Chrysanthemum in Zone 9A — Desert Southwest

Chrysanthemums are a fantastic addition to any Desert Southwest garden, bringing vibrant color and late-season blooms when many other plants are fading. Their versatility in bouquets and the satisfaction of extending your garden's beauty well into fall make them a worthwhile choice. Plus, they attract beneficial insects to your garden.

Our region's extreme heat and alkaline soil can be challenging, but with careful timing, you can successfully grow chrysanthemums here. Our long 298-day growing season gives you the flexibility to work around the worst of the summer heat and take advantage of our mild winters.

Starting chrysanthemum seeds indoors in the Desert Southwest isn't essential, but it can give you a head start, especially if you want specific varieties. If you choose to start indoors, aim for mid December through early January, about eight weeks before you plan to transplant. Use seed trays with a good seed-starting mix, provide warmth (a heat mat is helpful), and ensure adequate light with grow lights.

Remember to water from the bottom to prevent damping-off. Our very-early spring character means you need to be ready to transplant as soon as the weather allows, so starting indoors buys you some time.

Transplanting chrysanthemums outdoors in the Desert Southwest should happen mid February through mid March. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This prevents shock from our intense sun.

Space your plants 12-18 inches apart in well-draining soil. Keep an eye on the weather during this time – even though we have a mild winter, occasional late frosts can still occur, so be prepared to cover your transplants if needed. You can also buy potted mums in the fall or starts in the spring from local nurseries.

Watering chrysanthemums correctly is crucial for success in our arid Desert Southwest climate. They need moderate moisture, but overwatering can be just as harmful as underwatering. During the active growing season (spring and fall), aim to give them about 1 inch of water per week, increasing slightly during our monsoon season if rainfall is scarce.

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 warm, dry air. Signs of underwatering include wilting leaves and dry, crispy edges, while overwatering can lead to yellowing leaves and root rot.

In the extreme heat of summer, monitor your plants closely and water deeply but less frequently, allowing the soil to dry out slightly between waterings. A good layer of organic mulch, like composted leaves or wood chips, can help retain moisture in the soil and regulate soil temperature.

Pruning chrysanthemums is key to getting bushy plants loaded with flowers. Begin pinching the growing tips of your plants every 2-3 weeks, starting when they are about 6 inches tall. Pinch off the top set of leaves on each stem to encourage side branching.

Continue pinching until around July 4th. Stopping at this point allows the flower buds to develop for fall blooming. At the end of the season, after the first frost around early December, you can cut the plants back to a few inches above the ground.

You can expect your first chrysanthemum harvest in the Desert Southwest from late April through mid June, depending on the variety and when you transplanted. The flowers will last for weeks in the garden. Cut them for bouquets when they are fully open.

To harvest, use sharp scissors or pruning shears to cut the stems at an angle. Removing spent flowers regularly encourages continued production. As the first frost approaches in early December, you can cut any remaining flowers to bring indoors and enjoy.

Here are some common problems you might encounter with chrysanthemums in the Desert Southwest:

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, winding trails or blotches inside the leaves.
• What causes it: Larvae of small flies that burrow between leaf surfaces to feed. More prevalent during warm weather.
• How to fix/prevent it: Remove and destroy affected leaves. Use floating row covers to prevent adult flies from laying eggs. Neem oil can help deter them.

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.

Desert Southwest Specific Challenges: The extreme heat and low humidity of our region can stress chrysanthemums, making them more susceptible to pests and diseases. Regular watering, proper soil drainage, and providing some afternoon shade during the hottest months can help mitigate these issues. Monitor plants frequently, especially during monsoon season, for signs of fungal diseases.

Chrysanthemums benefit from companion planting in the Desert Southwest. Tomatoes and peppers are good companions because they deter pests like aphids that can bother chrysanthemums. Lettuce also works well, as it provides ground cover that helps retain soil moisture, which is critical in our dry climate.

Avoid planting chrysanthemums near aggressive plants that compete for water and nutrients.