Q. Why are the leaves of my Spanish lavender (Lavandula stoechas) turning yellow?
Lavandula stoechas, called both Spanish lavender and French lavender, is an evergreen shrub native to Mediterranean countries. This association with hot, dry, rocky, living conditions tells you something about the problems that these plants can have. Lavandula stoechas is more fragile than common lavender and frequently short lived if it is not given just the right growing conditions.
The keys to success with Spanish lavender are a warm, sunny location with plenty of air circulation and soil that is gritty and drains well. They need little feeding. The plants require regular watering but must not be allowed to stay soggy or stand in water. Over-watering, water retention in the soil and high humidity can all lead to fungal growth and root rot.
Most Lavandula stoechas and its cultivars require alkaline soil conditions, though some, such as Lavandula stoechas subsp. stoechas, grow well in acid soil.
Yellow leaves on a potted Lavandula stoechas plant may be the result of a moisture problem (humidity, drainage) or too much or too little nitrogen in the soil. If you have been feeding your plant, particularly with a high nitrogen product, you should stop feedings. If not, try giving the plant a dilute feeding, but only occasionally.
It is more likely that your plant is having a problem with moisture. If the plant is not too far gone, try removing any debris such as dead leaves around the base of the plant that may be leading to moisture retention at the roots. Lavandula stoechas has a vast fibrous root system and benefits from regular re-potting; move your plant to a larger container if it seems hemmed in by its own size or other plants in the container that may be preventing it from having good air circulation and it seems strong enough to withstand the move. Replace soil with a gritty, fast-draining, alkaline soil and a heat retaining mulch of pea gravel or builders sand. Make certain that your plant is in an airy position that allows it to receive sun and heat for most of the day.
These plants need winter protection in the New York City area if they are going to thrive. From November through April, they should be kept, ideally, in a spot that allows them to continue to receive plentiful sun but with night temperatures in the 40 to 50 degree F. range and only 5 to 10 degrees higher during the day.
Courtesy of NYBG Plant Information Service