Rosemary Dropping Leaves and Turning Brown
Answered by: Richters Staff
Question from: Jeanette Jobson
Posted: Before April 1998

I grew rosemary very successfully in a herb bed this past summer and as we live in a Zone 5 (or lower area) I potted it up and brought it indoors for the winter. In the past few weeks it has dropped nearly all its leaves and is turning brown. It has been watered sparingly and is in a cool room with light on three sides. Is this natural or will the plant die and I’ll have to start again next spring?

This is a common problem with rosemary. Unlike other, faster growing herbs like mints and chives, rosemary has a harder time adjusting to the lower light levels when brought indoors. All plants produce two types of leaves depending on the light it receives. In the strong light of outdoors plants produce ‘high light’ leaves which, among other things, has as a defining characteristic that they are inefficient in turning light into plant food. They don’t have to be efficient because outdoors they usually get more light than they really need for survival. Indoors, on the other hand, plants rely on a different type of leaf, the ‘low light’ leaf, which is much less efficient in converting light into plant food.

Now, fast growing herbs are able to change gears quickly when they are potted up from the garden and brought indoors. Rosemary, being a relatively slow-growing evergreen, can’t shift gears as easily. With a sudden new regime of lower light (even the brightest of windows is about one-tenth the intensity of outdoors) the inefficient ‘high light’ leaves can’t produce enough food for maintenance, let alone new growth. Low light levels for a ‘high light’ plant is a starvation diet and the plant has to make adjustments. It’s as if an Olympic sprinter has to make do with only one meal a day. The sprinter will quickly lose weight; likewise, a ‘high light’ plant will start to shed inefficient leaves because the food those leaves makes is less than needed for their maintenance. If the plant cannot adjust fast enough to produce new high- efficiency ‘low light’ leaves, the plant will eventually starve itself to death. Rosemary plants can look fine until mid-winter or so when suddenly they keel over.

So, yes, there is a good chance that you will have to start over again with new plants this spring. To avoid the same fate, try digging up your plants a month earlier than you did this year and gradually, over a period of 4-8 weeks, expose the plant to lower light levels until it is adjusted to indoor light levels. You can tell when a plant is adjusting when it sheds older leaves and produces new leaves. If the new leaves are developing well then you know that the plant is succeeding in its attempt to renovate itself.

For now, try increasing the light level as much as much possible. Move the plant right in the brightest window, placed. say, on the windowsill, not on a table near to the window. Take extra care not to overwater – examine the soil daily and only water if the soil feels dry below the surface. Examine branches that have lost most or all of their leaves: branches that snap easily (with stems that are dry and brown at the centre) should be cut back down to live tissue (green or white stem centres).

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