Yarrow.

• Air. Your herbs will thrive in a temperature between 65° to 70°F, about what is comfortable to you. Avoid rooms that are stuffy and have stale air. Herbs like fresh, moving air but not drafts; humidity is best around 50 percent.
• Soil. For your container potting mix, forget about soil. A soil-less medium of one part sphagnum peat moss and one part perlite makes a good mixture for herbs. If you prepare your own mix, add 3 to 4 tablespoons of horticultural lime per quart of mix; peat can be quite acid and the lime sweetens the mix. Stir ingredients while dry and dampen the medium before planting. Soil-less media with natural ingredients, but without soil, have been tested on herbs and found to be superior to other potting soils. For years we have used Pro-Mix BX, a commercial soil-less medium, with excellent results.
• Water. Watering is not as tricky and mysterious as you may have been led to believe. Here are a few general rules which might improve your green thumb: Don't let the pot sit in water and drown the herb's roots. During warm weather and rapid growth, plants are more tolerant of water. When the air is cool and light is poor, go easy with water and allow the surface of the soil to dry between watering. Water temperature should match air temperature. A fan moving air around and over your potted herbs will help dry the growing medium.
• Pots. Containers come in many sizes, shapes, colors and materials. I like plastic because it is light and cheap. Plants do not dry as quickly in plastic as they do in clay pots which can be a disadvantage during winter. Don't worry about stones or broken crockery in the bottom when the pot has lots of drainage holes; the stones actually hinder drainage and are a bad idea. Pot your herbs in a container 2 inches larger than the one it is transplanted from. Don't put a small plant in a huge pot, the roots cannot use all that moisture and they are likely to rot quickly. Because plants grow at different rates, planters with several different types of herbs will need careful attention; it is sometimes better to use the planter to hold individual pots which can make individual transplanting easier. You can keep your plants smaller by leaving them in small pots, but this restriction of growth means less foliage and it can harm the plant. When roots begin to fill the pot and circle its sides, it is time to transplant. Yellowing leaves or shedding leaves sometimes indicates the plant is pot bound.
• Light. Most herbs need 5 hours of full sun for optimum growth. Only a few herbs will grow well without direct sunlight. Don't despair if you lack a south facing, sunny window. Herbs will often adjust to less than the best conditions. A fluorescent light garden is one way to give your herbs and other plants near perfect light conditions. Commercially available 'Grow Lites' can be used but are more expensive. Fluorescent tubes should be no more than 3 to 5 inches above plant tops. Use 4 cool white tubes for each 10 inches of table width. Lights can be used to supplement natural sunlight. Use a timer to provide a total (sun and fluorescent) photo period of 12-14 hours.
• Food. It's better to give your herbs a little food often (every week at half strength, for instance) than a lot all at once. A slow release fertilizer like Osmocote 14-14-14 works well. If your pots are small and you like liquid fertilizers, choose a 10-10-10 or a 20-10-20 formula. Keep in mind that when you water more often, you should also feed your plants more often. Organic fertilizers such as fish emulsion are excellent for soil-less potting media.

• Pruning for shape. Some pruning will be required to keep any rosemary shapely, but the variety's growth habit dictates how much. Rosemary plants can be pruned to almost any shape but the most popular are globes and columns; the variety sometimes lends itself to one or the other shape, but rigorous pruning can overcome any plant's inclinations. The best time to prune for shape is when the plant is in full growth (in our area this is spring and summer); this will provide the plant enough time to recover before the rigors of winter. Remove the amount of stem necessary to create the shape you desire but try not to remove more than half the stem length. Leave some green leaves on the branch that is left. Pruning for shape is essential for rosemary grown into topiary forms. During periods of optimum growth such forms may need pruning every few weeks. When light levels are low indoors, pruning is useful to keep stems from becoming leggy.
• Pruning for health. Winter damage and disease are another reasons to prune. Dead wood caused by winter cold should be removed in early spring. Cut damaged stems back to green leaves or if the stem is dead, remove it. Fungus disease sometimes attacks low-lying branches and the inside of the plant where air circulation and light are poor; this is a particular problem in humid climates or when there is an unusual amount of rain. Lower branches that have dead leaves on them should be removed so that air can circulate under and through the plant to dry dampness. A mulch of several inches of sand or pea gravel dries quickly and radiates drying heat into the plant and is helpful to control diseases.
• Pruning for use. The most enjoyable pruning is for use. Snip branch tips to use when needed; no special method or procedure is necessary and it may be done at any time of year. Fresh growth, rather than hard, woody stems is best. Performed shrewdly, this pruning will shape your plant and keep it healthy.
• Root pruning. Pruning the roots of container-grown plants is a last resort; it revives and reinvigorates root bound plants when larger pots are unavailable. Root pruning is best performed in mid-spring after the plant has been outside for several weeks and soft indoor winter growth has been removed or has stiffened. After the plant is knocked from the pot, take a sharp knife and remove the bottom third of the root ball; then slice several inches from the sides. Add fresh growing medium to the container and return the plant, being careful to work the medium around the edges of the root ball. The root-pruned plant is likely to show some stress in the form of wilting at this time. A protective spray of Wilt-Pruf will help less foliage dehydration. Keep the plant in a spot sheltered from direct sun for a week before returning it to a sunny exposure.

Your plants have been carefully grown in my greenhouses and selected for form and vigor. Before you transplant them to the garden they should be conditioned to ready them for outdoor survival by a process horticulturists call 'hardening off'.

When you bring your plants home and outdoor temperatures are above 40°F, keep them outside in a partly sunny spot (direct sun can sometimes burn tender plant tissue) protected from wind. Bring the plants inside if temperatures are expected to drop below 40°F. After four to seven days of this regimen, the plants should be hardened enough to transplant outside. Less water should be given the plants during this treatment, but care should be taken to keep the plants from wilting. A weak solution of liquid fertilizer, applied to the plant at the time of transplanting, will also help get it off to a good start.

Early transplanting calls for vigilance by the gardener and attention to weather forecast. Danger of a frost or freeze means measures must be taken to protect the young transplants. Poly spun row covers (like Reemay) will provide up to 4°F of protection. Wall O' Water is an excellent choice for protecting tender plants as well as warming the soil prior to planting. Properly installed, the Wall O' Water can give up to 10°F of protection. Effective, home-made devices can be constructed from old plastic milk containers or styrofoam cups with the bottoms removed. An old sheet or blanket will also do in a pinch.

Although no special equipment is necessary to harden off your plants, a cold frame will be helpful. A cold frame is nothing more than a protective structure with a glass or plastic top that will open and shut. Sides may be of wood, masonry, straw, bundled newspapers, or poly sheeting stapled to a wooden frame. The top is usually slanted (usually towards the south) so that it will catch the sun and drain rain water away from the structure. The cold frame protects young seedlings and transplants from the ravages of spring wind and unsettled cold weather. Seedlings are hardened off for a week or two in such a structure to stiffen their stems and adjust them to temperature fluctuations that did not exist indoors on your window sill, under your grow lights, or inside the greenhouse, where conditions are ideal and encourage soft growth.