Why Are My Dahlia Flowers Fading in September? Fixing Late-Season Bloom Drop
Are your dahlias losing their zing long before the first frost? It doesn't have to be this way! Here's how to keep them blooming strong all summer long.
Dahlias are among the most lush, dramatic and beautiful of garden flowers, appearing in tropical-punch shades. I have heard the blooms called “swoon-worthy,” and though I have never in my life used that term, it does seem appropriate. The flowers appear delicate, almost fragile, but they are actually tough and durable. blooms in gorgeous “tropical punch” hues. These blooms look delicate, but they are tough and hold up well to handling.
Given the fragile beauty of the flowers, you might think that dahlia care is complex, but not so. They are surprisingly easy to grow and care for. However, your flowers may experience late-season issues that lead to small, dull, or dropping blooms. But if you give your dahlias appropriate late September care, you can help them shine right through the fall.
Why Are My Dahlias Looking Sad?
Dahlias are late-season favorites, blooming happily from mid-summer through the first frost in fall. But as summer shifts into fall, problems sometimes arise, with dull, wilting, or dropping blooms.
If you want to point a finger at the culprits causing these issues, you’ll need both hands. Some of the issues are simply seasonal, like the shorter day length and the hot sun of August. Some are fungal, like powdery mildew and tuber rot; some are nutritional like potassium and phosphorus shortages after months of growth. If this is the problem, you might need a dose of dahlia fertilizer, which you can get on Amazon.
Others are caused by pests, and still others by well-meaning but inept humans (like improper deadheading and too much fertilizer.)
How to Solve September Dahlia Problems
You can’t stop the day length from shortening nor the sun from blazing hot in August. But you can solve a lot of dahlia’s early-autumn blues by taking these five easy steps.
1. Change Your Irrigation Practices
As the hot dry summer transforms into cool, rainy autumn, your dahlias need less water from you. As the local rainfall increases, you need to adjust your watering. Water pooling around the dahlia stems can result in rotting stems and fungal issues.
This isn’t an exact science. You don’t have to measure the amount of rainfall and reduce the amount of water you provide by that amount. Just keep an eye out for weather, and if an appreciable amount of rain falls, water your dahlias less each week.
2. Just Say No to Nitrogen Fertilizer
You fertilize dahlias with nitrogen in order to encourage new growth. But the new growth is soft, weak, green growth that won’t do well in the cooling weather. This type of growth is a beacon for pests and also invites fungal issues. And you want the plants to start hardening off their stems in September. In addition, a dahlia’s primary job in September is to finish its bloom cycle, not to add new growth. So cease all nitrogen feeding by September.
However, low- or no-nitrogen fertilizer will help those beautiful dahlias if it is high in potassium and phosphorus. These nutrients have been depleted after months of growth. This type of feed will encourage robust final blooms.
3. Deadhead Properly
Deadheading dahlias in September can be a positive step toward extending flowering, but done wrong, it can be a very negative step. Improper deadheading in early fall can cause your dahlias to stop blooming prematurely or encourage disease.
“Bad” deadheading can be as simple as mistaking fresh buds for spent seedpods and snipping them off, reducing the amount of flowers. It’s equally bad (or worse) to adapt an “off with their heads” philosophy, pinching off the flower heads right below the bloom. This leaves the hollow stems standing. They fill with water from fall rains, then rot, inviting fungal issues or pests.
“Good” deadheading looks very different. You follow the spent flower stem down to where it meets a lateral leaf node. Make the cut just above that intersection at a slight angle. This helps the dahlias achieve peak production in September. Clipping off the dead blossoms this way stops the plant from wasting energy on seed pods so it has energy to produce new buds.
4. Improve Airflow
Improving airflow for dahlias in September is not just a “nice” gesture. It might be the single best way to keep those blooms coming while avoiding late-season dahlia powdery mildew.
September is a moist month. It ushers in cooler nights and heavier morning dew. The dahlia’s incredible, dense late-summer growth traps in all that moisture. That means party time dahlia disease and fungal spores.
Open up your plants through September by pruning off the leaves from the bottom 6 inches (15cm) of the stalks. This clears a wind "tunnel" at the base of the dahlias, letting a breeze pass gaily through the patch, rapidly drying out morning dew and rain. It also reduces the possibility that soil-borne fungal spores will splash up onto the leaves during heavy September rains.
This is incredibly effective, especially if you also thin out crowded interior stems. Stand above your dahlia plants and look down into the center. You are likely to see a tangled mass of weak, overlapping, or non-blooming stems. Trim away crossing or weak inner branches, remove yellowing or damaged leaves.
Balance Is Key to Managing Late-Season Blooms
Every gardener would like their dahlias to bloom right up until Christmas. But we all know that in order to have flowers the following season, the dahlia tubers must have time to refill their energy. The rest the tubers require is the very reason for dormancy.
The key to enjoying late season blooms without depleting the tubers’ energy is balance. Yes, you can still cut dahlias for vases indoors; just cut shorter stems instead of longer ones. When you take out long stems in fall, it causes the dahlia to waste energy rebuilding lost vegetation instead of storing sugars in the tubers
Second, taper off the water supply, letting the soil dry out more between waterings than you did in summer. This signals to the plants that winter is coming, prompting the dahlia to begin funneling its resources underground.
Finally, take advantage of the “killing frost.” Don’t cut back the plants until the first hard frost hits. This cold snap causes the dahlia to dive into dormancy, sending a final flush of sugar to the roots. At that point, cut the stalks to 5 inches (25cm) tall. Two weeks later, lift the dahlia tubers from the ground. The brief curing period toughens the tubers, ensuring that they store well and sprout vigorously in spring.
Dahlia Care Essentials
