Rosemary can be a tricky plant to grow, especially as the weather changes for the winter.
Q. I have a potted rosemary plant in my house, and ever since I brought it in for the winter (I live in Wisconsin) it has been turning brown and seems to be dying. I’ve been watering it once a week. Am I overwatering it? I try to keep it in the sun when it shines in the kitchen window. Does it need more sun? Please help.
A. Your rosemary’s symptoms indicate that death is near. Growing rosemary indoors is difficult because dry heat and other indoor conditions are contrary to the needs of this plant. Rosemary’s natural home is the sunny Mediterranean region where the climate is cool and wet during the winter. The best way to keep your plant alive in Wisconsin during the winter is to leave it outside, in partial shade, when the temperature is above freezing.
When wintering rosemary during extended freezing spells, add a grow light to the indoor home of your plant to give it the light it needs. Water it only when the top stem tips droop slightly—then shower the leaves and drench the root ball until it is saturated. This method of watering will wash the house dust off of the leaves, give it the humidity it craves and dampen all of the peat moss around the root system. Only water when the plant shows you it is in need. A light misting between watering days will help the problem of lack of humidity.
Tip: Arid indoor winter air is not pleasing to a rosemary plant. Their native Mediterranean environment is cool and wet during the winter.
Q. I think my rosemary bonsai tree is dying. The tips of the needles were turning black and brown and then falling off, so I thought I was overwatering it. I let it be for a day, and the next morning all the branches except one are dead-looking brown and black, turning gray. We cut off some branches and it appears to be alive at the main stem. Would you advise cutting it down to the trunk?
A. This rosemary is not likely to live. Cutting back to the trunk will not achieve any benefit. I would advise discarding the suffering plant. Keep the pot and the big rocks and start over with a young rosemary and raise it yourself.
Research rosemary’s native growing conditions to understand how to grow rosemary. Make the potting mix, then simulate a Mediterranean climate throughout the year. Your bonsai will need root and top pruning and/or shaping every three to four months during the growing season. Situate it outdoors in dappled shade where you can observe and enjoy it every day. Water it daily during the heat with a diluted fish and kelp fertilizer solution. However, adjust frequency to avoid overwatering plants on cloudy or cool days.
During the winter, keep the rosemary bonsai in the coolest part of the house or, better yet, in a sunroom or cool greenhouse. Night temperatures of about 40 degrees and mild days will help the plant thrive, rather than struggle through the winter in an indoor environment. It will still need a minimum of six hours of strong light. Because growth and transpiration slow down significantly in the winter, the plant will not use as much water as in summer. Watch for the slightest wilting of the leaves before watering—black leaf tips are an early indication that the plant is being overwatered. Brown and black leaves are a sign of root rot, and it is very difficult to heal this condition. It is better to be sensitive to the signals the plant gives you before it is too late.
Tip: Rosemary needs less water during the winter, so monitor your plant carefully for signs of overwatering: wilting or black tips on the leaves.
Tina Marie Wilcox has been gardening at the Ozark Folk Center State Park in Arkansas for more than 25 years. She is co-author of The Creative Herbal Home.