Beat Pests and Diseases By Spring Cleaning Your Greenhouse

, written by Jeremy Dore gb flag


A greenhouse is a wonderful addition to almost any garden.  It enables you to grow tender plants earlier, ripen crops that need more warmth and keep harvesting later in the season.  Unlike other garden protection such as cloches and row covers which need removing whenever you want to weed or water, greenhouses make tending your plants much easier.  However, it’s not just plants that enjoy the protection of a greenhouse – pests and diseases love them too, avoiding the harsh weather and frosts that usually kill them off!  So how do you stop them from enjoying a sheltered existence where they can strike at your new sown plants?  The answer is a good spring clean...

Cleaning has never really been my strong point (as my wife would gladly tell you) but when it comes to the greenhouse it is an essential part of the gardening year.  Once Christmas is out of the way and days start to lengthen, cleaning the greenhouse is a great way to feel connected with the garden again, even if the temperatures are too low to be sowing seeds.

Jeremy in his greenhouse

So what are the pests and diseases that try to invade the greenhouse?

  • Slugs: The number one pest for gardeners, they love to hibernate in hidden cracks, under old pots and any damp sheltered places.  As soon as Spring arrives you can be sure that young seedlings will be their favourite snack if they are left.
  • Aphids: These lay eggs before the winter becomes harsh, often on existing plants.  So it is important to remove all old plant material, inspect the underside of leaves of overwintering plants and check that there aren’t any weeds or leaves under benching.  Aphids are one of the main ways that viruses are transmitted between plants.
  • Red spider mite:  The hot dry conditions in greenhouses over summer are ideal for red spider mite which multiply rapidly, reducing the crop of many plants.  The tiny red female insects can overwinter on greenhouse walls and pots so it is important to clean them out thoroughly.
  • Mealy bug: Colonies of mealy bugs resemble blobs of sticky cotton wool and are largely a greenhouse pest, sucking the sap from plants like aphids.  They can often be found under the rim of pots or similar places, which need to be cleaned thoroughly.
  • Mildew:  Good hard frosts will usually kill off mildews but they can survive on old plant material, particularly the leaves of cucurbits such as cucumbers.
  • Botrytis (Grey Mould): This fungal infection can attack the foliage of plants left in damp conditions over winter.  It is important to fully ventilate the greenhouse and remove all dead plant material to prevent this.
  • Blight: The spores of blight infection can easily spread to tomatoes and potatoes from plant debris left in the greenhouse or garden, so it must be removed and disposed of or burnt.
Pots outside a greenhouse in winter

The solution to eradicating these potential problems is to do a deep clean of the greenhouse before the new sowing season begins.  I usually follow these steps:

  1. Remove everything from the greenhouse.  Slugs can hide anywhere and the best way to find them is to take out any removable benching, pots, tools etc so that every corner can be reached.  All old plant material should also be removed to the compost heap, or burnt if it suffered any diseases last season.   This year I found at least 15 large slugs hiding in the gap of a breeze-block which was propping up my bench.  Needless to say, they were removed far away!  I also leave out a beer filled slug-trap for a few weeks before sowing new plants as an extra precaution and always start the most tender plants off in pots on a bench.
  2. Clean the glass.  Light levels in the UK are usually poor around early spring and plants need as much sunlight as possible if they are to grow well without becoming straggly.  It’s amazing just how much algae can build up in the warm moist conditions, so a good clean inside and out with warm water and a sponge is essential.  Some people like to add some disinfectant but if you do, make sure that it is based on natural ingredients and is suitable for use near plants – many garden catalogs sell such products.   Use an old plastic plant label to reach the grime that forms between glass pains.  [Don’t forget to switch off any electrical supply before you start using water!]
  3. Scrub the surfaces: Again, all traces of mildews, grime and pests must be removed from any staging where your plants will be placed.  Any capillary matting should usually be replaced.  The greenhouse should then be well ventilated to allow everywhere to dry.
  4. Clean Pots and Trays: Emptying out all compost, I usually leave these a few days to dry out so that most of the dirt can be ‘dusted out’ before rinsing them.
  5. Replenish Soil: If, like me, you grow tomatoes in the same greenhouse bed every year then it is all too easy for the soil to become depleted of nutrients or diseases, such as blight, to persist.  The best solution is to dig out some of the spent compost each year and replace it with a fresh supply.  I usually cover this with a layer of sterilised potting compost/soil to keep weeds at bay.

Many people heat their greenhouses and over-winter tender plants in them.  In this case you will need to wait until the milder weather in order to do a cleanout, or find somewhere sheltered for the plants to stay while you work.  However, I like to completely empty the greenhouse and give it a few weeks of frosty ventilation to finally sterilize everything.  It’s not the most enthralling task in the gardening year but there is a certain satisfaction in knowing that the greenhouse, my operations centre for the new season’s growth, is well prepared for the coming year.  If only I could get such satisfaction from indoor cleaning...!

