Rosemary in Zone 5B — Mountain West

Rosemary is a must-have herb for any Mountain West garden. Its piney aroma and flavor elevate roasted meats, potatoes, and even cocktails. Plus, its evergreen foliage adds year-round interest to your garden, which is a real bonus considering our short growing season.

Growing rosemary in our high-altitude environment does present challenges. Intense sun, alkaline soil, and a short 149-day growing season means you have to be strategic. But with the right timing and techniques, you can absolutely enjoy homegrown rosemary.

Starting rosemary from seed indoors is possible, but honestly, it's a bit of a pain. Rosemary germination can be erratic, and the seedlings grow very slowly. If you're up for a challenge, go for it, but buying established plants is usually the way to go in our short season.

If you want to try starting from seed, sow indoors from mid February through early March, about 12 weeks before you plan to transplant. Use seed trays with a good seed-starting mix, provide warmth (a heat mat helps), and give them plenty of light from a grow light. Bottom watering is key to prevent damping off, so set the tray in a shallow dish of water and let the soil absorb it. Remember that our late spring means patience is key!

Transplanting rosemary outdoors should happen from mid May through early June, once the danger of frost has passed. Before planting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This prevents transplant shock from our intense Mountain sun.

Space your rosemary plants 24-36 inches apart in a sunny spot with well-drained soil. Even in late spring, we can still get unexpected cold snaps, so keep an eye on the forecast and be ready to cover your transplants if needed. Honestly, though, established plants are much easier to manage in our climate.

Rosemary thrives on neglect, which makes it perfect for our dry Mountain West climate. Once established, it's very drought-tolerant, so overwatering is a far bigger concern than underwatering. Poor drainage combined with overwatering will quickly lead to root rot.

During the growing season, water deeply but infrequently. Let the top 2 inches of soil dry out completely before watering again – use the finger test! In general, an inch of water per week is plenty, especially since we get so little rainfall (10-20 inches per year). Water at the base of the plant to avoid wetting the foliage, which can encourage fungal problems in our cool nights.

Signs of underwatering include wilting or drooping leaves. Overwatering shows as yellowing leaves, especially near the base of the plant. A light mulch of gravel or pine needles can help retain moisture without suffocating the roots.

Rosemary needs minimal pruning, which is good news for us busy Mountain gardeners. The main goal is to maintain its shape and encourage bushier growth.

Prune lightly after flowering, which usually happens in late spring or early summer. Snip off the tips of the stems, but avoid cutting into the old, woody growth, as it may not regrow. In the spring, give your rosemary a light shaping to remove any dead or damaged branches. As first frost approaches in early October, you don't need to do anything special. Just let the plant go dormant and wait for spring.

You can start harvesting rosemary from mid August through late September, about 90 days after transplanting. Look for healthy, 4-6 inch stem tips – these are the most flavorful.

To harvest, simply snip off the stem tips with clean scissors or pruners. Never take more than 1/3 of the plant at a time, as this can weaken it. The woody stems are too tough to eat, so stick to the newer growth. Even after harvesting, rosemary will keep producing as long as you don't over-harvest.

As the first frost approaches in early October, harvest any remaining stems you want to use fresh. You can also dry them for later use.

Here are three common problems you might encounter growing rosemary in the Mountain West:

Root Rot from Overwatering

• What it looks like: Yellowing leaves, especially near the base of the plant. Soft, mushy roots that are brown or black instead of white.
• What causes it: Overwatering and poor drainage. Our alkaline soil can sometimes exacerbate drainage issues.
• How to fix/prevent it: Ensure well-draining soil. Water deeply but infrequently, allowing the soil to dry out between waterings. Amend heavy clay soils with compost or other organic matter. Consider growing rosemary in raised beds or containers for better drainage.

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves and stems. Leaves may curl, yellow, and die.
• What causes it: Fungal disease that thrives in warm days, cool nights, and moderate humidity. Dry leaf surfaces actually favor it (unlike most fungi).
• 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. A milk spray (1 part milk to 9 parts water) is surprisingly effective.

Spider Mites

• What it looks like: Fine stippling on leaves (tiny white dots). Fine webbing on leaf undersides and between stems. Leaves 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 if possible. Neem oil or insecticidal soap can also be used.

Mountain West Specific Challenges: Our moderate-at-altitude heat combined with low humidity and dry conditions can stress rosemary, making it more susceptible to spider mites. Be vigilant about watering correctly and providing adequate air circulation to prevent these problems.

Rosemary is a great companion plant for many other herbs and vegetables in your Mountain West garden.

Good companions include sage, thyme, and lavender. These herbs share similar growing requirements (full sun, well-drained soil) and attract beneficial insects. Rosemary also helps repel bean beetles, making it a good companion for beans-green. Carrots also benefit from rosemary's pest-repelling properties.

Avoid planting rosemary near mint. Mint is aggressive and can quickly overcrowd rosemary, stealing valuable resources. Plus, mint prefers moister conditions than rosemary, so their needs don't align well in our dry climate.