Focus On Dahlia Fertilizer

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Dahlias can be grown from seed, plugs, liners or tubers. The newest dwarf varieties have brought intense color and appeal to our customers, which makes this crop a staple value-added selection for the early spring holidays.

Fertility Early On

In propagation, dahlias prefer to be kept in evenly moist media at a pH below 6 (5.5 to 5.8). Before the signs of first leaves, begin to fertilize with calcium-based formulas, like 15-0-15 or 13-2-13 at 75 ppm. The calcium in these formulas is associated with nitrogen as NO3, which is easily taken up by newly forming root systems. After seven to 10 days, increase your fertilizer rate to 100 to 150 ppm using the same formula for another seven to 10 days. After these initial two weeks, you can use the same formula selection or incorporate a more potentially acidic fertilizer like 21-5-20 as an alternative to keep the pH of the root zone in the target range of 5.6 to 6.0. Early targets for media EC are between 1.2 and 1.5 mS/cm.

The Building Blocks of a Strong Plant

Testing your water source is a fundamental step in choosing the fertilizer that will match up with the existing nutrients in your water. Dahlias are heavy feeders, especially when they enter into the long-day stage. Nutrient demand will grow as day length and light levels increase, therefore it is important to provide a balanced nutrient profile at the start of this stage. This includes providing all the major, minor and secondary nutrients from either your water source or your fertilizer selection. Calcium and magnesium may have to be supplemented to reach minimum levels of 60 to 80 ppm calcium and 30 to 50 ppm magnesium.

For dahlias, a strong and nicely branched root system will add stability to the container as the plant begins to produce large and colorful flowers.

Once a sturdy root system is established, it is recommended to increase the fertility levels to a much higher rate. How do you know if it is the right time? Examine the root system by tapping the plant out of the pot, being careful to support it so as to not break any stems or leaves. If you can see healthy, white roots extending to the edge of the pot, you can be sure that the root system can start to absorb a higher rate of fertilizer.

A Fantastic Finish

We have talked in previous articles about formula selection. Similar rules apply to the fertility choices for growing the different types of dahlias. Very briefly, high alkalinity and high pH water sources should use a potentially acidic formula, like 21-5-20, 25-5-15 or 20-3-19. If it is necessary to supplement with magnesium, you can add Epsom salts directly to your stock tank. A conservative rate of 4 ounces per 100 gallons of water will achieve an increase of 30 ppm magnesium in the nutrient solution.

Low alkalinity or pure water should aim to increase or maintain media pH by adding calcium at each feeding. Complete formulas such as 13-2-13, 15-5-15 or 15-2-20 are very good options since they add both calcium and magnesium in one balanced formula. However, they will raise your pH over time.

Neutral formulas such as 17-4-17 and 16-2-15 also add calcium and magnesium but are designed to keep the pH change in the root zone to a minimum. Compare these factors when selecting your fertilizer, so you can provide a balanced nutrient profile as well as keep within the pH target values.

When the root system is established, gradually increase your overall fertilizer rates to 250 to 300 ppm and maintain these EC and pH targets until right before ship or sale date. All programs should still use a lower-phosphorus formula to prevent internode stretch. Monitor growing media to avoid a gradual rise in pH and EC. Do not exceed a pH of 6.2 and EC of 2.5 mS/cm.

Taking the plant to finish, many growers can switch back to the early calcium containing formula (15-0-15 or 13-2-13) at a lower rate (100 to 150 ppm) to harden off the plant while also alternating with a higher phosphorus formula, such as 15-15-15 or 15-16-17. This routine will surround the roots with available phosphorus and stimulate the plant to stop focusing on leaf growth and start focusing on flower production.

Common Fertility Issues

Growing dahlias for the early spring window can easily boost your initial spring or Mother’s Day sales, but there are some common issues to be aware of.

Underfertilization due to overwatering or overall low fertilizer rates will cause soft growth and premature flowering. As light levels and temperature can spike in the early spring, growers must take care to maintain the higher fertility rates as the plant increases transpiration.

Need a little bit of green? As plants go to bloom, make sure you are maintaining dark-green foliage. Use a specialty iron fertilizer, like Jack’s Petunia Feed 20-3-19. This formula contains a proprietary blend of the three iron chelates (EDTA, DTPA and EDDHA) that will give your chlorotic tissues a greening boost, even if the pH is a bit on the higher side. Use a rate of 200 ppm in place of your potentially acidic fertilizer for two or three feedings, then return to your normal fertility routine.

Overfertilization will cause more vegetative or leaf growth and less flower production, especially at the end of the crop. At the early stages of growth, light levels are low, so pay particular attention to the nitrogen form in the fertilizer you choose. Keep ammonium (NH4) nitrogen levels in the root zone at less than 15 ppm by choosing high nitrate (NO3) based fertilizers like, 13-2-13, 15-5-15 or 15-0-15. These formulas are designed with a balance of nitrogen that is easy for the plant to absorb along with low, but readily available, phosphorus to encourage the growth and branching of a healthy root system.

Dr. Cari Peters is vice president at J.R. Peters. You can e-mail her at


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