold Weather

While cabbage plants can withstand cold weather and a bit of frost, extreme cold can damage the plants and cause the leaves to turn purple. The ideal temperature for growing cabbage is between 55-75 °F (13-24 °C).

When the temperature drops below this range, the plants start to experience stress. This stress manifests in the form of purplish or reddish leaves. However, this defense mechanism lasts for only a few days, and the leaves will eventually turn green again as the temperature rises.

If you live in a state that frequently experiences cold weather like Alaska, Minnesota, or Wisconsin, you can start your cabbage plants indoors 4-6 weeks before the last frost date. This will give the plants a head start and help them withstand the cold weather better.

You can also use row covers or plastic tunnels to protect your plants from extreme cold and frost. Add a layer of organic mulch like straw, hay, or leaves around the plants to insulate them and keep the roots warm.

3. Insufficient or Excessive Watering

Cabbage plants need about 1-2 inches (2.5-5 cm) of water per week, either from rainfall or irrigation. Water is essential for the growth and development of cabbage plants. It helps transport nutrients from the soil to the plants and keeps the leaves from wilting. However, too much or too little water can also cause problems.

If the soil is too dry, the leaves of the cabbage plants will become stiff and turn purplish or bluish. In severe cases, the leaves will wilt and eventually die. Conversely, when the soil is too wet or waterlogged, the roots of the plants cannot get the oxygen they need to function properly. The roots will start to rot, and the leaves will turn yellow, then brown, and finally die.

To avoid these problems, water your cabbage plants only when the soil is dry to the touch. Insert your finger 1-2 inches (2.5-5 cm) into the soil to check the moisture level. If the soil particles fall off your finger, the soil is dry, and your plants need water. Water the plants at the base, taking care not to wet the leaves as this can encourage fungal diseases.

However, if the soil particles stick to your finger, the soil is still moist, and your plants don’t need water yet. In hot climates like Hawaii, Texas, and Arkansas, mulching your plants can help conserve moisture and keep the roots cooler. Leave a 1-inch (2.5 cm) space from the stem of the plant to avoid rot.

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4. Soil pH Imbalance

The pH level of your soil indicates how acidic or alkaline the mixture is. Cabbage thrives in a soil mixture with a pH level between 6.0 and 7.5, which is slightly acidic to neutral. If the pH level of your soil is too high or too low, it can affect the uptake of nutrients by the plants, which in turn can cause the leaves to turn purple.

Before correcting your soil’s pH level, it’s crucial to get it tested first to determine whether it’s too acidic, too alkaline, or just right. It’s always best to send a sample of your soil to a local lab to have its pH level tested. Alternatively, you can always use an at-home pH test kit (which you can find online or buy at your local nursery).

If your soil is too acidic, you can add agricultural lime to raise the pH level. For every 100 square feet (9.3 square meters), add 5 pounds (2.3 kg) of agricultural lime. You can also spread a layer of mulch around the base of your plants. Wood ash has high levels of calcium and potassium, which can help raise your soil’s pH level.

To fix soil that is too alkaline, you can add sulfur, which lowers the pH level. Well-decomposed compost can also help to lower the pH level of your soil. Spread a layer of compost around the base of your plants and till it into the top 3-5 inches (7.6 cm-12.7 cm) of soil.

5. Black Leg

Blackleg is one of the most serious diseases that commonly affect cabbages. The root cause behind it is Leptosphaeria maculans (Phoma lingam), a fungus that enters the plant through wounds in the leaves or stems.

The bacteria then release toxins that kill the plant’s cells, causing the leaves to turn yellow and eventually die. The plant’s stem will also turn black and become soft and mushy. Blackleg is most common in warm, wet weather.

Early detection and treatment are crucial to preventing blackleg from spreading to your other cabbage plants. If you notice any symptoms, remove the affected plants immediately and destroy them. Avoid getting water on the leaves of your plants as this can spread the bacteria.

Water your plants at the base, taking care not to wet the leaves and stems. You should also practice crop rotation, planting your cabbage in a different location each year. This will help to prevent the build-up of bacteria in the soil.

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6. Transplanting Shock

Transplanting shock is a common problem that can occur when you transplant cabbage seedlings into your garden. It occurs due to the disturbance of the roots when the seedlings are moved.

The plants also suffer stress from changing temperatures and light conditions, causing the leaves to turn purple. Luckily, this is not a serious problem, and the leaves should return to their normal color within a few days.

To prevent transplanting shock, harden off your seedlings before planting them in the garden. Hardening off is the process of gradually acclimating your seedlings to outdoor conditions.

Start by placing them outside in a sheltered location for a few hours each day, gradually increasing the amount of time they spend outside for a week. This will help them to adjust to the change in conditions and prevent transplanting shock.

Transplant early in the morning or evening when the temperature is cooler. This will also help to reduce stress on the plants. Be cautious not to damage the roots when transplanting, and water the plants well afterward.

  1. Use a garden trowel or your hands to make a hole in the soil that is big enough to comfortably fit the root ball.
  2. Gently remove the seedling from its pot and place it in the hole.
  3. Fill in the hole with soil, being careful not to bury the stem.
  4. Water the plant well and apply a layer of mulch around the base to help retain moisture.
Why Are My Cabbage Leaves Turning Purple
Why Are My Cabbage Leaves Turning Purple

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Nutrient deficiency, extremely cold weather, and soil pH imbalance can all cause cabbage leaves to turn purple. Fertilize with nitrogen and phosphorus-rich fertilizers to boost the soil’s nutrient levels and amend with compost to help with drainage and water retention.

Mulching also helps protect the plant’s roots from extreme temperature changes. When transplanting seedlings, harden them off first to prevent them from undergoing transplant shock. Following these tips, you can prevent your cabbage leaves from turning purple and have healthy, bountiful plants.