Monday 5 January 2015

Why does a plant fail to flower?

Gardening myths discussed. “Give ‘em some potash and get ‘em to flower!

My abutilon has flowered all summer and continued up to Christmas

It’s a hoary old question on GQT and similar programs. Why does a plant fail to flower? 
Recently, after giving quite sensible advice the ‘expert’ hesitated and thinking perhaps she had not filled her time, rather unconvincingly added that you could of course give them some potash. I am not quite sure she believed it herself. When stumped by the question in front of an audience it just trips off the tongue. Shame on me I have done it myself!
I wonder how many people go out and actually apply sulphate of potash after such tosh. I am doubting today whether it will make any difference whatsoever whether a plant does or does not flower. 

Perhaps if the punter applies a high potash general fertiliser which contains a balance of nutrients, improved nutrition might very well increase the quantity of flower - but then it will often make a bigger healthier plant where all aspects of growth are improved.

I find myself thinking today that in seeking reasons why plants really fail to flower I might be promoting equally mythical gardening lore. It’s always a problem when what one assumes to be ‘a given’ is also untrue!

A little knowledge is a dangerous thing! At his Open day my friend Peter Williams was selling stunning rhododendrons in full and magnificent flower. A visiter commented that he would not touch them with a bargepole, they were destined to die! He had confused the situation where plants near to death sometimes produce a flurry of flowers in a last gasp attempt to procreate. Peter’s plants were flowering well as a result of being so healthy!

I described in a previous post that whilst I do not fear the effects of honey fungus in my garden I was suspicious that my very old lilac was showing signs of infection and would probably die. Despite Brenda’s own ominous prediction, last June it flowered absolutely magnificently. It did not make very much new growth last summer and the leaves dropped early. Brenda again predicts it’s imminent demise. My own hope is that at will again produce generous flowers. 
The strong basal shoot on the lilac is well on the way to making a new tree. (It’s not grafted, so the sucker comes ‘true’).

My lilac photographed in December. Already in January I can discern healthy buds

You can see the armillaria (honey fungus) fructification which is very close to the lilac and the Spartium junceum.

A second flowering in December, not because of stress, it is very healthy, but because of the mild autumn

Reasons for not flowering
There are very many varied and distinct reasons why plants do not flower. Most are matters of plant physiology where a plant is reacting to environmental stimuli to achieve efficient seed production.
For many perennial plants, especially shrubs and trees, there is the not so little matter of juvenility. Many woody plants have a juvenile phase when they are incapable of flowering. For some plants this period may last many years. Sometimes a gardener’s activities may prolong this phase. For example beech - not that you are bothered about flowering! Beech when grown as a hedge retains its dead leaves right through the winter and this characteristic is maintained indefinitely by regular clipping. When a beech grows as a tree it retains this character for several years but eventually achieves autumnal leaf abscission and normal leaf fall and capability to flower.

Often the mature flowering ‘adult’ form of a plant maintains its ability to flower when it is propagated by vegetative means such as cuttings or grafting. Ivy is a well known example where the difference between juvenility and maturity is very clear. The flowering tops of an old ivy will retain the ability to flower when cuttings are rooted, but not always the ability to climb. Juvenile ivy climbs remarkably well!

Adult Paddy’s Pride retains its ability to flower after propagating from cuttings. It no longer climbs but manages to scramble

The practical significance of the phenomenon of juvenility is that some seed raised  perennial plants will not flower for several years. Fear not, there are not very many of them that you are personally likely to sow. 
Wisteria is a classic example. I imagine that my first wisteria must have been a seed raised plant. It took fifteen years and when it flowered it was a huge disappointment! 
I know now I should have bought a grafted wisteria. Best to buy one in full flower at the garden centre and you can see what you are getting. Even then they are liable to sulk for a while. I would not bother with any extra potash!
Sometimes a wisteria graft fails and you buy the sprouted rootstock. That will be another fifteen years sir, to achieve inferior flowers!

For brevities sake here is a list of examples of other factors that might control flowering!
(sorry about the numbering, it's beyond my technology)
  1. Stress does not always promote flowers, sometimes the opposite. If magnolias, camelias and rhododendrons suffer drought stress in late summer and autumn they fail to make flower buds.

