When and How to Fertilize Dahlias (Essential Guide)

Newcomers to flower gardening often start with Dahlias. While they’re not the easiest flower to grow, especially from seed, they’re not too hard either and reward the patient gardener with eye-popping colors, shapes, and symbolic value. If you’re trying to make your Dahlias pop this summer, you’ll need to learn the ins and outs of fertilizing this plant. Dahlias are heavy feeders and will rely on you to supply them with a steady dose of fertilizer throughout the flowering season.

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Fertilizing Dahlias – The Essentials

Dahlias prefer a fertilizer somewhat low in nitrogen, such as a product with a ratio of 6-24-24. Slow-release fertilizers are preferred, but almost any product with the right ratio of NPK will work.

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Botanical Overview

Name:	Dahlias
Scientific Name:	Dahlia spp.
Native Range:	Central America, especially the mountainous meadows of Mexico and Guatemala
Growing Zones:	Hardy for overwintering in USDA zones 8 to 11, treated as an annual in zones 3 to 7
Flowering Season:	Mid-summer to the end of fall.

The Role of Fertilizing in Plant Health, Growth, and Development

Fertilizer is simply all the nutrients that a plant normally takes from the soil holding its roots. 

While plants are technically fed by the sugars they produce through photosynthesis, these nutrients are still crucial for growth. Without nitrogen, phosphorous, and potassium, plants like Dahlias can’t grow strong stems or develop flower buds. 

Micronutrients are also essential, but even the poorest soil tends to provide more than enough of these nutrients. It’s the major NPK compounds that plants generally need supplementing throughout the year.

Common types of fertilizer include:

• Water-soluble powders that are applied during regular watering
• Pelletized or spike-shaped formulas designed for slow release
• Dry powders designed for sprinkling on the soil so the next rain can wash it down to the roots
• Organic products like manure or bone meal that may come powdered, pelletized, or as a liquid.

Fertilizers are generally made either from a natural waste like manure, blood meal, or fish by-products or from refined chemicals. 

Some chemical fertilizers are mined out of the ground, and others are created through industrial reactions. 

Organic fertilizers are less impactful on the environment to produce, but they don’t always work as well for demanding plants like Dahlias because they may supply the wrong proportions of nitrogen to other nutrients.

Signs Your Dahlias are Lacking Nutrients and Need Feeding

Dahlias need more fertilizer than most other flowering plants you might have in your garden. 

When they go too long without the three macronutrients they need, the plants tend to stall out in growth first. You may not see any new leaves or buds forming on the plant despite it being still in the middle of summer (for more, see our in-depth guide to how tall dahlia flowers grow).

If the problem persists, the foliage will change color and become pale and wilted. Leaves may drop off, along with buds that haven’t opened yet. Weak stems, color issues, and susceptibility to pests and disease all indicate a lack of nutrients.

When and How Often Should You Fertilize Dahlia Plants

Dahlia plants need fertilizer every month during the growing season. 

They go dormant after the first frost, so there’s no need to fertilize them after that point, even if you keep them indoors in pots over the winter. 

Don’t start fertilizing Dahlias in pots or in the ground before they sprout new leaves either. Wait until there is at least three to four inches of new shoots on the plant before applying the season’s first dose of fertilizer. This ensures delicate feeder roots aren’t burned by unnecessary nutrients.

The Best Fertilizer for Dahlia Plants

Like many other flowering and fruiting plants, Dahlias need less nitrogen than potassium or phosphorous. A product that offers a balance of 6-24-24 or a similar proportion is the best choice. 

A higher dose of nitrogen can be helpful at the very beginning of the growing season when the plant is focused solely on leaf development. However, you may want to switch to a 0-10-10 formula by the start of blooming to build up the best possible color display. 

Dahlias can handle full-strength applications of fertilizer, but consider half-strength use if your dahlia soil isn’t depleted or poor. Too much fertilizer leads to yellow, weak leaves and floppy stems. Well-composted manure is one of the best options, and it’s something you can easily create at home if you keep a few chickens or rabbits for pets.

How to Apply Fertilizer

Spread pellets, powders, and organic manure around the base of each Dahlia plant. It may be tricky to get to the base under the bushy growth, so try lifting the leaves before spreading each cup full of fertilizer. 

Water-based products can be easy to apply for this reason, especially foliar sprays that are absorbed through the leaves. If using spikes or slow-release pellets, make sure they’re near the main stem, so they reach the majority of feeder roots. 

Consider spacing Dahlia plants out further apart for easier fertilizing, especially smaller dahlia varieties that tend to grow tightly together.

Key Considerations

Too much nitrogen is the most damaging mistake to make when fertilizing Dahlias. This can lead to a bushy, well-developed plant that simply never grows buds or flowers. Dark green foliage and a lack of stems indicate this problem. 

If you’re using the right fertilizer but applying too much, the leaves may turn light green to yellow and go splotchy. Weak stems and wilting that aren’t due to sunlight or lack of water also indicate over-fertilization. There isn’t any way to flush excess fertilizer out of open soil, but container plants can have their soil mix replaced to reduce these negative effects. 

Be careful during application since applying too much fertilizer can be an irreversible process.

Fertilizing Dahlia Plants FAQs:

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Wrapping Up

Giving your flowers a little fertilizer here and there won’t compare to the results from a strict monthly routine. Once your plants start receiving a steady supply of NPK, you will see a difference in their growth and blooming. Give yourself a chance to enjoy the biggest and most colorful Dahlias this year by committing to a fertilizer routine. For more, see our in-depth guide to growing dahlias at home and cutting dahlia flowers for a vase or bouquet arrangement.

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