Cosmos in Zone 9B — Gulf Coast

Cosmos are worth growing here in the Gulf Coast for their bright, cheerful blooms that keep coming all season long. They're fantastic cut flowers, adding a burst of color to any arrangement, and they attract pollinators like butterflies to your garden, which is always a plus. Plus, they handle our long growing season like champs.

Sure, we've got our share of challenges with the high humidity and potential for fungal diseases, but don't let that scare you off. With a little attention to timing and airflow, you can enjoy cosmos from late spring all the way into early winter, thanks to our 319-day growing season.

Starting cosmos indoors isn't usually necessary here in the Gulf Coast, as they direct sow so easily. However, if you're itching to get a jump start, you can start seeds indoors from early to late January, about four weeks before you'd like to transplant them outside.

Use seed trays with a good seed-starting mix and keep them in a warm spot with plenty of light – a sunny windowsill or under grow lights works great. Make sure to water from the bottom to avoid damping off, a common problem with seedlings. Our early spring warmth means you'll need to be ready to harden them off pretty quickly.

Transplant your cosmos seedlings outdoors from mid February through late March. Before you plant them, harden them off by gradually exposing them to outdoor conditions for about a week.

Choose a sunny spot and space them about 12-18 inches apart. Keep an eye on the weather forecast; those late-season cold snaps can still happen, so be ready to cover them if needed.

Direct sowing is the easiest way to grow cosmos in the Gulf Coast. You can sow seeds directly into the garden from late January through mid October. The soil needs to be at least 65°F for good germination.

Prepare the soil by loosening it and removing any weeds. Sprinkle the seeds lightly over the surface and cover them with a thin layer of soil. Keep the soil moist until the seedlings emerge, which usually takes about a week. Space plants 12-18 inches apart, thinning as needed once seedlings appear.

Cosmos are pretty low-maintenance when it comes to watering, which is great for us here in the Gulf Coast where we get plenty of rain. In fact, overwatering is more of a concern than underwatering. During our rainy season, you might not need to water them at all!

During the drier periods, especially in the heat of summer, check the soil moisture with the "finger test." Stick your finger about two inches into the soil; if it feels dry, it's time to water. Aim for about an inch of water per week, watering deeply at the base of the plant to avoid wetting the foliage and encouraging fungal diseases in our humid climate.

Signs of underwatering include wilting leaves and stunted growth. Overwatering can lead to yellowing leaves and root rot. A layer of pine straw or other organic mulch can help retain moisture in the soil and suppress weeds, but don't overdo it – good airflow is key in our humid environment.

Deadheading is key to keeping your cosmos blooming all season long. Simply snip off the spent flowers just below the flower head. This encourages the plant to produce more blooms instead of putting energy into seed production.

Pinching young plants when they are about 6 inches tall can also encourage bushier growth. In late fall, as our first frost approaches around mid- December, you can let the last blooms go to seed if you want to collect seeds for next year.

You can expect your first cosmos blooms about 50 days after sowing, so from late March through mid December, depending on when you planted. Cut the flowers when the petals have just opened fully for the longest vase life.

Use sharp scissors or pruners to cut the stems at an angle. Removing the lower leaves that will be submerged in water helps prevent bacterial growth. To encourage continued production, keep deadheading those spent blooms. As the season winds down and frost threatens, you can collect seeds from the dried flower heads by shaking them into a paper bag.

Here are a few common problems you might encounter with cosmos in the Gulf Coast:

Flopping in Rich Soil

• What it looks like: Tall, leggy plants that fall over easily, especially after rain or wind.
• What causes it: Overly fertile soil leads to excessive foliage growth at the expense of strong stems. This is common in our area where people amend the soil too much.
• How to fix/prevent it: Plant cosmos in average to poor soil. Avoid over-fertilizing. Stake or support plants if necessary.

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).

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves, stems, and sometimes fruit. Leaves curl, yellow, and die. Reduces plant vigor.
• What causes it: Various fungi that thrive in warm days, cool nights, and moderate humidity. Dry leaf surfaces actually favor it (unlike most fungi). Our humidity can really exacerbate this.
• How to fix/prevent it: Improve air circulation through spacing and pruning. Remove affected leaves. Neem oil or potassium bicarbonate sprays help. Milk spray (1 part milk to 9 parts water) is surprisingly effective. Plant resistant varieties.

Gulf Coast Specific Challenges: The hot heat, very-high humidity, and wet-year-round rainfall in the Gulf Coast create a perfect environment for fungal diseases like powdery mildew. Ensure good air circulation around your cosmos and avoid overhead watering to minimize these problems.

Cosmos make great companions for several plants in the Gulf Coast garden. They attract pollinators, which benefit tomatoes, peppers, and squash. The pollinators increase fruit set.

They also work well with green beans. Cosmos attract beneficial insects that prey on bean pests. I wouldn't plant them near anything that prefers very dry soil, as cosmos like consistent moisture, which is a natural fit with our rainfall.