The Foetus as Parasite

or

Why We Deadhead

by M.J. Harvey

At the time of starting to write this I had just been to Hazel Vanslyke’s to photograph the red Paeonia rockii hybrid she had acquired for the Government House gardens in Victoria, BC. As I was shortly due to give a talk on peonies to the Hardy Plant Conference at the University of Victoria, a photograph of this rare form would add to my presentation. The only trouble was – she had just cut off the flower buds.

Now the point of this scribble is to consider why Hazel cut off the flowers. The immediate reason was that she had been advised to by Richard Fraser of Fraser’s Thimble Farm on Saltspring Island. A more proximate reason was that flowers (and fruit) sap the strength of any plant, especially one not yet established. You all know the above. The need to deadhead all sorts of plants is one of the ‘accepted truths’ of gardening, although it is occasionally debated in Rhododendron circles. Well, I am an iconoclast so let’s analyse the idea. Exactly why do we deadhead? What are its origins? And is it a good idea?

In the misty past, when I was a college student, the idea came out of zoology that in mammals the foetus was in effect a parasite on the female. This use of the word “parasite” was designed to shock, got into the science headlines briefly, then faded from the public mind. But being a biologist I remembered it. Don’t misunderstand this sensationalist approach. No one is saying that your children are parasites but science is a cutthroat, highly competitive profession and those who get the headlines get the money.

This embryo-as-parasite concept was then extended to plants. After all, the argument goes, seeds are similarly parasitic on the plant. They are an energy drain. Thus, cutting off the flowers or fruit is a sensible precaution preventing the diversion of energy from shoot formation to the seeds.

There are a few holes in the above argument. Not least being that flowering plants are not mammals. There are vast differences. In particular the ovary, which becomes the fruit, is green. Being green means that it can produce some of its own energy requirements – but how much?

Now I know of no research on the energetics of seed production in peonies or rhododendrons, but in agriculture the literature is chock-a-block with experimental evidence of where the sugars come from that go into the developing grain. I should explain that in biology, experiments are done on one or a few plants or animals and then their results are extrapolated to other plants or animals. So I will summarise briefly some of the results from wheat and then try to consider in what ways rhododendrons differ.

In wheat a large percentage of the sugars that are used to produce the starch in the grain are produced either in the head itself or the flag (upper) leaf. The lower leaves hardly matter, they serve to produce stem and roots. One surprise was that in bearded wheats and barley the awns (bristles) supply quite a percentage of the grain energy supply. These experiments served to establish that the nearer the green parts are to the ‘sink’ (seeds) the greater the percentage of the stored energy they contribute.

So how do rhododendrons differ from wheat plants? There are some broad ecological differences between them. Cereals belong to a general ecological category called ‘Ruderals’. These are plants which have a short lifespan and devote most of their energy resources to seed production. A lot of what we call weeds are Ruderals.

By the way of contrast, rhododendrons belong to a category called ‘Stress-tolerators’, (J.P. Grime, Plant Strategies and Vegetation Processes, Wiley, 1979). Stress-tolerators are those slow-growing, long-lived plants whose first priority is to put all their energy into building up a substantial structure of roots, shoots and leaves. They frequently delay flowering until they are many years old (from seed that is). Stress-tolerators also differ from Ruderals in that the percentage of their energy from photosynthesis going into seed production is very small. You might contrast a rhododendron seed, which is tiny, with that of wheat, which is maybe a thousand times heavier. By now you should be able to see where I am going. The “energy drain” justification for deadheading rhododendrons doesn’t have a leg to stand on. It seems likely that most or all of the photosynthesis required to nourish the growing seeds goes on in the walls of the seed capsules. The seeds do not inhibit shoot growth by “sucking up” the available energy supply. In rhododendrons the seed is not a “parasite” on the plant.

Deadhead or not Deadhead?

Don’t misunderstand the above. I’m not saying that deadheading is pointless. What I’m saying is that the justification many of us have used for deadheading may be wrong.

When you compare new shoots sprouting just below a flower truss with those coming from vegetative shoots on the same plant, the shoots below the flowers will usually be shorter or start growth later. What developing seeds do, apart from absorbing a microscopic amount of foodstuffs, is produce hormones that then diffuse down the stem. I suspect this is a hormonal effect that slows shoot growth.

At a certain time in the growing season a shoot has to decide whether or not to initiate a flower bud for next year. This decision time is quite early, I don’t know when, but probably June or July. It is long before the tip swells to indicate a flower bud. If a shoot is small or delayed in its growth it will decide to not flower next year.

So the reason for deadheading becomes one of trying to promote shoot growth. You can do this by deadheading immediately the petals shrivel. I doubt if deadheading later in the summer or fall will do anything to increase the abundance of flowers.

You will have realized that gardeners were deadheading long before pseudoscientific reasons for it were made up. Even gardeners in Victorian times did it. So what was their reason? Well, labour was cheap and the bushes looked neater, and I think that is why most of us do it.