Chrysanthemum in Zone 5B — Mountain West

Chrysanthemums, with their vibrant fall blooms, are a welcome sight in our Mountain West gardens as other plants fade. Their late-season color provides much-needed cheer, and the cut flowers last beautifully in bouquets. Plus, those cool nights we get even in summer enhance the flower color, giving you richer, more intense hues.

Our short growing season and alkaline soil present unique challenges for high-altitude gardening, but with careful timing and attention, you can absolutely grow beautiful mums. We're working with a 149-day window, so planning is key to get the most out of your Chrysanthemums.

Starting Chrysanthemum from seed isn't the most common approach around here, but it can be done if you want a wider variety than you find at local nurseries. If you're up for it, sow seeds indoors from early to late March, giving them about 8 weeks before transplanting.

Use seed trays with a good seed-starting mix, providing warmth and bright light – a grow light works wonders, especially with our intense sun. Remember to water from the bottom to avoid damping off, and keep the soil consistently moist. The late spring means you'll need to keep an eye on the weather before moving them outside.

Most of us in the Mountain West buy potted mums in the fall or starts in the spring—it's just easier with our short season! If you've started your own or bought starts, transplant them outdoors from early May through early June, after the last expected frost.

Harden them off for about a week before planting, gradually exposing them to the outdoors. Space them 12-18 inches apart in a sunny spot with well-draining soil. Keep an eye on those late spring temperature swings and protect them if a cold snap threatens.

Chrysanthemums need consistent moisture, especially during bud formation, but overwatering is a quick way to invite root rot in our alkaline soil. Aim to water deeply but infrequently. During our moderate-at-altitude summer heat, this usually translates to about 1 inch of water per week, depending on rainfall.

Use the "finger test" – stick your finger about 2 inches deep into the soil. If it feels dry, it's time to water. Always water at the base of the plant to avoid wetting the foliage, which can encourage fungal diseases in our dry climate.

Signs of underwatering include wilting leaves and dry soil, while yellowing leaves and soggy soil indicate overwatering. A layer of mulch around the base of your plants will help retain moisture and regulate soil temperature, a plus in our variable climate.

To encourage bushy plants with lots of blooms, pinch the growing tips of your Chrysanthemums every 2-3 weeks up until around July 4th. This forces the plant to branch out, resulting in more flowers later in the season.

You're pinching new growth, not removing flower buds, so don't worry about sacrificing blooms. After July 4th, stop pinching to allow the flower buds to develop. As the first frost approaches around early October, you can cut back the foliage to tidy up the plant.

Chrysanthemums are fall-blooming, so expect your first harvest from mid-July through early September, depending on the variety and when you transplanted. The flowers last for weeks in the garden, providing a beautiful display.

Cut the stems for bouquets when the flowers are fully open and the petals are firm. Use clean, sharp scissors or pruners to avoid damaging the plant. Cutting the flowers encourages the plant to produce more blooms, so keep harvesting throughout the season.

As the first frost approaches in early October, harvest any remaining blooms to enjoy indoors. The plant itself is hardy, so it can withstand a light frost, but the flowers will be damaged.

Chrysanthemums can be susceptible to a few common problems, but vigilance and quick action can keep them healthy. Here are the ones we see most often 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. Populations explode 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.
• 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: Tan squiggly trails inside the leaves. Leaves may appear distorted or blotchy.
• What causes it: Larvae of small flies burrowing between leaf surfaces.
• How to fix/prevent it: Remove and destroy affected leaves. Cover plants with row covers to prevent flies from laying eggs. Beneficial nematodes can help control larvae in the soil.

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 low humidity in our region can actually help prevent some fungal diseases on Chrysanthemums, but it also means spider mites can become a bigger problem. Our intense UV radiation can stress plants, making them more susceptible to pests, so provide afternoon shade if needed. Watch for hail damage, which can shred the leaves.

Chrysanthemums make good companions for several garden vegetables. Tomatoes and peppers benefit from the Chrysanthemum's pest-repelling properties, as they deter aphids and other insects. Lettuce also enjoys the partial shade that Chrysanthemums can provide, especially during our intense summer sun.

Avoid planting Chrysanthemums near plants that are susceptible to root rot, as they prefer well-drained soil and don't like to be waterlogged.