Dr. Mike Pryor is considered by many to be one of the top exhibition dahlia growers in the NADC and perhaps the country. His detailed research in all phases of dahlia culture needs to be saved and made available to dahlia growers throughout dahlia land. As a scientist you can rest assured his conclusions are tested under the very strict rules of investigation.

by Dr. Mike Pryor

The new dahlia grower can be in tough shape when it comes to getting reliable information on fertilizing. Assuming that he (or she) has joined a local dahlia society someone will usually give a talk in late winter on fertilizing. This information can be good, bad, or more frequently a mixture of the two, depending on who is dispensing the advice. Because the dahlia is not a flower that has major commercial importance there are very few reference books published and even these, in part, tend to reflect the cultural prejudices of their authors. The A. D. S. through the medium of its quarterly Bulletin tries to fill the gap but, in my opinion, with only partial success. As a result there is too much erroneous information around which, by constant repetition, becomes almost institutionalized. This is unfortunate because a new grower following unsound cultural advice will undoubtedly get disappointing results. In this and other articles I shall try to identify a few of these widely believed myths and hopefully lay them to rest for a while.

Now the foregoing is not intended to sound presumptuous. My basic training is in chemistry and I spent my professional career in research. Therefore I did not find it difficult to use my dahlia garden as a test bed for investigating various fertilizing practices. Controlled experiments require subject dahlias, whose fertilization is modified in some way, and control dahlias of the same variety. The control must be separated from the subject by at least two spaces in order to prevent the fertilizer addition to the subject dahlia from affecting the control dahlia, the later being subjected to normal fertilization. It is necessary to repeat a successful experiment at least once and preferably twice in order to try to minimize the influence of uncontrolled factors like the weather. Patience is clearly required since it takes a whole growing season to yield an individual datum point. I have conducted many dozens of experiments over the years and have incorporated favorable results into my normal dahlia culture. It is on the basis of these experimental results that this article is written rather than on my unsubstantiated opinion. Always bear in mind that if a different cultural practice seems to make sense it may or may not be advantageous. In this case it may be worth a few controlled experiments to check it out. If a modified cultural practice does not make sense it is surely worthless.

When I first started growing dahlias the late Paul Smith was my mentor. In this I was somewhat fortunate. Paul suggested a basic initial fertilization of around 2 lb. of 5-10-10 per hill spread uniformly and tilled in. This was a good suggestion and I use it with only minor modification today. He also suggested side dressing with a handful of a strange fertilizer called Electra in mid July. This fertilizer had a 5-10-10 formulation, was labeled "Mainly Organic" and contained a great deal of soot. It was popular with a number of old time growers in the NADC and was supposed to enhance color (it did not).

I grew for around eight years with this fertilization scheme and had very mixed results. I had some modest success on the show table with BB's and to a lesser extent B's. In contrast my A's and AA's (they were combined at that time) were a disaster. A Connecticut dahlia, WALTER HARDISTY, was fairly new at that time and I wanted to grow it. Unfortunately it would only grow to a diameter of around 8" in a good year. Accordingly I started to increase my fertilization in a series of progressive steps. I hesitated to use more 5-10-10 since I was concerned with the possibility of burning the feeder roots early in the season (by that time I was growing primarily from plants) Instead I used dried cow manure as an auxiliary fertilizer. It started to increase bloom size progressively until I reached around 6 pounds per hill after which no additional size advantage could be discerned. Thereafter my initial fertilization became the 2 pounds of 5-10-10 and the 6 pounds of dried cow manure spread uniformly and tilled in. From the above you may see that my objective was improving fertilization for the larger flowers. What I determined is not applicable to the smaller flowers like miniatures and poms where diminutives is desirable. These clearly require and benefit from much lesser fertilization.

Somewhere along the way I was able to dispense with Electra. One year I ran out of it mid way through side dressing. Since it was not easily available locally I finished the garden with conventional 5-10-10. To my relief there was absolutely no difference in color intensity. I had expected this result because soot (graphite) is essentially insoluble in water and could, therefore, not be absorbed by the root system. I was glad to abandon it because it smelled awful and was very fluffy. If it stayed on the foliage for more than a couple of minutes it burned the leaves to a crisp.

Now achieving acceptable bloom size in A&AA dahlias brought along an unfortunate complication particularly in WALTER HARDISTY. That side effect was quite willowy stems. It didn't impact single bloom entries but was a major problem with 3 blooms and baskets. First a 3 bloom entry was a nightmare to set up and once set up the outside blooms would usually collapse overnight in a two day show. So then I started to get it in the neck from the old gurus. "Too much nitrogen Mike. You should know better". "Fertilizing to the hairy edge of disaster" said another. But now I was the victim of a classic "Catch 22". If I reduced the amount of dried cow manure, allegedly to strengthen the stems, I would forfeit bloom size. So I decided to continue with my heavy fertilization and suffer with my willowy stems and make less multiple bloom entries in hope that something would turn up from future experiments to guide me.

And indeed it did in fact happen when I conducted a series of experiments on the effect of supplemental fertilizing after a pea size bud had formed. The objective was to see if this would impact bloom size. There is quite an amount of speculation on this subject in the dahlia literature and little of it made sense to me. Accordingly I set up a major program with a wide variety of fertilizers (12) high nitrogen, high phosphorus, high potassium, balanced fertilizers both granular and soluble in at least a couple of different concentrations. Many of the additions were massive. The main result was not surprising to me. No additional fertilizer irrespective of type had any detectable beneficial effect on bloom size. In other words once a pea bud appears the maximum size of the flower is established. Only inadequate watering will reduce the size.

The auxiliary results from the foregoing experiments were even more important to me. Those cultivars that had received high phosphorus fertilizers grew tremendous tubers particularly at the highest phosphorus levels. The cultures receiving high nitrogen fertilizers grew heavier foliage and much stronger stems thereby silencing the "too much nitrogen" prophets of doom. When I thought about this at leisure it made a lot of sense. My good friend Roger Miller was fertilizing at that time with cow manure only. According to the best guesstimate I could make he was using at least three times as much nitrogen as I was. Yet he was growing stems like broom handles on A's and AA's. With things finally falling into place I was able to solve my stem problems by heavier side dressing with 5-10-10, a practice I still use.

In summary direct experimental evidence deflated two dahlia myths as follows:

1. Weak stems are not the result of too much nitrogen. In fact stems can be strengthen by fertilizing with additional nitrogen.

2. Bloom size cannot be increased by fertilizing after a pea size bud has formed.

These conclusions may well be at variance with cultural practices used by many good dahlia growers. In this case I would suggest that a few well chosen, controlled experiments in the garden will lead to the truth.

Copyright 2009 Mid Island Dahlia Society