Saturday 12 April 2014

Garden myths discussed: do you need to clean dirty pots?

I also make special reference to damping off disease.

Blogger Sue Garrett recently challenged fellow bloggers to publish pictures of less flattering parts of their garden. If you believe this picture was specially staged you are wrong!

I try in my blog to provide correct factual information. When I fail to do so please let me know. Inevitably in some cases there is a degree of speculation and opinion. You might decide that is the situation today. My scientist friend Peter Williams agrees with the thrust of my submission  and agrees dirty pots are essentially the same with regard to plant health as putting new plants harmlessly into outdoor  soil.  He is adamant however that he will continue cleaning his pots although I don’t think he actually goes so far as washing them.

I am indebted to blogger Rick Nelson for alerting me to this myth. He empathised with me when I  previously described washing clay pots outside on a cold winter morning when I worked at Hartlepool Parks and Recreation Department. In those days pots were scrubbed to remove algae and lime. New pots were also soaked and washed to remove salts left from manufacture. Attitudes change, and I remember displaying Dicentra cucullaria in a large clay pot at the Harrogate Spring Flower Show. The dirty earthenware pot was covered with white lime and algal green. It was a star.

It is now fifty years since I washed a pot. I cannot remember even giving a pot or container as much as a light dry clean. Most, but not all, of my pots are plastic and have been reused numerous times. I do recommend  to gardeners to have a store of pots and sturdy seed trays to re-use several times rather than buying new fiddly flimsy containers and modules.
I had thought not cleaning my pots and seed trays was part of my slovenly nature. I am grateful to Rick for informing me that I am not unique.

Ammi was all over the place at a rather hyped flower show last year. I have succumbed to fashion.
Plant health is an essential part of plant management. There will be rare occasions when a dirty  plant pot harbours pests such as root aphid or root mealy bug. Heaven forbid, vine weevil larvae may lurk in some peaty compost. You might have even grown cabbages with clubroot . Perhaps your plants were dripping with red spider mite and some have gone into hibernation round the rim of your pot but I doubt it. In none of these events would I clean the pot. I would go further and throw them away.

You may be worried about the fungal fungus disease of seedlings called damping off when the contents of an infected seed-tray keels over and dies. This disease is associated with dirty conditions and unsterilised soil. I have unconventional views about this very important and ubiquitous disease.

We used to teach our students that damping-off was an infectious disease and not merely a disorder caused by bad management. We used to emphasise hygiene, clean pots, clean irrigation water, no drips from the glass and to always use sterile or sterilised compost. The principle was if you avoid the infection it will be impossible to get the disease. This is the conventional view. It is successfully achieved in commercial horticulture where potent chemical help is at hand.

For most of we ordinary gardeners the water carried spores of this disease are almost everywhere - albeit not in the air. In spite of this I think that the disease will not be a problem if we grow our plants well. I argue that It is impracticable to avoid infection by such things as washing pots, the disease will find another way in. The significant factor is that only If the plant is in a susceptible condition will it succumb to the disease.

Gardeners can do dreadful things to their plants. They sow them too early with excessive artificial heat when there is inadequate light to support them. Some even imagine that small plastic mini greenhouse-like structures standing on a shelf or only slightly better, on a  light windowsill in the house, are somehow suitable for seed-propagating plants. Gardeners are advised to cover their seedlings with a sheet of glass, all that humidity, no wonder they get drawn and keel over with rot!
The worst mischief is poor watering, This is a real problem because to the inexperienced gardener it is easy to go wrong. Knowing how much and how often to water is not always obvious. Too much water and high humidity are the conditions where damping-off thrives.

It is not always practicable for certain difficult plants but for all of the many plants I personally grow from seed the germinating seedlings are fully open to the air and I do not cover them. Very high humidity can be fatal. I thoroughly water at first sowing and for very small seed actually water the compost before scattering the seed. For other than very fine seed like antirrhinum I like the compost surface to get quite dry before I thoroughly water again. Damping off really does thrive when it is wet and humid. There will be some infrequent occasions where for very small seed a compost is dry at the surface and yet wet below. In these circumstances a light watering rather than a thorough soak may be in order but not every day!

Some gardeners do not realise that very thin layers of compost lie very wet after drainage and damping off may be more likely to occur.

There are two kinds of person when it comes to our own domestic hygiene. Some are constantly cleaning and every surface is regularly swabbed with some useless anti-bacterial product. Any food falling on the floor goes straight in the bin. Others amongst us are more cavalier and like to keep our immune systems primed. Some people wash pots and others don’t.

Healthy Corydalis lutea seedlings have ‘volunteered’ in my seed tray of recycled ‘compost’. I have now weeded them out and the germinating seedlings of Dictamnus fraxinella are now doing very well.

Superb dwarf Lilium formosanum pricei was sown six months ago in my unheated greenhouse. I will have hundreds of dwarf lilies flowering next year.

Sown two years ago, fresh seed of Fritillaria meleagris  germinated last February and were grown-on undisturbed in their tray. They are sprouting again. 
I have now patched them out on the village plot. The inevitable liverwort has not done any harm.

