7 Reasons Why Your Basil Leaves Are Turning Yellow

Healthy basil leaves are bright to deep green, with a few edible varieties producing foliage in purplish red. When leaves start to take on a yellowish color, it's a sign your basil plant's growing conditions are wrong in some way. Yellow basil leaves quickly lose flavor and texture and you could end up losing foliage altogether, so it's important to identify the problem and correct it quickly to save your harvest.

Here are seven common reasons why basil leaves turn yellow, plus tips on how to fix and prevent the problems.

Overwatering

Overwatered basil is prone to root rot, which causes the plant to wilt and lower leaves to turn yellow.

To know if overwatering is the cause of yellow basil leaves, check soil moisture by poking in a finger or using a moisture meter. Potted plants may need to be moved into a new pot with fresh, dry potting mix. For in-ground plants, hold off on irrigating until soil has had a chance to dry out.

To prevent root rot, allow the top 2 inches of soil to dry between waterings and make sure potted plants have plenty of drainage holes. If you're growing basil in a pot, be sure to drain off all excess water before returning the plant to its saucer.

Poor Drainage

Poor drainage also leads to root rot and yellowing of leaves.

Loosen compacted soil with aged compost or sand prior to planting. A mix of compost and potting soil also allows for good drainage in potted plants. Leave plenty of space between plants; 12 to 16 inches in the garden bed.

Avoid crowding too many plants in one pot. Basil grows best in large deep pots with about 3 gallons of soil for each plant.

If the soil in your potted plant is compacted, you can break up the soil by gently poking in a dowel.

Nutrient Deficiency

Feed basil with balanced NPK fertilizers like 10-10-10. Nitrogen deficiencies can cause leaves to yellow. Inadequate levels of magnesium, iron, and some other micronutrients may lead to chlorosis, which is a yellowing of leaves due to poor photosynthesis.

Add compost to garden soil before planting to boost available nutrients. Potted plants benefit from regular feeding every two to three weeks throughout the growing season.

Incorrect Light

Basil is a warm-weather annual that grows its best leaves with 6 to 8 hours of sunlight daily and temperatures in the 50 to 80°F range. It does not tolerate frost. However, extended periods of temperatures above 80 degrees F. can cause leaf burn. Edges turn brown, and leaves yellow, eventually die, and drop off.

In hot, dry climates with extended periods of temperatures in excess of 80 degrees F., a little afternoon shade helps avoid leaf burn.

Inadequate Air Circulation

When properly pruned, basil grows into a bushy form with plenty of dense foliage. This makes it important to give each plant plenty of individual space and to practice good harvesting habits throughout the season to maintain good circulation within the plant.

Keeping the center of the plant open prevents leaves from compacting together which causes wilting and yellowing and reduces the harvest.

Bolting

Although basil is a warm weather herb, hot summer temperatures quickly spur the plant into bolting. When leaves start to take on a yellow caste, this can be a signal your plant is preparing to produce flowers and go to seed.

During warmer periods, check your basil daily and pinch out buds and flowers forming at the top of the plant. This, in turn, promotes branching and more leaves.

Pests and Plant Disease

Basil is not particularly vulnerable to insect pests, although a spider mite infestation causes yellow stippling in the leaves, which eventually turn brown and fall off. Look for webbing underneath or between leaves. Depending on how bad the infestation is, several treatments with neem or a horticultural oil might be needed.

Downy mildew causes yellow splotches on leaf tops with a grayish spore-like mold on the undersides. Plants affected with this fungus can't be saved, although unaffected leaves can still be harvested and are safe to eat. Pull the infected plant, remove any usable leaves and put the rest of the plant in the trash.

Fusarium wilt is another fungal infection identified by yellowing leaves with brown spots. There is no cure for fusarium wilt, so plants should be removed and disposed of.