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Growing Rosemary Inside

The proper conditions for growing Rosemary indoors

By: Sandy Feather 2008
Penn State Extension


Q. I had two spectacular rosemary plants that I grew in pots on my deck all summer. I brought them into the house for the winter and put them in a sunny window, but now one rosemary does not look very good. About of the plant has gotten brown foliage. Do rosemaries require a certain type of indoor care during winter?

A. Rosemary (Rosmarinus officinalis) is a tender, evergreen shrub native to the Mediterranean, popular in herb and kitchen gardens as well as ornamental landscapes where it is winter hardy. Since it is only reliably hardy to USDA Zones 7-8, it should be overwintered indoors in Pennsylvania. Rosemary grown in very sheltered locations through a mild winter can survive, but don't count on it.


Average Annual Minimum Temperatures in Pennsylvania

(Click to enlarge)
Pennsylvania is in hardiness Zone 5 and 6

While it has been problem-free in our demonstration gardens (even Bambi does not bother it), growing it indoors is more of a challenge. The dead area on your plant could be due to a number of factors, including overwatering, underwatering, not enough sun or powdery mildew.

When it comes to growing rosemary indoors, the more sun, the better. A south- or southwest-facing window is best. Be sure to give the plants a one-quarter turn frequently to expose all parts of the plant to as much sun as possible. This also keeps them from growing lopsided.

Rosemary in a pot
Proper watering of Rosemary is important

Another factor is that rosemary overwinters best indoors between 50 and 60 degrees, cooler temperatures than people find comfortable in their homes. I've had good success overwintering rosemary in a bright attached garage where winter temperatures stay between 40 and 50 degrees.
  
Watering is perhaps the most critical factor in successfully overwintering rosemary indoors. The trick is to keep the soil evenly moist without overwatering. You want the soil to dry slightly in between waterings, but you never want it dry out completely. Overwatering kills rosemary with root rot, but if you allow it to dry out completely, it will die almost overnight.


Many factors work together to determine how often you should water any houseplant:

Potting soil texture - Light artificial mixes dry faster than those that contain soil mixed with amendments such as perlite, vermiculite, compost, bark, grit, sand and/or peat moss. An ideal potting mix for rosemary would have sharp drainage while holding some moisture.

The ratio of pot size to plant size - Small pots with large plants require more frequent watering than larger pots. Large pots with small plants may hold too much water and create perfect conditions for root rot to start.

 

Type of pot - Plastic pots hold more moisture than clay pots and do not need to be watered as frequently. However, clay pots "breathe" through their sides and create a more favorable environment for the roots of many plants, including rosemary.

Air temperature - Plants grown in cool (50- to 60-degree) temperatures use less water than those grown at warmer temperatures (70 to 80 degrees).

Whether or not the plant is actively growing - Plants that are putting on new growth use more water than those that are just sitting there. New growth is more tender and succulent and loses moisture more rapidly than older growth that has hardened off.
  
Rather than watering on a set schedule, every Monday for example, get into the habit of feeling the soil before you water. If the top inch or two of soil is dry, go ahead and water; if it is still moist, hold off until it is drier.

 

Rosemary foliage
Evergreen foliage of Rosemary

Powdery mildew is a fungal disease characterized by a white coating on the leaves. Since the underside of rosemary leaves and its new growth are covered with very fine white hairs, powdery mildew may go unnoticed until it is severe. At that point, the plant can be defoliated pretty quickly and may not survive. Powdery mildew seems to be more of a problem for growing indoors than outdoors because air circulation is not adequate indoors. Space plants to allow as much air circulation as possible, and monitor them closely throughout the winter. You can also position a small fan to blow on the plants for a few hours every day to create some air circulation.

Ultra-fine horticultural oil applications easily control powdery mildew on rosemary without using stronger fungicides. Be sure to use ultra fine horticultural oil rather than dormant oil. Horticultural oil is more refined and less likely to burn the foliage. Read and follow label directions as to application rate and intervals between applications. More is never better, and too strong a solution or too frequent application can also burn the foliage.


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