Addendum, do we have another myth?

Since penning this post six weeks ago I have agonised as to why our students when raising plants in college greenhouses for their plot projects almost always suffered from damping off! (The plants not the students). I have not personally seen the disease since I ‘retired’ from college more than twenty years ago and as this post makes clear my conventional plant hygiene is appalling! (I will however explain in a future post that in some aspects of plant hygiene how I am extremely strict). You will have gathered by now that not only do I not wash pots, I also recycle compost and soil!

I have now remembered that the students, who I hasten to add, were not under my charge, almost invariably covered their glasshouse sown seeds with polythene or glass. I have now seen the light and am certain that covering was the cause of their problem. Instructors ensured the students exerted the most extreme hygiene you could imagine yet the seedlings constantly keeled over.

Plants sown outside in the garden, either by the gardener or nature are almost never covered by anything but soil. You (normally) never get damping off outside. It suddenly dawns on me that covering glasshouse seed with plastic or glass is normal practice for many gardeners. I had somehow cut myself off from such technique and perhaps I have even regarded my own methods as rather more primitive. I now notice in magazines and gardening blogs people envelope seeds with plastic covers much of the time. How strange, I even spotted someone covering brassica seed with a glass sheet!

As is my wont to take things to extremes, for the last six weeks I have tried to get my uncovered seed and seedlings to succumb to damping off. I have watered some more often than normal, some with heavy drenches others with daily light trickles. As usual I frequently fail to find find my rose. Perforce they were unwatered for the five days when we were away. The only watering sin I could not bring myself to do was to leave my seed trays standing in water all day! 
Not a seedling has died and I have got healthy seedlings aplenty. 

Cathi came round last night and pointed out  that I was not a proper scientist and I did not even have the experimental control of actually covering any of my seed!  Shame on me but this would have been a bridge too far!


  1. Great post Roger, I am sure you will have many differing opinions/responses. I am a pot-washer. Do I need a 12-step programme?

    Seriously, I wash all pots before I put them away because, when I go to use them again, I like picking up a clean pot. Washing means the little cupboard I store pots in does not get dusty and muddy and with clean & dry pots, I've never found slugs lurking in them before re-use.

    1. No there are only ten!

      I dread to think what Brenda would say if I brought mine inside!
      I completely understand your sensible approach Jayne. I suppose I am handling my pots a great deal at this time of year. I am not very good at clearing them away and they tend to stay lying around until Brenda shouts at me or more often tidies them up in desperation. She has not volunteered to clean them!

  2. 50 years since you washed a pot was that child labour you would have only been a toddler!

    We don't suffer by washing pots but we do have a dry brush which we brush out pots with if they have crusty rings or left over compost in the bottom. This is usually just before we use them again.

  3. I am not a pot washer or cleaner too but to keep it tidy I stack and sort the pots under the sowing table. From clay pots I remove the algae in spring.

    1. It is fascinating to learn about everyone's gardening secrets, Janneke

  4. I am a pot-brusher, not a pot-washer. I re-use my pots and seed-trays again and again, and very seldom experience any damping-off. I actually LIKE the white deposits on an old terracotta pot. It somehow makes it look more natural.

    1. I completely agree with you about the white deposit! And thanks for mentioning your experience of not having a damping off!

  5. I will use a power hose to wash soil splash from my glazed ornamental planters, but that is the extent of my cleaning. What is more, all of my spent compost goes onto my compost heap, and my seed and potting compost comprises 1 part molehill soil (yes, I have lots of molehills), 1 part new peat based multi purpose compost and 1 part sieved (well rotted) compost, together with a sprinkle of ground limestone (or flowers of sulphur if it's for ericaceous use). No sterilising anything, no washing and no brushing. Apart from a cursory inspection for slugs or snails, anything that gets stuck in the pot from last use is food for this planting to utilise.

    My growing is guided by one principle - "What is good for my worms is good for the health of my growing media, and what is good for my media is good for plant root health, and healthy roots means healthy plants"

    Based on this, I looked at adding earthworms from my very healthy compost heap (nettles that invade my compost heaps will easily grow to 6ft tall) to my 3" seed pots. To my surprise, the worms took the very first opportunity to leave, usually on the first night, this despite the fact that I knew that the worms were quite happy to set up home in a large pile of my premixed compost or in 10" pots.

    I used to plant seed into 3" pots and stand the pots in plastic trays. When I looked closely at the media in the pots, I noticed the waterlogged bottom layer you have also mentioned Roger. The bottom inch was waterlogged, the top inch quickly dried out and this left only the middle inch suitable for the worms - no wonder they left sharpish.

    To get rid of the waterlogged bottom layer, I took the pots out of trays and stood them on capillary matting and immediately the worms were happy to set up home in the 3" pots.

    Oh, and I don't get any damping off even if I water from the top with dirty old rain butt water. So I am with you on this one Roger - here is to good old dirty, healthy, plant pots.

  6. One experiment worth noting though, I have a large Vermicomposter outside my back door and every few months it produces a tray of rich worm casts. I reasoned that adding the worm casts to my seed trenches would pep up the good bacteria in the soil and get my seedlings off to a good strong start - but there was a bigger picture I was not aware of...

