Dahlias are summer-blooming superstars, beloved for their bold blooms and diverse range of forms. For cut flower gardeners, few flowers can rival these tuberous wonders. From tiny pompoms to enormous dinnerplate varieties, dahlias never cease to surprise and thrill.  

I would love to tell you that growing dahlias and getting great blooms from them is an easy task. The truth is, they’re as fussy and high-maintenance as they appear, but they’re totally worth it. With their long, stiff stems and flamboyant flowers, they make excellent cut flowers. There is a definite satisfaction in watching them bloom. 

To get the most exuberant flowering season from your dahlias, they need fertilizer. But you don’t want to use just any fertilizer. Let’s talk about the type of fertilizer that will give you the most robust blooming season. We will cover all you need to know about fertilizing dahlias, as well as some alternative and additional supplements to keep them popping all summer long.

Use the Right Fertilizer

The right option can make all the difference when growing these flowers in your garden.

For starters, using the correct type of fertilizer makes all the difference when you’re growing these plants. Use the wrong one, and you could end up with lots of vegetation and few flowers. That is a huge disappointment that I don’t want you to experience. 

A vast majority of plants will thrive ideally if you provide them with an all-purpose, balanced fertilizer. Dahlias, ever the drama queens of the garden, don’t fall into this group. These queens need a formula that is lower in nitrogen and higher in potassium and phosphorus. 

Nitrogen is responsible for leafy vegetative growth. When fertilizing dahlias with more nitrogen than they need, you’ll get huge, lush, green plants. That’s it. While they apply all that energy to foliage, very little will go toward developing flowers, and you could have a disappointing season. 

Phosphorus is an essential nutrient for fertilizing dahlias. It supports strong root development as well as flower production. Not only does it encourage the formation of buds and promote larger, more vibrant blooms, but it also enhances the overall appearance of the plant. It also helps the plant establish a strong root system and develop those all-important tubers. 

Potassium is also critical for fertilizing dahlias. It enhances the color, size, and longevity of your blooms and helps reduce stress by causing the plant to manage water efficiently. It helps protect against pests and diseases and strengthens cell walls, leading to sturdy stems. 

In your fertilizer’s formula, the first number represents nitrogen, the second is phosphorus, and the last is potassium. Choose a formula where the first number is lower and the second two are the same. Something along the lines of 5-10-10 or 10-20-20 is best. 

Start Early

It is beneficial to apply an adequate option during planting.

Dahlias are heavy feeders from the start, so don’t wait until you see growth to apply an initial dose of fertilizer. At the time of planting is the time to get started. Mix a small amount of granular or slow-release fertilizer into the soil at planting time to give your tubers a strong start. 

After planting, wait until your dahlias sprout to fertilize again. Let them grow to approximately six inches, and then begin your regular fertilizing routine. If you give them too much before they get growing, you can burn the tubers.

Stick to Your Schedule

A regular routine yields healthy blooms.

Once your plants start growing, it’s a good idea to establish a solid routine so that you can ensure they receive what they need. I like to keep track of fertilizing on my garden planner. I’m a visual person, so using a planner with visuals in addition to reminders helps me stay on track. 

Continue using your nitrogen-light formula throughout the summer. The phosphorus and potassium will become even more critical as blooming time approaches. Starting when your plants are six inches tall, fertilize every three to four weeks throughout the growing season. 

For granular fertilizers, sprinkle about three to six inches away from the stem, in a circle around it. For liquids, use according to the label, and make sure you water your plants first, so the soil is moist. When you get to about a month before your first expected frost, you can stop fertilizing. 

Don’t Over Fertilize

Using too much can harm the plant.

We discussed the effects of excessive nitrogen, which can result in gorgeous, lush, and leafy dahlias, but few or no flowers. Over-fertilizing can also lead to stems that grow too quickly and become weak and floppy. Dahlia flowers are large and heavy, and if the stems aren’t strong enough, they can snap easily. 

Excessive dahlia fertilizing may also delay blooming by causing the plant to prioritize growth over reproduction. Even if your stems are thick and sturdy, if they grow too tall, they are even more vulnerable to snapping and falling over. Support is a vital aspect of growing these.

Signs of over-fertilization include leaf burn, where leaves turn yellow and eventually brown at the edges due to salt buildup. It can also damage the roots, particularly if you apply it to dry soil. Too much fertilizer applied to the roots can burn them, ultimately killing the plant. 

Try Alternative Supplements

Use organic fertilizers like fish emulsion, seaweed, and compost.

If you prefer to combine other organic fertilizing agents for your dahlias, create a custom-tailored formula that will encourage optimal performance. Here are some options that work well:

Fish Emulsion: Nitrogen-rich, so use sparingly, especially early in the season. Boosts early growth and is also rich in trace elements. 

Seaweed/Kelp: Contains ample potassium and other micronutrients, which encourages strong roots and flower production. Combine with water and use as a foliar spray, or use as a soil drench. 

Compost: Amend the soil upfront to improve soil texture, enhance moisture retention, and add balanced nutrients. You can also use it to side-dress throughout the season. Breaks down slowly, reducing the risk of fertilizer damage.

Wood Ash: Good source of potassium and calcium; however, it raises the pH of the soil, so only use it if you have acidic soil. 

Bone Meal: A great source of phosphorus to encourage root and flower development. Mix into the soil at planting time in place of fertilizer. 

Worm Castings: Gentle, nutrient-rich compost that enhances the soil structure and life. 

For an ideal combination of nutrients that is tailored to dahlias’ specific needs, create a mixture of compost, kelp meal, and bone meal. This gentle, slow-release mixture is ideal for promoting healthy, balanced growth and robust flowering.