Bugs, Beneficial Insects and Plant Diseases

< All Guides

Garden Planning Apps

If you need help designing your vegetable garden, try our Vegetable Garden Planner.
Garden Planning Apps and Software

Vegetable Garden Pest Warnings

Want to Receive Alerts When Pests are Heading Your Way?

If you've seen any pests or beneficial insects in your garden in the past few days please report them to The Big Bug Hunt and help create a warning system to alert you when bugs are heading your way.

Show Comments


"I found your explanation of the needs and methods of cleaning a home greenhouse downright inspiring. As I look out the window onto the two feet of compact snow and ice over the gardens it's encouraging to think about the coming year of garden activities. "
Clarice McKenney on Friday 16 January 2009
"I have chickens which were allowed to mooch in my new greenhouse. The dug around in the soil and found lots of creepy crawlies to munch on... My other greenhouse is heated and full of overwintering plants so they werent allowed in there. I want to start planing seeds now which ones can i start now..... I have already planted a few tom seeds but really dont want to be wasting seeds and time if it really is too early... i have a conservatory for raising them."
Sue on Wednesday 28 January 2009
"Chickens sound like just the ticket because they're natural predators that would eliminate "creepy crawlies" in the greenhouse. - Clarice"
Clarice McKenney on Friday 30 January 2009
"what do you use to clean a greenhouse after tomato blight?"
j. budden on Wednesday 2 September 2009
"I would use a product such as Citrox which is a disinfectant made from extract of citrus fruits and is suitable for greenhouses and garden uses. It is meant to be good for clearing fungal diseases, so it should be effective against blight. However, remember that blight spores are mostly spread from old diseased plant material, so removing that and any residue on the top of the soil should be the priority."
Jeremy Dore on Thursday 3 September 2009
"Very helpfull. I have just had my first greenhouse season and all direction is much appreciated. I have a gravel bed in the greenhouse - should I sprinkle Jeyes fluid over it?"
James Wilson on Sunday 3 January 2010
"as a novice gardener, these tips and comments are immensely helpful. Presently my greenhouse only harbours a lot of junk and I am inspired to begin clearing my greenhouse this weekend. Is it alright to jet wash the greenhouse in extremely cold conditions? We've had a lot of snow this Jan and the garden is covered by a white blanket. "
jacklyn on Thursday 7 January 2010
"Depends on the material used to construct the greenhouse as to whether jet washing would be good for it. Our 8' by 10' greenhouse is made of fiberglas imbedded with reflective fibers that prevent shadows on plants. It would survive just fine, I think. Unless a glass house is made of tempered glass, couldn't a jet blast of water during extreme cold shatter it? Could such a stress also crack clear plastics?"
Clarice McKenney on Thursday 7 January 2010
"I didn't think of that. Thanks. My greenhouse is old, so will respect its age and not subject it to more torture. I think I'll just wait for the big freeze to abate. It's too cold at the present. "
jacklyn on Friday 8 January 2010
"James, I would be hesitant about using Jayes fluid as this is now not recommended for use with edible plants. There are some natural alternatives such as Citrox which would be better for disinfecting the gravel bed, though I can't say whether they are as effective."
Jeremy Dore on Wednesday 13 January 2010
"Unusually, I am a cat loving gardener. Please, if you do use Jeyes fluid, make sure that cats do not have acccess to the area, as phenols are extremely toxic to them."
Geg Williams on Saturday 16 January 2010
"CATS Hmmmmmm ! I am a cat lover but as far as lotties and gardens are concerned, I feel that they NEED boundaries ! There is nothing more off putting that to go happily gardening barefoot, and find a cAT-ASS-TROPHY squelching up between your toes ! I have been 'oop t'lotty this morning, and the greenhouse is green with slime on the panes. Next month, when the weather is more benign, I shall clear out the greenhouse, clear out everything, and scrub, scrub everything, as bio friendly as possible,, having left the greenhouse open to the elements all over this freeze time. A glorious time this, for reclaiming the patch for another season of producing, and trying out new methods and promising oneself to be more strict with labeling of pots and rows of seeds ! I have NEVER succeeded yet, to fulfill this resolution ! "
PETER BALLAN on Sunday 17 January 2010
"A big thank you to Jeremy for his wonderful blog and to all my fellow garderers commenting on this blog. Following all your tips, I have now managed to completely wash and disinfect my greenhouse on Sunday. Everything has been cleaned, all spiders and creepy crawlies ejected out and pots and containers washed and dried over the weekend. Also bought my fresh stock of seeds and compost in preparation for Spring. Now, I wait and watch for good weather to show up to innaugurate my backyard green initiative. Thanks again everyone. "
jacklyns on Monday 18 January 2010
"Thank you for sharing your knowledge. The help and guidance is much appreciated. "
