Chrysanthemum in Zone 6B — Mountain West

Chrysanthemums are a fantastic addition to any Mountain West garden, adding vibrant color and late-season blooms when many other flowers are fading. Their cheerful flowers brighten up the garden and make beautiful cut arrangements. Plus, the cool nights even in summer here seem to enhance their colors and longevity.

Our high altitude and short season can present challenges, but with careful timing and a bit of planning, you can successfully grow Chrysanthemums. The 163-day growing season means starting early or choosing quicker-maturing varieties is key.

Starting Chrysanthemum seeds indoors in the Mountain West is optional but can give you a head start, especially with our short growing season. If you want to try it, begin in late February through mid March, about 8 weeks before you plan to transplant.

Use seed trays or small pots with a good quality seed-starting mix. Keep them consistently warm and provide plenty of light—a grow light works best. Water from the bottom to avoid damping off, especially in our dry air. Remember, spring here often has a late start, so patience is key.

Transplant your Chrysanthemum seedlings outdoors in late April through late May, after the risk of hard frost has passed. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week.

Space them 12-18 inches apart in a sunny spot with well-draining soil. Keep an eye on the weather forecast; late spring snow or hail is not uncommon in the Mountains, so be ready to cover your young plants if needed. Alternatively, you can buy potted mums in the fall or starts in the spring.

Consistent watering is essential for healthy Chrysanthemums in our dry Mountain West climate. While they need moderate moisture, avoid letting them sit in soggy soil. Our low humidity means you'll need to pay close attention.

During the hottest part of the summer, especially in July and August, give your Chrysanthemums about 1 inch of water per week. Use the "finger test" to check soil moisture—if the top 2 inches feel dry, it's time to water. Water at the base of the plant to avoid wetting the foliage, which can encourage fungal diseases in our cool nights. Signs of underwatering include wilting leaves and dry, crispy edges, while overwatering can lead to yellowing leaves and root rot. A layer of mulch around the base of the plants will help retain moisture and keep the soil cooler.

Pruning is important for encouraging bushier growth and more abundant flowers on your Chrysanthemums. Start pinching the growing tips every 2-3 weeks through July 4th. This encourages side shoots to develop, resulting in a fuller plant with more blooms.

Simply pinch off the top set of leaves on each stem. After July 4th, stop pinching to allow the flower buds to develop for fall blooming. As the first frost approaches in early October, you can cut back the plants to a few inches above the ground to prepare them for winter dormancy.

You can expect your first Chrysanthemum harvest in early July through late August, depending on the variety and when you transplanted. The flowers last for weeks in the garden, so enjoy them!

Cut the stems for bouquets when the flowers are fully open and the colors are vibrant. Use sharp, clean pruners to avoid damaging the plant. Regular harvesting encourages continued production, so don't be afraid to cut them often. As the first frost approaches in early October, harvest any remaining blooms to enjoy indoors.

Here are some common problems you might encounter with Chrysanthemums in the Mountain West:

Aphids *What it looks like:* Clusters of tiny soft-bodied insects (green, black, or white) on stems and leaf undersides. Sticky honeydew residue. Curled or distorted new growth. *What causes it:* Rapid-reproducing sap-sucking insects that thrive in warm weather. Ant colonies may farm them for honeydew. *How to fix/prevent it:* Strong water spray knocks them off. Encourage ladybugs and lacewings (natural predators). Insecticidal soap for heavy infestations. Neem oil works systemically. Avoid excess nitrogen fertilizer (it attracts them).

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, which is common here. *How to fix/prevent it:* Strong water spray (daily for a week) is the best treatment. Increase humidity around plants. Neem oil or insecticidal soap. Release predatory mites (Phytoseiulus persimilis). Avoid broad-spectrum pesticides that kill predators.

Leaf Miners *What it looks like:* Noticeable tan winding trails on leaves. *What causes it:* A small fly lays eggs on the leaves, and the larvae eat the leaf tissue as they tunnel through the leaf. *How to fix/prevent it:* Remove and destroy affected leaves. Cover plants with floating row covers to prevent the adult flies from laying eggs. Insecticidal soap can help control infestations.

Rust *What it looks like:* Rusty orange, yellow, or brown pustules on leaves and stems, mostly on undersides. Leaves yellow and drop prematurely. *What causes it:* Various rust fungi that spread via wind-borne spores. Favored by warm, humid conditions with moisture on leaves. *How to fix/prevent it:* Remove affected leaves. Avoid overhead watering. Improve air circulation. Apply sulfur or copper fungicide. Don't work among wet plants. Rotate crops.

Mountain West Specific Challenges: The moderate-at-altitude heat and low humidity of the Mountain West can stress Chrysanthemums, making them more susceptible to spider mites. Our dry climate also means consistent watering is crucial to prevent drought stress. The intense UV radiation can sometimes scorch leaves, so providing some afternoon shade during the hottest part of the day can be beneficial.

Chrysanthemums benefit from companion planting in the Mountain West. Tomatoes and peppers make good companions because they enjoy similar growing conditions (full sun and well-drained soil) and can provide some shade to protect the Chrysanthemums from the intense sun. Lettuce is also a good companion, as it can help suppress weeds and keep the soil cool and moist. Avoid planting Chrysanthemums near anything that might compete for nutrients or water.