My camellia has made nice fat autumn buds
And so has the rhododendron 

And the Magnolia is wanting to flower in January!
  1. Some plants respond to day length. Chrysanthemums are short day plants and flower when the day length is shorter than a critical number of hours. Spinach is a long day plant and unfortunately goes to seed in the middle of the summer.
  2. Some plants require critical temperatures to flower. Early autumn chrysanthemums respond to higher temperatures rather than day length.
My pom-pom chrysanthemum flowered for ten weeks from mid August.
  1. Some plants such as campsis as are said to need ‘ripened wood’. I am not sure what this means and I fear it might be a myth. Or might it just be low temperature or even low light intensity that prevents flowering?
  2. Winter cold promotes ‘bolting’ on such as overwintered onions and too-early sown beetroot.
  3. Traditional fruit growing practices such as ring barking and root pruning may induce flowering.
  4. Inexperienced gardeners prune shrubs such as forsythia, philadelphus and weigela at the tips and in the wrong season and cut away next years flowers!

Brenda was a little over eager to tidy back the Garrya elliptica last summer. Fortunately she did not prune over most of the plant
  1. Rarely light intensity might be significant - I think!  I have never known an avocado pear flower and set fruit  in the UK although it grows very well (inside). My dipladenia failed to flower all summer in our conservatory and I concluded it was too shady. I went to Italy and in a very dark courtyard it was flowering magnificently! Wrong yet again!
  2. Actually light intensity for house plants can be very significant indeed where a plant does not photosynthesise enough to make sufficient sugar to support flowers. My Hoya carnosa merely made healthy foliage but no flowers for ten years at my previous home but now in a light position it flowers superbly.

Hoya carnosa in our east facing conservatory
  1. And what climate conditions create mast years where such as oak and beech produce prolifically. What synchronises flowering of bamboos over very wide areas?
  2. And how can you prune fruit to avoid fruit biennial bearing?

If you can add to my list, please do so in my comments column. I welcome participation!
On the subject of participation it is always me that does the spouting. Please feel free to argue!

My Aster amellus gave me an extra flower in the wrong phase.
So what about potash?
I am comfortable with the idea that high potash might promote quality of flowering and fruiting. With reservations I might even accept the notion that potash  balances excessive nitrogen that might otherwise promote excessive leafy growth. What I do doubt is that potassium ever induces flowers where they would not otherwise arise.

And a little story

‘A Woman, a dog and a walnut tree, the more you beat them the better they will be’.

Many myths surround walnuts. Real Greek and Roman ones! 
Most walnut myths seem to be about throwing sticks to knock the nuts down! This seems to have evolved into the idea of beating the bark to encourage fruiting.
(One might explain this claimed fruit promotion in terms of plant stress or translocation of sugars).

I am more inclined to believe the man who wrote on a well known gardening blog that the first two strictures did not work and he therefore doubted the third


  1. Made me chuckle at the end there! Happy New Year.

    1. Glad my comment tickled your fancy, perhaps you are a bullied male like myself! Happy New Year to you too!

  2. With our radio 'expert' the answer to most plant ills is to apply Epsom Salts.

    Both my sister and I wish we could crack how to get our moth orchids to flower again. We have read everything we can some advice directly contradictory. We have in turn tried to apply most pearls of wisdom and nothing. A friends seem to be permanently in flower without him doing anything - we've tried that too.

    So my theory is that plants being living entities have individual personality traits. Some are too lazy to flower, some just downright stubborn, some enjoy winding us up and others just don't like us enough to want to please us. Mine have discussed their options and decided that they are on strike. Probably Skyped my sister's plant when we were not looking. Maybe it's time to threaten them with the compost heap.

    1. Our moth orchids do flower in our east facing conservatory for several years but do decline over about five years - so our conditions are not ideal, possibly too dry atmosphere. I think temperatures are critical especially in winter. Mine do receive some direct sunlight in the morning despite advice I read to the contrary.
      Better tweet you warning about the compost heap.

    2. Mine have spent most of their lives on an east facing windowsill and for this winter have Moves to south facing. I tried daily misting for about a year!

    3. Daily misting does very little--another myth, and forget that tray of water under the plant myth. Orchids need neglect-they live very tough lives in nature. My phals flower constantly, in an east window. I only water when dry--if I remember, fertilize very little. They do get colder temps in winter being near the window, but here in Canada we tend to keep house temps higher than in the UK. I used to grow about 1,000 orchids--never had a phal that did not flower.

    4. I've done the neglect too but not the colder temperatures.

    5. I go away for five minutes and you are all having a chat. Great!
      I remember Robert, when Brenda's eldest lived on Andros in the Bahamas ( yes they get everywhere) and visited I was amazed at the tough conditions the bromeliads withstood growing as an epiphyte on trees. Yes I know not an orchid but same principle, they like it tough

  3. As usual a most instructive post. Talking about potash, what is your opinion on wood ash. I have access to a great deal of it but spread some on the garden only every 2 to 3 years, afraid to overdo it. I suppose I should have the soil analysed rather than guess how much to apply.
    You mention Campsis. A lot of them were grown where I had my first garden and some did beautifully. I would say that they bloom best and for the longest time in the sunniest, hottest corner. They are attractive but I do not know any plant that suckers so viciously - everywhere in a radius of 50 feet away from the mother plant and hundreds of them!
    Happy New year to you and Brenda Roger!

