Watering and Fertilizing
Indoors or out, avoid overwatering rosemary. Let plants in pots dry out between waterings. If you water rosemary too much its leaf tips begin to turn brown, then entire leaves die and drop off, starting at the base of the plant. Let soaked soil or container mix dry thoroughly before watering again.
Rosemary does fine in good soil with just the all-purpose slow-acting granular fertilizer added to the soil in the spring. However if you harvest stems often, it benefits from supplemental sprays of a liquid seaweed or kelp-based plant tonic product two or three times during the growing season to supply extra micronutrients to the plants. Overfeeding causes excessive leaf growth, which looks good but reduces the desirable flavorfulness of the leaves.
Unless your soil is really infertile, lacking organic matter, avoid mulching rosemary plants with organic materials. They tend to harbor fungus diseases, such as botrytis blight, that flourish in humid environments and threaten densely growing dry-land plants such as rosemary. Instead, to keep down weeds mulch with crushed stone, pea gravel, or rocks, which dry out quickly and reflect warm sun onto the plants and reduce humidity still further.
Pruning or Grooming Rosemary
Pruning herb plants keeps them healthy and productive. Prune to shape rosemary almost anytime except during flowering. However, do any severe cutting back of outdoor plants prior to 45 days before frost. Rosemary plants like to sprawl, so trim a few inches of new growth regularly with hand pruners to maintain the plant's shape and provide sprigs for cooking. In the spring prune off any winter damaged or diseased stems.
Rosemary withstands fairly severe pruning and is a good candidate for standards, hedges, bonsai and topiary training. Prune any time of the year except during flowering. Potted rosemary plants should thrive for 3 to 4 years if both the foliage and roots are routinely pruned. Rosemary roots do not mind being crowded, so occasional root pruning is preferred to transplanting to a bigger pot.
Most rosemarys cannot withstand temperatures below 20° F. They can handle light frosts, but not hard freezes. In cold climates bring plants indoors and place them by a sunny window. Shelter those left outdoors from the effects of winter weather.
Set pots of rosemary into the soil or mulch the soil over their roots well. Set them in a cold frame or fashion a cover, or cloche of some sort from natural burlap or white polyspun floating row cover to protect against winterburn from drying winds and the heavy weight of snow. Spraying their foliage with an anti-transpirant spray gives additional drying protection.