Dahlias are treasured for their stunning, diverse blooms that flower from midsummer until the first frost They come in virtually every color except blue, with flower forms ranging from tiny 2-inch pompons to giant 15-inch dinner plates
But to nurture these beauties to their full potential dahlias need rich fertile soil and steady feeding throughout the season. Organic fertilizers are ideal for providing dahlias with slow-release, gentle nutrition without chemicals.
When selecting an organic fertilizer, seek out options specifically formulated for flowers and vegetables. Avoid quick-release synthetic fertilizers which can burn delicate dahlia roots.
Here is an overview of some of the best organic fertilizer options to help your dahlias put on a spectacular show this year:
Cow Manure Compost
Cow manure makes an excellent organic base fertilizer for dahlias. Composted cow manure provides a gentle nutrient boost over an extended period. It is relatively low in phosphorus, with an NPK ratio around 3-2-3.
Cow manure compost fosters beneficial microbial activity in soil. It also improves moisture retention and adds organic matter to raise nutrient levels over time.
Work aged compost into garden beds before planting dahlia tubers. Top-dress containers with 1-2 inches of compost. Mix a handful or two into each planting hole as well.
Compost and Compost Teas
Homemade compost and compost teas offer an easy organic fertilizer solution. As compost breaks down, it releases nitrogen, phosphorous, potassium, and micronutrients.
Use compost tea bags to steep quality compost in water, then apply the resulting “tea” as a nutritious foliar feed and immune booster for dahlias.
Mixing your own compost allows you to control the inputs. Avoid adding potential contaminants like pet waste or pesticide-treated plants.
Fish Fertilizers
Fish fertilizers like fish emulsion provide fast-acting nutrition to prompt lush dahlia growth. The nitrogen, phosphorous, and potassium in fish fertilizers are readily available to plant roots.
Use fish emulsion diluted per package instructions to give dahlias an occasional mid-season boost. Alternate with milder organic fertilizers to prevent overfeeding.
If using fish emulsions, be sure to choose natural, unscented formulas. Artificial scents may attract pests to your dahlias.
Seaweed Fertilizer
Seaweed is prized in organic gardening for its array of micronutrients, vitamins, amino acids and growth promoters. Dried seaweed meal provides sustained nutrition.
Liquid seaweed fertilizers offer a quick nutrient infusion. They are mild enough to use every 2-4 weeks on dahlias. Kelp meal or liquid kelp extract are easy to find commercial versions.
Seaweed strengthens plants against stress. It supplies key elements like iron, zinc, and manganese vital for robust dahlia growth.
Earthworm Castings
Earthworm castings, also called vermicompost, contain concentrated nutrients worms process from compost and organic matter. Using castings reduces the risk of high salt levels sometimes found in uncomposted manures.
Blend worm castings into potting soil for containers. Top-dress garden beds with 1-2 cups per square yard before planting. You can also brew castings into compost tea.
With their exceptional nutrient density, a little worm castings fertilizer goes a long way. Avoid over-applying.
Organic Slow Release Granular Fertilizers
Many brands offer organic granular fertilizers that slowly and evenly distribute nutrition. Options with an NPK ratio close to 5-10-5 work well for dahlias.
Scatter granular fertilizer over the soil surface around dahlias according to package directions. Rain and irrigation water will move nutrients down into the root zone.
Reapply every 4-6 weeks. Reduce frequency for potted plants or in rainy climates to avoid overfeeding.
Organic Flower and Vegetable Formulas
Seek out organic fertilizers designed specifically for blooming plants like dahlias. These provide extra phosphorous for abundant flowers and plenty of potassium for strong roots and stems.
Use an organic vegetable formula for dahlias you plan to use for cut flowers. The extra potassium benefits keeping cut blooms fresh longer.
Dust flower or veggie fertilizer lightly around each plant, gently scratching it into the soil surface. Reapply monthly.
Natural Soil Acidifiers
Dahlias prefer slightly acidic soil with a pH between 6.5-7. Occasional watering with coffee grounds, citrus peels, pine needles or purchased soil acidifiers like sulfur dust keeps soil conditions ideal for dahlias.
Acidifiers lower soil pH, making nutrients like iron, zinc and manganese more plant-available. This prevents deficiency symptoms like yellowing leaves.
Use acidifiers sparingly to avoid over-correction. Test soil pH yearly and only acidify if needed.
Tips for Feeding Dahlias
Keep these tips in mind to get the most out of organic fertilizers:
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Test soil before planting and amend as needed with compost or natural additives to optimize fertility.
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Read labels carefully and follow package instructions to avoid over-applying fertilizer.