    For the experiment, I planted two rows of carrots and into the bottom of the first drill, I sprinkled a small quantity of worm casts. Sowed the carrot seeds, covered and tamped down.

    But ! The mole homed in on the worm casts like an exorcett missile, hunting for worms. Not finding any, but certain they were there, it must have gone up and down that row a dozen times, scattering the planting all over the place.

    My lesson - don't put worm casts in the bottom of seed drills.

    1. I love reading your rambles Mr Mole-Hilly.
      I suppose it’s some kind of definition of an optimist when he looks at his lovely lawn covered with molehills and says with delight “ oh look at all that friable compost material which the mole has kindly brought to the surface and he has thoughtfully picked out all the grubs!”
      i mulch mow my lawn and the sandy soil is therefore highly organic and is very suitable for making up seed and potting composts. This does help to assuage my anger somewhat when I suffer mole visits. They generally deposit the soil in the edges of my lawn, thank goodness and I can collect it for compost. Where it is completely on the flower borders I just kick it level - it even gives the illusion that I am a digger!

      Where a soil is for example a heavy clay it is an illusion that it has suddenly been converted into a suitable ingredient for composts. Moles don’t make silk purses out of sow’s ears! Other than that proviso, I fully approve of using mole soil for compost. I swear that my lawn is lowering and the borders rising as the years pass!
      Although I do not see any advantage of introducing worms into potting composts, I do not fear the inevitable invasion of earthworms into pots as some gardeners do. Having said that, one of my very large display tubs must have had enough worms in it to entice a mole visit a week ago. There was a mole hill at the top of my pot!

  7. Many years ago, I did an evening class course on 'A' Level Biology. It was a memorable course because one of the 'students' was a gentleman well into his eighties, his claim to fame was that he was Norfolk's last mole catcher. His tales were amazing. As a young man, moles were prolific and he would catch hundreds which were in great demand by wealthy farmers for moleskin trousers. He told us though that in more recent years, moles had become so scarce, that he would catch them alive and release them where they would not get themselves into trouble.

    He told us of how he would hunt the moles, standing quiet and listening and watching for mole activity, then pouncing to squish the tunnel behind the mole, then with a deft flick of the hand he would spill the mole out onto the surface where he could quickly scoop it into a can before it dived back into the land.

    But getting back to your mole in your display tub. He told us that in his career, he had come across two pure white moles which he used to keep in two very large pots, one either side of his back door. When he sat by his door, they would both come out onto the top for food. Apparently they were both tame and lived happily in their pots for many years.

    During that unique course, I not only learnt some Biology, but I also learnt a lot about a country life that had long since changed, but that could in part be recaptured if we are prepared to allow nature to integrate into our gardens. Wildlife enchants me and lives in abundance in my garden, but my garden will never win me any show prizes and earns me the scorn of my neighbours for the 'rubbish' I encourage in order to ensure food and shelter for what I consider to be my real prize - a garden full of birdsong and butterflies, stripey snails and bats and occasionally a Barn Owl gliding silently through the evening twilight (and of course the moles).

  8. Life's too short to wash a pot.

  9. I am pleased to have suggested a subject for you to explore in your excellent series of myth discussions. We are not alone when it comes to not washing pots as I have noticed recently that some of the gardening press are taking a similar line and you never know this could lead to a trend. I can see it now; buy your pre-soiled plant pots here, packs of ten, specially treated.
    More importantly one of my favourite books by someone I regard as a true authority is Peter Thompson's Creative Propagation in which he describes pot washing as "an unnecessary and time-wasting activity" and goes on to say that any losses which may possibly be incurred are minimal when balanced against the time saved which can be used for further propagation. He also recommends that if the gardener is particularly concerned over what may be termed a "high risk" plant then use a new plastic pot.
    On the question of damping off, I agree wholeheartedly. Seedlings in the wild do not have their own little plastic bag to protect them, likewise when we try to emulate the conditions for germination the emerging seedlings should have a large volume of free air around them. I have used mini-propagaters (1/4 tray with dome) but only to maintain a damp surface to the sowing medium which interprets as saving them from neglect. As soon as germination takes place the covers come off and the seedlings have to fend for themselves. I should point out that this is only done with half-hardy species, all my hardy stuff, which is by far the majority, is cold-sown.
    Another thing for you to muse over, which is also mentioned in the book although I have done it for years is to use an innoculum of used compost in your mix to build up immunity, which is probably as effective as many of the preparations available on the market.

  10. I always find it reassuring after my wilder posts to have some confirmation from old hands such as yourself Rick! I think most of my readers are too polite to tell me when they think I am (talking a lot of rubbish).
    Thank you for the mention of inoculation with old compost. I stepped back in this post considering the fact that unsterile soil might be a good thing, although I did mention recently in the comment column, tongue in cheek, about reusing old cyclamen compost because it would probably contain mycorrhiza. In a future post I intend to explore this issue but chickened out this time!
    You might not be writing to support my next myth Rick which will be about bonemeal!


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