Rosemary in Zone 4B — Mountain West

Rosemary, with its fragrant, needle-like leaves, is a rewarding herb to grow in our Mountain West gardens. The flavor is unmatched by anything you'll find in a store, and it brings a touch of the Mediterranean to your high-altitude cooking. Plus, the satisfaction of harvesting your own herbs, even with our short growing season, is hard to beat.

Our climate definitely presents challenges, from intense sun to alkaline soil and those unpredictable late frosts. But with careful timing and a little extra attention, you can absolutely enjoy fresh rosemary throughout our 128-day growing season.

Starting rosemary from seed indoors is possible, but be warned: it requires patience. Germination can be erratic and the seedlings grow slowly. If you're up for the challenge, start your seeds indoors from late February through mid March, about 12 weeks before you plan to transplant them outside.

Use seed trays with a good seed-starting mix and keep them consistently warm (around 70-75°F) using a heat mat. Provide plenty of light with grow lights, as our spring sunshine can still be weak. Bottom watering helps prevent damping off. Frankly, given our late spring and cool nights, you might find it easier to buy established plants.

Transplant your rosemary outdoors in late May through early June, after the danger of frost has passed. Before planting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week.

Choose a sunny spot with well-drained soil and space your plants 24-36 inches apart. Our intense sun means rosemary needs a sheltered location to avoid leaf burn. Be prepared for unexpected cold snaps that often occur in early summer at altitude. If you started from seed, it might be easier to buy established plants.

Rosemary thrives on neglect, so resist the urge to overwater. Once established, it's very drought-tolerant and prefers dry conditions. Overwatering is the quickest way to kill rosemary in our Mountain climate.

During the active growing season (summer), water deeply but infrequently. Check the soil moisture with the finger test: stick your finger about 2 inches into the soil. If it feels dry, it's time to water. Usually, about 1 inch of water per week is sufficient, given our low humidity and moderate-at-altitude heat. Water at the base of the plant to avoid wetting the foliage, which can encourage fungal diseases.

Signs of underwatering include wilting or drooping leaves. Overwatering, on the other hand, leads to yellowing leaves and potentially root rot. A light layer of mulch around the base of the plant can help retain moisture and suppress weeds, but don't overdo it, as rosemary prefers well-drained soil.

Rosemary needs minimal pruning. The key is to avoid cutting into the old, woody stems, as they are unlikely to regrow.

Lightly prune after flowering to maintain the shape of the plant and encourage bushier growth. Shape your rosemary in spring, removing any dead or damaged branches. Stop pruning around mid- September as the first frost approaches to allow the plant to harden off before winter.

You can start harvesting rosemary from late August through late September, about 90 days after transplanting. Look for stems that are at least 4-6 inches long.

Harvest by clipping the stem tips, taking no more than one-third of the plant at a time. Avoid harvesting the woody stems, as they tend to be too tough to eat. Regular harvesting encourages continued production throughout the season.

As the first frost approaches around mid- September, harvest as much rosemary as you need and dry or freeze it for later use. Consider potting up a small plant and bringing it indoors for the winter, but remember that indoor conditions are often too humid for rosemary, so provide good air circulation.

Rosemary is generally low-maintenance, but here are some common problems you might encounter in our Mountain West gardens:

Root Rot from Overwatering

• What it looks like: Yellowing leaves, wilting even when the soil is moist, and a general decline in plant health. Roots may be brown and mushy.
• What causes it: Overwatering and poorly drained soil. Our alkaline soil can exacerbate drainage issues.
• How to fix/prevent it: Ensure well-draining soil. Water deeply but infrequently, allowing the soil to dry out between waterings. Avoid overhead watering.

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves and stems. Leaves may curl, yellow, and die.
• What causes it: Fungi that thrive in warm days, cool nights, and moderate humidity. Although our humidity is low, dense plantings can trap moisture.
• How to fix/prevent it: Improve air circulation by spacing plants properly and pruning. Remove affected leaves. Neem oil or potassium bicarbonate sprays can help.

Spider Mites

• What it looks like: Fine stippling on leaves (tiny white dots), fine webbing on leaf undersides and between stems. Leaves may turn bronze or yellow.
• What causes it: Tiny arachnids that thrive in hot, dry conditions. Populations explode in drought stress.
• How to fix/prevent it: A strong water spray (daily for a week) is the best treatment. Increase humidity around plants. Neem oil or insecticidal soap can also be used.

Mountain West Specific Challenges: Our moderate-at-altitude heat, low humidity, and dry rainfall mean that rosemary is generally well-suited to our climate, but overwatering is the biggest threat. Be vigilant about drainage and avoid excessive watering.

Rosemary thrives when planted near certain companions in your Mountain West garden. Sage, thyme, and lavender are excellent choices, as they share similar needs for well-drained soil and full sun. These herbs also repel certain pests that might bother rosemary.

Green beans benefit from rosemary's ability to deter bean beetles. Carrots also appreciate rosemary's pest-repelling qualities. Avoid planting rosemary near mint, as mint's aggressive growth can quickly overwhelm rosemary and compete for resources. The different watering needs of mint and rosemary also make them poor companions.