Chrysanthemum in Zone 3A — Mountain West

Chrysanthemums bring a burst of late-season color to our Mountain West gardens, just when everything else is starting to fade. Their bright blooms are perfect for bouquets, adding a touch of warmth to our homes as the cool nights roll in, and the satisfaction of extending the growing season with these beauties is hard to beat.

Our short growing season and intense sun can be a challenge, but with careful timing and a little extra attention, you can absolutely grow vibrant Chrysanthemums. Starting seeds indoors and providing adequate water will help you make the most of our 96-day window.

Starting Chrysanthemums indoors is a good idea in our climate to get a head start. Begin in late March through mid April, about eight weeks before you plan to transplant them outdoors.

Use seed trays with a good seed-starting mix. Keep them consistently moist by bottom watering to avoid damping off, and provide a warm location with plenty of light – a sunny windowsill or grow lights work well. Our spring weather can be unpredictable, so indoor starts give your Chrysanthemums a fighting chance.

Transplant your Chrysanthemum seedlings outdoors in early to mid June, once the risk of frost has passed. Be sure to harden them off for about a week beforehand by gradually exposing them to outdoor conditions.

Space the plants 12-18 inches apart in a sunny spot with well-draining soil. Keep an eye on the weather forecast, as we can still get some surprise cold snaps or even hail this time of year, so be ready to cover them if needed. Buying potted mums in the fall, or starts in spring, also works well for our short season.

Consistent watering is key for healthy Chrysanthemums, especially given our dry climate. Aim to water deeply at the base of the plant, providing about 1 inch of water per week, depending on rainfall. Use the "finger test" – stick your finger about 2 inches into the soil; if it feels dry, it's time to water.

Our low humidity means you can usually water at the base of the plant without worrying too much about foliar diseases. During bud formation, ensure they receive consistent moisture to prevent bud drop.

Signs of underwatering include wilting leaves and dry soil, while overwatering can lead to yellowing leaves and root rot. A layer of mulch around the base of the plants can help retain moisture and regulate soil temperature, which is especially helpful with our intense sun.

Pinching is your best friend when it comes to getting bushy Chrysanthemums loaded with flowers. Pinch off the growing tips every 2-3 weeks until about July 4th. This encourages side shoots to develop, resulting in more blooms later in the season.

After July 4th, stop pinching to allow the flower buds to form. As the first frost approaches in early September, you can either bring your Chrysanthemums indoors to extend their bloom time or leave them in the garden as an annual.

You can expect your first harvest of Chrysanthemums from early August through mid September, depending on the variety and when you transplanted them. The flowers should be fully open and showing their vibrant colors.

Cut the stems at an angle, leaving several inches of stem on the plant to encourage continued production. Deadheading spent flowers will also help prolong the blooming period. As the first frost looms, harvest any remaining blooms to enjoy indoors, as they won't survive the freeze.

Here are some common problems you might encounter with Chrysanthemums in our region:

Aphids

• What it looks like: Clusters of tiny green, black, or white insects on stems and leaf undersides. You might also see sticky honeydew and curled or distorted new growth.
• What causes it: These sap-sucking insects reproduce rapidly, especially in warm weather. Ants often "farm" them for their honeydew.
• How to fix/prevent it: A strong spray of water can knock them off. Encourage natural predators like ladybugs. Insecticidal soap or neem oil can help with heavy infestations. Avoid over-fertilizing with nitrogen.

Spider Mites

• What it looks like: Fine stippling on leaves (tiny white dots), webbing on leaf undersides and between stems, and leaves turning bronze or yellow.
• What causes it: These tiny arachnids thrive in hot, dry conditions, so they can be a problem in our dry climate.
• How to fix/prevent it: A strong water spray daily for a week can disrupt them. Increase humidity around the plants. Neem oil or insecticidal soap can also help.

Leaf Miners

• What it looks like: Tan squiggly lines or blotches in leaves.
• What causes it: The larvae of small flies tunnel between the upper and lower leaf surfaces, feeding on leaf tissue.
• How to fix/prevent it: Remove and destroy infested leaves. Cover plants with floating row covers to prevent adult flies from laying eggs. Insecticidal soap or neem oil can help with severe infestations.

Rust

• What it looks like: Rusty orange, yellow, or brown pustules on leaves and stems, mostly on undersides. Leaves may yellow and drop prematurely.
• What causes it: Rust fungi spread via wind-borne spores and are favored by warm, humid conditions with moisture on leaves.
• How to fix/prevent it: Remove affected leaves. Avoid overhead watering and improve air circulation. Apply sulfur or copper fungicide. Rotate crops to prevent buildup of the fungus in the soil.

Mountain West Specific Challenges: Our intense sun can stress plants, making them more susceptible to pests and diseases. The low humidity can help reduce fungal issues, but it also means you need to be diligent about watering. Hail can also cause significant damage, so consider using netting during severe weather.

Good companions for Chrysanthemums include tomatoes and peppers. Chrysanthemums repel some pests that bother tomatoes, and tomatoes provide some shade that can protect Chrysanthemums from our intense sun. Lettuce also makes a good companion, as it enjoys the partial shade provided by the Chrysanthemums, and the Chrysanthemums help deter aphids that can plague lettuce.