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Use mild organic fertilizers like compost, kelp, and fish emulsion more frequently (every 2-4 weeks).
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Reserve higher-nitrogen fertilizers like alfalfa meal for before planting or the earliest growth stages.
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Switch to natural potassium sources like kelp or wood ash as buds form to support flowering.
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Fertilize consistently throughout the season, tapering off in late summer.
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Pay attention to plant cues like leaf color and adjust your feeding regime as needed.
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Water in fertilizers after applying to prevent burning roots and help nutrients penetrate the soil.
Signs Your Dahlias Need More Fertilizer
Watch for these clues that your dahlias are hungry for fertilizer:
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Leaves are smaller than expected or yellowish in color
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Few flowers or small sized blooms
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Weak stems that flop over or break easily
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Small tubers when you dig up the roots
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Stunted plants that don’t fill out well
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Overall lackluster performance
If you notice any of these signs, increase organic fertilizer, taking care not to overdo it. The ideal regimen depends on factors like soil, climate, and variety. When in doubt, err on the side of less fertilizer to avoid risk of burning tender dahlia roots.
The Bottom Line
Feeding dahlias with the right organic fertilizers is crucial for growing them to their maximum potential. Organic options like compost, manures, fish emulsion, seaweed, and worm castings slowly release nutrients that dahlias crave, without chemical additives.
Pay close attention to your plants and soil, adjusting your fertilizing routine as needed. With a little trial and error, you will settle on the perfect organic feeding program to nourish your dahlias for show-stopping performance.
Observations from experiment and some of my recommendations for dahlia growers getting started:
1. Soil test is needed to determine nutrients available.
2. When dahlias are first planted, they don’t need any fertilizer because the roots usually have all the nutrients they need. Fertilizer isn’t useful until there are feeder roots. As seen in groups #1 & #2 with fertilizer in the soil around the root, I surmise the fertilizer applied prior to planting slowed the growth initially in group #1 and burned the roots in group #2.
3. Dahlias do not need high quantities of P (over 100 ppm). Once applied phosphorus does not move through the soil or leach. From readings and websites, I know that too much P might lead to a lack of potassium and make it harder to take in some other nutrients (but these sources don’t say how much P is needed to cause this condition).
4. Initially dahlias do not need lots of water to get started. Moist and warm soil will get dahlias growing and wet soil will tend to cause rot. This is true when starting dahlias in a greenhouse and in the ground.
5. Mulch around the dahlias to conserve moisture and to keep the soil uniformly moist. With our sandy soil once each week, when there is no rain, is usually adequate. In some places, mulch will attract many different kinds of bugs and other pests, which need to be removed.
6. It’s possible that clay soils will need less nitrogen because the nutrients (nitrogen and potash) will be held on to better by the clay particles than by lighter soils.
As was already said, the dahlia garden soil had over 750 ppm of phosphorus four years ago, and no phosphorus has been added in that time. During this period the phosphorus has dropped 230 ppm which is equivalent to 430 pounds per acre. Based on this, I found that the dahlias in our garden need about 110 pounds of phosphorus per acre per year, which is about 25 pounds per 10,000 square feet. I was giving them about twice or three times this amount every year.
I’m going to dig up and store some roots from each group separately so I can test how well they grow and store. Will advise next spring.
Some growers will question the use of nitrogen on the bigger cultivars, so I’m going to keep doing this next year with the bigger cultivars, applying nutrients again and keeping the areas separate.
– Mac Boyer 06/2001
Best Fertilizer for Dahlias | How to Bloom your Dahlias
FAQ
What is the best fertilizer for dahlias?
Are coffee grounds good fertilizer for dahlias?
How to make homemade fertilizer for dahlias?
What does Epsom salt do for dahlias?
Should you fertilize dahlias with potash?
Potash, the third number listed on fertilizers, aids in overall plant functions like the development of roots. When it is time to fertilize dahlias, the composition you use can greatly impact the growth of plants. In general, dahlia growers should avoid fertilizers with high nitrogen numbers, as this will result in bushy plants without many blooms.
How do you fertilize a dahlia plant?
When fertilizing dahlias, seek out fertilizer with low nitrogen and higher amounts of phosphorous and potassium. Manure and commercial slow release fertilizers are both popular options. I recommend blending composted manure into the soil before planting, then feeding your dahlias once a month during blooming season with a highly rated fertilizer.
How often should you fertilize dahlias?
When it is time to fertilize dahlias, the composition you use can greatly impact the growth of plants. In general, dahlia growers should avoid fertilizers with high nitrogen numbers, as this will result in bushy plants without many blooms. It is most often suggested that dahlia fertilizer be applied to new plantings once a month, until mid-summer.