Rosemary topiaries shaped like a Christmas tree are great gifts to give and receive. If you are the lucky recipient, you should know a few things about how to care for rosemary indoors.
First of all, rosemary is tricky to grow inside. A rosemary topiary has been grown under controlled conditions in a greenhouse for months and won't be thrilled to be away from those conditions. Remove the foil wrapper or anything else that may be holding in moisture and make sure the pot has at least one hole for drainage-more is better. To reduce the effects of climate shock, it will need full sun. Rotate the pot weekly so that all sides of the plant get sunlight. Rosemary is native to Mediterranean climates so it prefers a hot and sunny location with its roots kept on the dry side but the air around it humid.
Place the pot on a shallow pebble-filled container or tray. Add water so that it does not cover the pebbles. You want the rosemary plant to have increased humidity but not soggy soil and roots.
Water when the soil dries out a bit but not to the point where the plant is wilting. No fancy tools needed...just stick your finger about an inch down into the soil. If it feels soggy, wait a couple more days.
Water the plant by placing the pot in a larger container and then add water to the larger container. Let the plant absorb water for about an hour. Remove the rosemary pot and let it drain before returning it to sit on the pebbles. Overwatering will cause root rot.
Prune out dead or browning stems as soon as you can, especially toward the inside of the plant. Trim new growth to keep the shape of the topiary, if desired.
You may notice a white coating on the leaves, a sign of powdery mildew fungus. Powdery mildew is common on rosemary grown indoors and when growth is crowded. Powdery mildew usually disappears when plants are moved outside. It is really not necessary to treat.
If you notice white dots on the leaves and webbing on the plant, you probably have spider mites. Dense foliage and poor air circulation contribute to this problem.
If the container is small enough, you could put the entire plant in the bathtub and give it a shower. This should reduce the population and it can be repeated as needed. This could also be done outside on a warm winter day.
When warm weather arrives, you can plant your rosemary outside. Mediterranean herbs like rosemary, thyme, oregano, marjoram, and lavender should be planted in a sunny location, in very well-drained soil, and protected from desiccating winter winds. If this is not possible, you should protect it with a wrap of burlap or floating row cover. Consider planting rosemary in a raised bed if your soil is heavy and wet. Do not add organic matter to the soil and don't fertilize. Rosemary performs best in poor soil. Remove the plant from the pot and break up the root ball with a sharp knife before planting. Cut away dead and circling roots. New roots will grow where the old roots were cut.