Don’t Fear Nitrogen: Feed Your Dahlias Right from the Start

Dahlia growers often worry that applying too much nitrogen will lead to tall, leafy plants that bloom poorly or fail to form good tubers. That concern is valid—if the timing is wrong. But early in the growing season, nitrogen is not the enemy. In fact, it’s exactly what your dahlias need.

Like all plants, dahlias function as an interconnected system. Each part—leaves, stems, roots, and blooms—plays a role in the plant’s overall health and productivity. And like all systems, dahlias require the right inputs to perform well.

The three primary macronutrients needed for plant growth are nitrogen (N), phosphorus (P), and potassium (K). A useful shorthand to remember their roles is this:

N for “up” (leaves and stems),
P for “down” (roots), and
K for “all around” (general vigor and resilience).

Unlike phosphorus and potassium, which tend to accumulate in garden soils over time, nitrogen is highly mobile and easily lost to rain or irrigation. That’s why many soils—even those regularly amended with compost—end up short on nitrogen, especially after a wet winter.

Early in the season, dahlias need nitrogen to build the green infrastructure—stems, leaves, and nodes—that will later support flowering. Skimping on nitrogen at planting time can lead to stunted plants that never reach their full potential. On the other hand, overapplying nitrogen later in the season, once buds have formed, can lead to excess foliage and fewer flowers. The key is giving dahlias what they need when they need it.

To illustrate this, I recently ran a small experiment with dahlia seedlings. I planted two identical sets of seedlings: one group in my standard Dahlia Doctor soil blend amended with feather meal (a slow-release organic nitrogen source), and the other in a 50/50 mix of that soil plus well-aged cow manure sourced from a local farm.

After six weeks, the difference in foliage color was dramatic. The manure group showed noticeably deeper green leaves—an unmistakable sign of more available nitrogen. Both groups grew at the same rate, but the color variation told the story.

It’s worth noting that I used the 50/50 manure blend not as a garden recommendation, but to test for residual herbicides in a bulk load of farm manure. I would never use that ratio in the garden. But even at this exaggerated level, the takeaway was clear: early nitrogen availability makes a visible difference.

In the garden, I usually add one heaping handful of well-composted manure to each planting hole, or one tablespoon of a granulated high-nitrogen fertilizer . The specific brand doesn’t matter. Many of these fertilizers are marketed as lawn food and typically have an NPK ratio such as 29-0-4 or 32-0-4. These are slow-release formulas that break down gradually, making them ideal for supporting vegetative growth in the first six weeks.

Fish emulsion and other liquid feeds can also be used, especially by organic growers, but they must be reapplied regularly. For large plantings or those who want to “fertilize once and be done,” granular products are simpler.

Be cautious with bagged “manure” from garden centers. These often contain sawdust or other wood byproducts that dilute the nutrient content. I’m fortunate to live in an agricultural area where I can source true composted manure from a farm—just aged manure mixed with straw or wood shavings that have absorbed urine and been broken down over time.

If your dahlias start out weak and slow, nitrogen deficiency may be to blame. Starting them off with a nitrogen-rich foundation builds the leafy structure and stem strength needed for abundant flowering later. As the season progresses, you can back off on the nitrogen and focus on bloom boosters or balanced feeds.

There’s no need to be afraid of nitrogen—just be thoughtful about when and how you use it.