DAHLIA-ITUS

Barry Kenyon

This is not a dictionary word but more one that has been made up (I think) by Johny Parky. It is said that it describes a phenomenon that exists whereby when Dahlias are grown in the same place year after year their quality deteriorates.

Well we have all come across the term “Crop rotation” normally associated with the growing of vegetables and there are a number of reasons why this has been carried out commercially for hundreds of years. The principal one being “Soil pests and diseases tend to attack specific plant families, so by rotating crops life-cycles are broken and build-up is reduced”.

To my knowledge there is nothing proven with Dahlias to show that the Dahlia-itus phenomenon exists but I am told something similar definitely does in Gladiola? For most of us a move to a different plot is out of the question and any way what does it matter when it is not proven .Winning National Trophies is however becoming increasingly difficult as the quality of the growers is getting better and better so any small advantage is a good one.

How can we achieve a similar result without moving to a different plot or replacing the soil?

Answer “Soil Sterilisation.”

At the start of the 2006 season I sterilised most of my Dahlia Beds. Funnily enough I did not do it for the reasons above, but because it cuts out nearly all weed growth and Wendy has been moaning about how long it takes her each year to weed the dahlias. (I don’t use any form of mulch or soil cover as I like to see the soil so that I know how much to water!). I also treated Wendy to a good “Mist Propagation System for our anniversary” and I would recommend this to everyone. The mist propagator that is.

Before I go on to describe how I did the sterilisation there are a number of other good reasons why you might want to try it yourself. Whilst I don’t know of any proof of the deterioration of bloom  quality caused by build up of unknown soil diseases. There are a number of known soil diseases that do strike Dahlias that we can well do without. One of these is “Verticillium Wilt”   Single branches or the entire plant wilts and dies. Vascular tissue in affected stems is black, dark brown, or greenish-brown. Remedy is to discard tubers and sterilise soil.

There is another one called “Scleratinia Scleratiorium” a particularly difficult one to get rid of. Symptoms of this are collapse of the whole plant or a single stem. Inside the stem you will find a sort of black bean. The disease starts from a mushroom type growth in the soil,  pale brown in colour with tops as big as a 50p piece and when the time and conditions are right for it, it puffs spores into the air that then infect all plants they land on. This particular disease is prevalent in Oil Seed Rape. The dahlia growers that I know have had it, have all used farm manure (could be a coincidence though).

“Soil Sterilisation” is a simple process using “BASAMID”. This is a white powder and when rotovated into damp soil gives off Methyl Isothiocyanate. This gas kills Bacterial and Fungal Diseases, Weeds and Weed Seeds plus soil born pests.

I treated at the beginning of April. I had left my covers on over winter to increase the soil temperature. It works down to 10 C (48F) soil temperature.( Soil temp can be increased initially by covering with black polythene for the previous month) I made sure the soil was damp first of all ( Soil just about held together when you made a ball of it and squeezed.) I rotovated it in to the beds at a rate of 50 gms per square meter, then covered the beds with thin gauge clear polythene sheet.

 My soil temperature was 10C so I needed to leave the  polythene on for about 12 days, planting time can then be 24 days later. (These times vary with temp, for instance at a temp of 15C ( 57F) Gas activity is cut down to 8 days and replanting can be 15 days after that.) After 12 days I removed the polythene to let the gas come out of the soil. After a further 10 days I gave it another quick rotovation and then did a germination test. This is a simple way of testing that the gas has dispersed and that no residue is going to harm my plants.

A sample of soil is taken (I took a teaspoonful from various places on the plot about 20 samples in all, and mixed these in a bucket. Tacking my final sample from this mix) and put in the bottom of something like a jam jar about 2 inches in depth. Another soil sample is taken from somewhere else in the garden that has not been sterilised and this is placed in another jam jar. The soils must be again damp. Garden cress seed is now sprinkled on the surface of the soil in each jam jar and the lids put pack tightly in position. The seed should germinate in about 2 days and should be the same in each jar. It worked just as explained for me. If it had not have done I would have given another rotavate in another 3 days and  sampled again. The cost of a 5kg bag of Basamid £35.25

The results were great, only the odd weed and hardly any earwigs this last year. I can’t say that the blooms were noticeably better but I did win the Individual Championship at the National again so who knows?