More Nutrients for Dahlias

Last year we grew plants of one cultivar (M-FD) in four groups with different applications of fertilizer for each group which produced various results; some never made it through the soil, there were growth differences during the season and the weight of the roots when dug as reported in the June ADS Bulletin. Soon after planting we realized a mistake of not planting any of the same cultivar in our show garden to gain another comparison as the soil analysis of that area indicated levels of P and K considerably higher there.

The primary purpose of the comparison this year was to evaluate the effects of growing named cultivars in soil containing considerably less P & K than presently in our show garden. The area was planted late May, divided again in four groups with plants of (started roots) EDNA C, HAMARI ACCORD and DANUM METEOR which were also planted in our show garden. About 7 days after planting I applied to the soil around each plant (this was not worked in) two tablespoon 12-12-12 to each plant of group #1 and two tablespoon 20-5-10 to each plant of group #2. Groups #3 and #4 each plant received one tablespoon Urea (46% N) and no additional fertilizer. The Urea used is not coated and is very soluble and will dissolve in water quickly.

Sometime soon after the initial fertilizer application I changed plans to have a broader comparison of the experimental area to the show garden. About three weeks after planting I applied about one tablespoon Urea to each plant, scattered on the surface and continued through late August at three week intervals to all groups at the same time application was made to the show garden. The area was mulched and an irrigation system laid down.

All plants (show garden and experimental) were topped and grown with the same number of laterals, disbudded and disbranched the same. One notable difference of the two areas is 30% shade cloth was pulled over the show garden about mid August.

Some time in September while working with the plants some of the stems of HAMARI ACCORD appeared to me to be somewhat larger in the experimental area, I dug out micrometers of my engineering days to record accurate measurements.

HAMARI ACCORD average stem diameter of flowers in bloom. Show garden -0.259 in. dia. avg.(range 0.215” to 0.285”) Experimental – 0.293 in. dia. avg.(range 0.260” to 0.360”) An average stem diameter difference of about 1/32”, 13% larger with about 10% of the phosphorus in the soil compared to the show garden and the same amount of applied nitrogen. Stems were always strong, straight and well proportioned with the foliage. All of the plants and the stems in the experimental area were not quite as tall and long but this I attribute to the cloth over the show garden. I should have proceeded to make measurements of the stems of additional cultivars but did not, there is always next year. How many times have you read of heard that dahlias needs lots of P for strong stems and nitrogen will make the stems weak and rubbery?

Blooms of HAMARI ACCORD and ENDA C in both areas were of approximately equal diameter, depth, number of ray florets and bloom position, I did ring some first bloom of HAMARI ACCORD in the experimental area at about 8 1/4” – 8 3/8” .

Blooms of DANUM METEOR grew to about 12” – 13” diameter on plants with four canes, strong, straight stems and well proportioned with the foliage in the experimental area. Root clump comparison, Show garden soil test (year 2000) P-514 ppm, K-205 ppm; Exp. Area P-82 ppm, K-83 ppm. Root clump average weight as dug with soil removed;

HAMARI ACCORD – show garden – 20.6 oz. avg.
experimental – 37 oz. avg.

DANUM METEOR – show garden – 8 oz. avg.
experimental – 20 oz. avg.

EDNA C – show garden – 26 oz. avg.
experimental – 64 oz. one clump only

While all ENDA C grew reasonably well, I culled the remaining plants to keep only the best.

My conclusions for growing dahlias in our garden with the general surroundings, the soil, growing conditions and weather that we had this year. There are many variables, just as the weather next year may not be similar and the results in growth, blooms and roots might be radically different.

Dahlias do not large need quantities of P & K to produce show quality blooms. Large quantities of P &/or K in the soil appear to inhibit root growth in our garden.

Limited quantities of N uniformly applied during the growing season does not produce weak, rubbery stems and N appears to be beneficial to overall growth (this is not new information as I have been applying N to the show garden for about 6 years). At this time I am not certain if the increased root growth of the cultivars in the experimental garden is attributable to the nitrogen or the reduced amounts of P or K. The unknown, are excessive amounts of P and K acceptable to attain optimum root growth or is this a deterrent? Possibly a test growing in the area during the season without any nitrogen added would give an indication.

There was not any noticeably differences of color in the foliage or the blooms.

Many times I have read or have been told that dahlias are heavy feeders of primarily P, next was K but be careful with N as it will give you soft growth. In my last article I referred to a question that I often asked years ago, “what are dahlias taking from the soil that we are not replacing?” My conclusions now indicate to me that we (at least in our garden) have applied considerably too much P and K to the dahlia garden and inadequate amounts of N through the years. At the present rate of usage I should not need to apply additional P for 6 to 8 years.

Some growers may argue that dahlias will take up only the nutrients needed for good growth regardless what is applied or already in the soil. But does excessive amounts of P or K in the soil possibly have a similar effect on the availability of some nutrients as when soil ph that is too high or too low? We know when the soil ph is not within a specified range many nutrients begin to be locked up. I have found references that have indicated a trace element was less available with high P, but again the ppm was not specified.

Additional information gleaned from various sources but usually did not find any reference to nutrient levels; 1) Balanced nutrition is a must for realization of the full phosphorus response potential of crops. 2) Insufficient levels of other nutrients can substantially reduce response to phosphorus. 3) Adequate supplies of other plant nutrients tend to increase the absorption of phosphorus from the soil. 4) Application of ammonium forms of nitrogen with phosphorus increases phosphorus uptake from a fertilizer as compared to applying the phosphorus fertilizer alone or applying the nitrogen and phosphorus fertilizers separately. I will take samples for another soil test in the Spring, regardless of the results I plan to not add additional P or K to either the show garden or the experimental area but may experiment with some of the minor nutrients in the area of the later. I believe our test garden may double in size next year and with the results that I have seen I probably should plant more of that area to be the show garden.

My recommendation; have your soil tested by a professional firm. If your analysis indicates very high P availability switch to a lawn fertilizer analysis but apply limited amounts (25-3-3, two tablespoon per plant) at regular intervals during the growing season. I used one tablespoon of Urea which is 46% N, almost twice the amount of N as in a 25-3-3 lawn fertilizer. What next? Possibly work with some of the minor nutrients.

 – Mac Boyer