Nutrients for Dahlias

We have been growing dahlias for over forty years, about 30 years ago we moved to our present location to have space around us and continued with the dahlia growing. Through the years we have met many respected dahlia growers and often the topic of discussion was fertilizing dahlias. The majority of growers recommended fertilizing with phosphorus and potash but go easy on the nitrogen and so that is what I did for probably 20 some years. I would apply possibly 200 – 250 pounds of 6-24-24 or 0-20-20 to about 10,000 – 12,000 square feet during the course of the growing season. After years of applying large quantities of this type fertilizer the soil tested at over 750 ppm phosphorus (many knowledgeable soil people contend that 60 ppm is very high).

As I recall the early years of growing in our present location soil tests were not employed, we would exhibit and usually get a few on the Court of Honor each year and at digging time there was an abundant root harvest. Storage was root clumps of the same cultivar in a brown grocery bag with some newspapers over the top of the bags in a room that was cool and dark, the roots were divided in the spring with little loss.

Through the years the dahlias seemed to be slower growing initially in the spring when set out and the root crop diminished noticeably along with the keeping qualities and I asked many growers “what are dahlias taking from the soil that we are not replacing?” Often growers would tell of adding trace elements each year, that did not seem to be beneficial in our location.

About 6 years ago in early July the dahlias again seem to be growing slowly until there was a heavy thunderstorm with considerable lightening and rain. Within one week the dahlias made noticeable growth and I surmised this was due to the nitrogen in the rain and purchased a 50 pound bag of urea (46% nitrogen) and began experimenting with applying nitrogen in solution (urea is very soluble) to the ground to a row of dahlias. I did not observe any burning of the foliage after about a week and proceeded to expand the application of N.

Through the years I have talked with others about our use of nitrogen on dahlias, this year Dan Franklin (Heartland Dahlias) suggested an experiment as we have considerable ground that has been idle for many years, possibly 50 or more. The results of a soil test of the area for the experiment; P – 82 ppm (Bray P1 test – very high range. This test indicates the amount of available P in the soil), K – 83 ppm (medium range), Mg – 140 ppm (high), Calcium 1200 ppm (high), pH – 7.7, CEC – 7.4. The soil is sandy loam (but more sand than loam), the garden has about 5 hours of full sun and partial sun the remainder filtered through trees on both the east and west sides of the garden.

The ground was tilled and Dan supplied roots of one cultivar (M FD R), all roots had sprouts about 2 – 3 inches long. Dan has taken to growing dahlias in pots, about 1 to 1 1/2 gallon size and we planted 2 potted roots in each area along with 8 bare roots. The area for the test was divided into four groups and planted May 20 with about 5 feet between the groups, 30 inches between stakes and two roots per stake. Following is the soil preparation for each group;

Group #1 – 2 pounds 12-12-12, 1 pound 35% magnesium, 1 pound lime and 1/2 cup captan. This was was broadcast over about 45 square feet and then worked into the top 2 -3 inches of soil. The stakes were set and the roots planted.

Group #2 – 2 pounds 20-5-10, 1/4 pound sulfur was broadcast over a similar area and worked into the soil. Stakes set and the roots planted.

Group #3 & #4. Nothing added initially, stakes set and the roots planted.

Group #3 & #4. June 1. Sprouts breaking though the ground of these two groups. Applied Urea in solution (1/2 tablespoon per gallon) to the ground around each plant of each group (about 1 gallon to each plant).

Group #3 & #4. June 5. Applied 1 heaping tablespoon Urea to each plant (about 8 – 10 inch radius), this was not worked into the soil but left on the surface.

Group #3. June 9. Applied 1 heaping tablespoon K (60%) to each plant.

Group #3 & #4. June 9. Applied 1/4 – 1/3 pound sulfur to each group and worked into the soil lightly to lower the pH.

Following the June 5 application of N to groups #3 & #4, 1 heaping tablespoon N was applied at 3 week intervals through August 20 and always left on the surface of the soil or the mulch to be watered in.

No additional fertilizer was applied to group #1 or #2 during the growing season.

I have noted in groups #3 & #4 on June 1 sprouts were breaking through with all 10 roots growing in each of the groups. Group #1 began breaking through about 10 days later, all 10 roots of this group grew. Group #2 about 1 week later than group#1, 4 of 10 roots grew in this group.

All plants were topped at about 4 sets of leaves with no disbranching on any plants. Plants tied when needed and disbudded and secondary shoots removed on the next set of leaves and sometimes the next set also.

The plants were mulched and irrigation laid down.

Plants bloomed about; July 15 – groups #3 &#4, July 22 – group #1 and August 5 – group #2

Plant height & bloom size and number of plants growing at end of season (including the 2 dahlias of each group grown in pots);

Group#1 – 4 feet – 3 1/4” – 10 of 10
Group #2 – 3 feet – 3 1/4” – 4 of 10
Group #3 – 5 1/4 feet – 3 5/8” – 10 of 10
Group #4 – 5 feet – 3 5/8” – 10 of 10

Observations: In groups #3 & #4 the foliage was noticeably darker green, group #3 with the added potash being slightly darker. As noted above the plants were taller and the blooms about 10% larger, in both of these groups there was considerably more branch growth and with the additional branches there were more blooms, the stems were longer (10% – 15%) and the foliage somewhat larger but with the larger foliage and longer stem the overall proportion of bloom/stem/foliage was good and acceptable.

Bloom position on all groups was typically good, the stems at no time during the growing season were weak, soft or rubbery, the blooms were always held erect with good bloom position. I did not make an actual petal count of any of the blooms, although there was the size difference of the blooms there was little if any difference in the form, substance, number of ray florets or quality of the blooms.

Observations from experiment and some of my recommendations for dahlia growers getting started:

1. Soil test is needed to determine nutrients available.

2. No fertilizer is needed to start dahlias, usually the root has adequate nutrients needed to start the plant and until there are feeder roots the fertilizer is of no value. 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. I have learned from articles and web sites that high quantities of P possibly may cause potassium deficiency and also inhibit the intake of some additional elements (but these articles do not state the level of P 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 areas mulch will harbor all sorts of insects and pests and one must attempt to control them.

6. Possibly in clay soils less nitrogen will be required as the clay particles will retain the nutrients (nitrogen and potash) much better than the lighter soils.

As stated previously, the dahlia garden soil tested at over 750 ppm phosphorus four years ago and during the past 4 years there has been no phosphorus applied. During this period the phosphorus has dropped 230 ppm which is equivalent to 430 pounds per acre. With this I calculate the dahlias as planted in our garden need about 110 pounds of phosphorus per acre per year or about 25 pounds per 10,000 square feet and I was applying about 2 to 3 times this amount each year.

I plan to dig and store separately some roots from each group, to evaluate the root making and storage qualities. Will advise next spring.

As there are some growers that will question the use of nitrogen on the larger cultivars, I plan to continue this next year with larger cultivars repeating the nutrient application and keeping the areas isolated.

 – Mac Boyer 06/2001