Dec 2022
- My thyme plant has green & yellow leaves. What does this mean?
Thyme is a hardy, aromatic, perennial, evergreen herb in the mint family (Lamiaceae) and the fifty or more varieties with many different fragrances and flavours have culinary, medicinal, and ornamental uses. The species commonly cultivated and used for culinary purposes is Thymus Vulgaris or Common Thyme which comprises a woody stem with paired leaf or flower clusters ("leaves") spaced 15 to 25 millimetres (1⁄2 to 1 inch) apart. It is hardy enough to thrive in the garden and will also grow as a pot plant in a bright, sunny window. There are several varieties that have variegated green and yellow leaves, but I am assuming that this is more of disorder that has developed with the plant rather than a varietal characteristic.
The three main reasons Thyme leaves turn yellow are: 1) Root rot: Root rot (rhizoctonia) is the most common reason for thyme developing yellow leaves and is caused by too much moisture around the roots from either over watering or slow draining soils. It is essential that the soil in which Thyme is planted drains well and it will grow well even in poor, stony or sandy soils or on dry slopes. If the plant is in an area of ground that does not freely drain and you gently disturb the soil around the base of the plant, you may smell the decay from rotting roots. Gently dig up the plant and cut away any rotted root tissue (remembering to disinfect the blades of your cutters before using them on another plant) and re-site the plant to an area with good drainage and full sun. This is a drought tolerant plant having evolved in a hot dry climate with infrequent summer rain and does not require much watering once established. Water only when the soil is very dry and in the garden, plant with other drought-tolerant perennials such as Rosemary to avoid over watering in that area. It is also pollinator friendly. Let some thyme plants flower, since the herb attracts bees; 2) Too much or too little Nitrogen: Nitrogen is fundamental to the growth of leaves and plant tissue, and an important food for the many soil organisms that convert other nutrients into forms that are available to plants. It is also a component of chlorophyll, so it is central to basic photosynthesis, and therefore needed by all plants. It is absorbed as nitrate and ammonium and is one of the three most important macronutrients for the healthy growth of plants, the other two being phosphorus and potassium. These three elements are the central components of most fertilizers, and they form the N-P-K ratio that is shown on fertilizer packaging. A lack of nitrogen can result in plants that are stunted with yellowing leaves, particularly the lower, older leaves of the plant, as any available nitrogen is routed to the newer growth near the stem tips to the detriment of the older growth. Conversely, too much Nitrogen can cause plants to have spindly, weak growth. Often the leaves are at more widely spaced intervals on the stem and can turn pale yellow or brown. An overabundance of Nitrogen can also reduce the potency of the aromatic oils in the plant .Thyme is a tough little plant that needs very little in the way of additional nutrients or soil amendments. The addition of organic matter, like compost, in the early spring is usually sufficient for the plants needs. You can use a balanced fertiliser such as 10-10-10 at half strength if the soil is poor but organic matter is preferable. A neutral or slightly alkaline soil is perfect for Thyme so a very light addition of lime in the fall will help counteract our acidic soils in the Pacific north west; 3) Spider mite infestation:
These tiny (1/50th of an inch) eight legged creatures resemble minute spiders. They feed by sucking the juice from plants and when present in sufficient numbers they can weaken, stunt or even kill infested plants (or parts of them). The first signs of damage are small light-colored specks on the leaves, which may become yellow or brown and eventually die and fall off. There may also be downy webs on the undersides of the leaves. The most important control for spider mites is a healthy population of predatory insects, so do everything you can to encourage these by planting lots of nectar producing plants near to your Thyme plant. If you have an infestation that requires more direct action, try washing the spider mites off your plants with a strong jet of water from a hose nozzle, taking care not to cause too much water to drain into the soil at the base of the plant.
Sources:
https://www.almanac.com/plant/thym
https://www.westcoastseeds.com/products/english-thyme
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