    1. Thanks for your excellent advice on Campsis Alain. Brenda was proud that she got two flowers last year! We have planted the now seven year old plant in the sunniest corner we can find.
      Bit of a waste of time in our cold climate if you ask me, but Brenda is determined!

  4. Interesting post again. I had a Wisteria which did not flower, then I tried it with a gift of potash, after seven years we got wonderful flowers (so glad). As far as I know potash is good for flowering, so sometimes I give a small hand to my Agapanthus and they are flowering well, I just do all by trial and error. But now about wood ash, I saw the question of Alain in above comment. We have a woodburner so I have three times aweek a drawer with ash which I give to my roses, Every rose gets about half a tray in winter. I 'think' it's good for the roses, but I am not sure, I do this already for some years and the roses are doing fine, but I do not know if this is due to the woodash. What do you think? I also spread sometimes the tray over the borders.

    1. reply to Alain and Janneke, happy new year to you both!
      Yes wood ash is undoubtedly a beneficial fertiliser containing lime and potash (potassium). It's not so good if it's from logs Janneke as the potash is rather less than from nutrient-rich twiggy growth.
      It would be quite difficult to give too much potassium from normal sources as it is relatively weak. Always use it before rain washes away the potash which is highly soluble. I think soil analysis is a bit of a waste of time in such circumstances Alain.
      I think you both know that I now think burning a garden bonfire right through burns up the beneficial charcoal which I now avidly collect from my fires after dousing with water as I described on my post on terra preta.
      Not appropriate advice for a wood burning stove Janneke!

  5. In my experience the general health of the plant is the biggest factor, though I think the amount of light does make a difference - though I can't prove it.

    1. Yes when I wrote my note on my hoya a lot of other examples of poor light on plants came to mind!

  6. I think the most widely asked question from gardeners must be how do I get my Wisteria to flower! I bought one exactly as you suggest, in bloom. It was the only time it bloomed! 8 years later I banished it to the bin. I like to think I pruned exactly as suggested but couldn't swear to it!
    I think the myth of Agapanthus requiring to be pot bound is another question that gets answered wrongly. Having spent years waiting on my Agapanthus to get pot bound, I've given up, split it down and started again. I am waiting to see what happens in 2015. I've kept records so I can post about it if I have flowers this year.
    Great post, full of your usual wit Roger :)

    1. Thanks Angie, I agree about agapanthus and discussed this in my agapanthus post last year.
      Sorry to hear about your wisteria, they do seem to have a mind of their own - just like Sue's orchid!

  7. The biggest problem with advice such as add potash, is that the gardener does not know there is a deficiency. It is quite pointless adding any specific nutrient without knowing it is needed.

    A plant is naturally driven to flower--that is it's main goal in life. I would suspect that it if it was short of potassium, and felt the need to flower, it would move excess potassium from where it is in the plant, to where it needs it--that is just a guess.

    About hitting the walnut. A very close friend of mine, who is also a professor of Botany, says this is common practice for grape vines and that scientific research has shown that whacking grape files increases the amount of fruit. I have a hard time believing that--but he is the expert!

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  9. Sorry for the deletion stupid typo!

    Fascinating post as usual Roger, I must admit I have never considered potash to be of any benefit until the flower buds are initiated but have no proof of this either way.
    Your reference to Chrysanthemums as "short day" plants reminds me of commercial Pot-mum production under glass where the plant's growth and bud initiation is manipulated by a combination of either lighting or shading depending on the time of year so that the actual marketing date can be arrived at from the planting date and the variety.
    I grow several camellias but had a problem with one specimen in particular, which was growing under a general canopy of deciduous trees but was also within 10' of a fairly tall conifer. Although the plant was quite a well established specimen it only managed three or four flowers a year. I chopped down the conifer several years ago and the plant, after a slightly better show the following year, is now covered in bloom every year but the question is, bearing in mind that there are still plenty of tree roots in its vicinity, is its revitalisation due to the conifer no longer extracting the water from the ground or is it simply that more light was made available or a combination of both? Either way it was a good result!
    Many of the local "mow and blow" brigade are pretty good at pruning at the wrong time, in fact many of them seem to only prune in the winter when they have little else to do, the customer is then at their wits end when half their treasured shrubs don't flower the following year.

    1. Perhaps I am imagining myths but I do seem to have heard the potash one so many times!
      Your camellia example is a wonderful illustration of many things in gardening being mult-factorial. I don't know whether to put my money on the water or the light angle, almost certainly both.


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