Saturday, 20 July 2013

Reasons not to dig 7: to grow healthy plants



What are the effects of cultivations on pest and disease?

The traditional view of birds following a tractor is that they are eating pests. Come to think of it, if you see a tractor in a field these days you blink and it’s gone! The birds may well be eating harmful ‘grubs’ such as wireworms, cutworms, swift moth larvae, millipedes and all, but they are also eating beneficial predators such as centipedes and ground beetles. They are also eating worms.
Even in my lifetime students have been taught that cultivations control pests. Winter diggers tell me that exposing the soil to the elements reduces pests!

When I read on some of those fascinating vegetable blogs written by diggers, I often see concerns expressed about soil-living pests. I cannot help thinking that constantly cultivating the ground and perhaps working in organic materials and expensive garden centre potions actually creates the excessively aerated conditions where grubs thrive. I have no evidence for this and it might be a foolish notion. Perhaps it’s just that I don’t dig and don’t see them! My instincts tell me that where the natural balance in the soil is undisturbed by cultivation, pests like symphilids, springtails, millipedes and woodlice are less of a problem. A complication to my argument is that sometimes springtails, millipedes and earwigs are sometimes beneficial predators!

Don’t get me wrong, I have my fair share of sick plants. Only yesterday I cut down my plum tree as it had acquired silver leaf disease (I was actually pleased to have an excuse to give Brenda for me to get rid of it). My delphiniums this year are pathetic (all that leaching of nutrients with all that rain last year). Shame on me, I even have some glyphosate damage, my favourite Aster amellus and Aster nove-anglae are so sensitive to drift.

Silver leaf on plum, early signs. The silvering is due to fungal toxins ascending from the trunk.

Silver leaf fungal infection in the trunk eventually kills the tree. 

Glyphosate damage on phlox. I hang my head in shame, I have been very careless with the ‘roundup’ in my cemetery garden.
Never-the less when I survey my gardens I see thousands of healthy plants. I am of course completely blind to minor blemishes, cuckoo spit and small aphid infections. I know  my brassicas will be covered in cabbage white fly in August and I might have to abandon my carrots to carrot fly later in the season. None of my vegetables would pass supermarket standards but they do taste good!

But yes, I do claim that most of my plants are healthy. I believe plant health to be the norm if plants are well grown. I have used no insecticides or fungicides so far this year. I do confess to a few slug pellets under protective fleece which is there to prevent the collared doves and pigeons from decimating my young brassicas.
(My post on slugs suggests my minimum cultivation methods which involve leaving plant debris on the soil surface contribute to minimizing the damage of slugs and snails. Contrary to the popular view).

So why do I think that minimum cultivation helps to grow healthy plants?
The essential thesis is that if plants are grown well they will be less likely to succumb to pest and disease. To me this is a firmly accepted principle in pest control. Of course there are many situations where either because of pathogen virulence and/or plant susceptibility, no matter how well you look after your plants they might become infected with pest or disease.

For over a year now I have hammered away at why minimum cultivation grows healthy plants. All these matters are inter-related
  • Soil organic matter is elevated
  • Soil density is reduced by spaces and pores. Soil structure is improved.
  • Soil aeration is improved.
  • Drainage is improved and rainfall penetrates into the soil rapidly.
  • Roots grow more deeply.
  • Walking on wet soil is less damaging than feet compacting cultivated ground.
  • Plant roots and dormant vegetative structures are undisturbed by cultivation.
  • It easier to control weeds. Some weeds may be hosts for pest and disease, on the other hand they might support beneficial predators and parasites!
  • There is more biological activity in the soil.
  • Worm populations are enhanced. Apart from contributing to the fertility listed above harmful organisms are digested in the worms’ gut, worms’ coelomic secretions have anti bacterial and nematicidal properties and worm-casts enhance nutrient availability. My post on worms
  • Beneficial fungi are increased. Apart from  mycorrhiza that aid nutrient uptake, nutrient sharing and drought tolerance, other species of fungi kill nematode pests (eelworms). It his fascinating how some fungi ‘lasso’ and digest their prey.
I would have liked to show you some pictures of soil grubs from my garden but could not find any!
This seems to be a popular post, if you found it of interest you might like this one

11 comments:

  1. Is silver leaf an airborne fungus? What can we do to protect from it. We have three plums and two gages on the plot and I would hate to lose them. Looks like this year we will have a bumper crop.

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    1. Airborne spores Sue, but it's not that common. I have never had it before and the only other time I have seen it in real life is as sporing brackets on some old fencing posts!
      The common idea is to do the pruning in summer or autumn when cuts quickly heal. Not in winter. Some say use tree paint on cuts, I say a waste of time.

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    2. I only chop bits off during summer (sorry I mean prune). Our problem with peaches and nectarines is peach leaf curl!

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  2. Hi Roger! I experiment with dig and no dig and the beds that I 'no dig' have a better consistency but I can't truly tell if I have better crops there or less pests, but I have noticed the change in soil texture and that's good for me! My heavy clay has definitely improved and I would recommend no dig to anyone who needs to improve their soil too.

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    1. Good to hear this Anna! Good soil translates to healthy plants!

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  3. Sorry to read you had to remove your plum tree Roger - I hope Brenda wasn't too upset. I had to remove a few mature Rhododendrons earlier this year as Cushion Scale was affecting them but only had myself to convince it was for the best!
    I commented on a blog of yours earlier in the year that I would turn part of my garden over to 'no dig' so far, so good! I hope I'm in a better position next year to comment on my findings.

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    1. I look forward to hearing about your experience Angie
      I will be blogging sometime about the wisdom of eliminating nasty pests like scale and mealy bug by the simple expedient of chucking out the host!

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  4. Hi Roger, I'm a friend of Sue's from GLA and have been meaning to say hello for ages.

    I don't dig. Period. I can't - my back and a damaged hip won't allow it. We've been creating a new garden and once a bed is prepared it isn't getting touched again. Preparation consists of (someone else) digging over the soil and removing perennial weeds. On top of this we add copious amounts of rotted pony poo, autumn leaves (they can rot to leaf mould in situ), a bit of top soil, old bark chip from the paths and then we leave things alone.

    During the recent heatwave the only plants which have needed water are new perennials which had been in the ground just a few weeks. Everything else was fine and on examination the soil had plenty of moisture from an inch or so down. Perhaps another reason for your list is 'ground which has not constantly been messed around with holds moisture better and enables plants to tolerate extreme conditions' ?

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    1. Got to know your namesake 'Baggins' very well when I read The Hobbit and later 'Lord of the Rings' to my son when he was ten, a long time ago! it took six months!
      Sounds as if you have a great water conserving mulch with all that organic matter, I like your phrase 'ground not messed around with!
      Hope your initial digger got rid of ALL the perennial weed!
      Look forward to saying hello again.
      Roger

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    2. Have just popped over to your own blog, I will be visiting again. I had a little difficulty at first - not being very clever with these things and going to your older blog rather than 'View from Bag End'.

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    3. Roger, thanks for the heads-up that I needed to update my profile ... have done so now :}

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