Rosemary plants, of the mint family Lamiaceae, are fragrant evergreen herbs used as culinary condiments and for their health benefits. Native to the Mediterranean, these plants have fibrous root systems and prefer hot climates with intense sunlight. Like most plants, they also suffer from certain conditions that cause yellowing leaves.
Rosemary leaves turning yellow: Common reasons
Nutrient deficiency or excess nitrogen
Rosemary plants may develop yellow leaves because they lack certain nutrients like iron, zinc, nitrogen or manganese. It is likely an iron deficiency if the yellowing starts on the young leaves and spreads to the older ones. However, if the first leaves to turn yellow are the oldest ones, it could be a nitrogen, manganese or zinc deficiency.
To fix the issue, add fertilizer at least monthly or every two months during the growing period, but not more than twice during winter dormancy. Water-soluble fertilizers with micronutrients are ideal; just mix one teaspoon of fertilizer with one gallon of water.
Inversely, yellowing rosemary leaves could also be attributed to a soil enriched with manure that is high in nitrogen, or because of too many fertilizers. Rosemary plants thrive in sandy or stony soil and have adapted themselves to these conditions. Excess nitrogen can cause the leaves of the plants to become yellow.
Other signs of nitrogen stress, aside from yellowing leaves, include weaker-smelling and tasting leaves, and more abundant foliage but with fewer flowers. Studies attest that rosemary planted with more perlite or sand, along with lower levels of fertilizer, produce more oils in the leaves and have increased aroma and flavor.
Planting rosemary in a sandy soil mix that is fast-draining recreates their native soil type and allows good drainage while creating a low-to-medium nutrient environment that is ideal for these plants.
Root rot and water problems
Rosemary leaves turning yellow could also be attributed to root rot and water problems. Rosemary thrives in well-draining soil and too much water could trigger root rot. Check your plant’s roots by removing it carefully from the pot and exposing the roots.
If you notice that the roots are black and slimy, this indicates root rot; healthy ones are white and firm. You can still save your plant if only a few roots are affected, by cutting back on watering and increasing drainage.
To resolve yellowing leaves due to water sensitivity:
- Cut watering back to once every two weeks.
- If the soil is slow-draining, improve it with sand or grit.
- Transfer the plants to pots with good drainage.
- Move the plants to an open area of the garden for more air flow.
Rosemary plants that are already established may only require watering once every two weeks with a generous soak. This encourages the roots to establish well and to make sure that the plants are resilient to drought conditions.
Spider mite infestation
Another reason rosemary leaves turn yellow is the presence of pests like spider mites. They feed by sucking the plants’ sap or juice, which weakens the plants. If left untreated, they could eventually kill the plants.
To treat the problem, wash down the plants with a forceful spray of water and use neem oil to keep the spider mites away. Make sure that the plants do not get too dry between watering to avoid attracting spider mites.
Insufficient light
Rosemary plants thrive when exposed to full sun for around six to eight hours daily, while indoor ones need plenty of supplemental light. If these plants do not get sufficient light, the leaves will start to turn yellow.
To fix this, place the plants in a sunny, south-facing or west-facing window where there is enough sunlight. Indoor plants should have a grow light or standard fluorescent lighting. Position the light at least six inches above the rosemary and leave it on for about 14 hours a day. It is ideal to use a timer to automate the light switch.
Conclusion
Rosemary plants are evergreen and fragrant with needle-like leaves. They are cold-hardy plants, but also suffer from disorders such as yellowing leaves. This could be due to nutrient deficiency or nitrogen toxicity, as well as watering problems, spider mite infestation or insufficient light.
Image: istockphoto.com / Ivannag82