James Wilson on Monday 18 January 2010
"I really found the advice here helpful and want to get on with cleaning the Greenhouse this week if possible. I wonder if anyone has any advice about "sulphur" candles - Ive seen recomendations for these to get rid of all bugs in the greenhouse. But looking at the above I wonder if it is really necessary? and if it is should it be done before or after the cleaning?"
Crofty on Sunday 7 February 2010
"Personally, I wouldn't use a sulphur candle as I think normal cleaning is adequate (they are normally burnt after cleaning the greenhouse). Sulphur candles produce Sulphur Dioxide which is one of the constituents of acid rain - in other words it reacts with any moisture in your greenhouse to produce sulphuric acid, killing pretty much anything (so you have to remove any plants apart from dormant vines). Although considered safe if all people and pets are kept away, I just think it's overkill if you follow normal greenhouse good practice - good cleaning, good ventilation and adequate humidity in hot periods."
Jeremy Dore on Monday 8 February 2010
"Thankyou Jeremy for that. I think I will just clean it well, like your article says above. Can't wait for the better weather though, as its sowing again a little today!!! I think this is a great site and am glad I found it. Thanks again"
Crofty on Monday 8 February 2010
"Yesterday was a beautiful day (almost 40 degrees Fahrenheit), so I raked some of the soggy leaves that fell extra late last fall. I inspected my four composters and constructed my little portable greenhouse in a bright southeast corner of the house and cleaned and prepared the work benches in my basement and tested the grow lights over them. I also organized my seed packets and located the little seedling trays I washed last summer (they're in the fiberglass greenhouse with all my oversized pots and decorative pieces for the gardens. I'm taking a 60-mile trip to our nearest big city today to buy large bags of seed starting soil. Next weekend, Lord willing, I'll be planting the trays. Like all of you, I'm itching to start. Here in northernmost Idaho of the US we've had a real weird winter. Usually we're a few feet deep in snow much of it, but this year we're hurting for snow here and in the mountains around us. Come spring we won't be seeing the old road washouts, at least, but summer will be mighty dry and expensive to water our gardens, I fear."
Clarice McKenney on Monday 8 February 2010
"Please can you tell me how to burn a suphar candle. I have lit the blue paper and the sulphar is still in tact. "
dawn rees on Monday 12 April 2010
"Just the right amount of info, thanks"
Peter Grice on Saturday 9 October 2010
"Very interesting info about greenhouse cleaning. I had late blight on my toms last year. Will cleaning with Citrox be sufficient. (I will be using all new grow bags for this season.)"
Christine Montrose on Wednesday 19 January 2011
"After the unusually harsh winter, the greenhouse has taken a battering with broken panes and sliding door that barely slides now. I have allocated time to go up t'lotty, to clean and scrub, and go into a reckless mode of renewing old pots, seed trays, and clearing out old seed packets that remain unused after several years. Replacing cracked or disintegrated pans can be quite therapeutic, with a good glass cutter, good gloves and professorial safety specs. I can only hope that the blueberry bushes I planted out last October, have survived the winter. They don't look too joyful at present, but will buck up later on in the season. Northumberland can be unforgiving on a north sloping face, but this only means that we are about a month behind the rest of the country, and possibly three weeks shorter growing season. The soggy ground will soon dry out to entice me in to thinking that I can start sowing a month before it is wise to. Listen to the old fellers and when THEY start digging and sowing !"
Pete Ballan on Wednesday 19 January 2011
"Christine, citrox should help remove the blight spores but the main problem will be any tomato seeds in the soil or surviving plant matter from the blight-infected tomatoes. See our article on blight for details:"
Jeremy Dore on Wednesday 19 January 2011
"Regarding immaculate greenhouse care, I find that actually REMOVING the panes and then scratching out the moss from the frame with a stiff plastic seed label is oft the best way of getting getting resistant gunk out of the less accessible nooks and crannies. I like the hiring outside labour in to get the grubbies out of the soil, ie, hens. I think I'll look into this option, providing the Duke of Northumberland doesn't object to 'wildlife' on his allotment."
Pete Ballan on Wednesday 19 January 2011
"I have recently become the proud owner of a green house so all help is apreciated I intend to install a heater probably electric Any advice regarding which heater to buy and how warm a green house should be kept at would be helpful. "
Mary James on Sunday 27 February 2011
"Any tips for Non Glass green house purchased from B / Q, What type of cleaner would be safe so as not to react with the moulded plastic"
Peter Grice on Monday 28 February 2011
"Jeremy thanks for all the great info on cleaning your greenhouse. It is still too cold here but in the next month or so with my pail and cleaning articles, I'll be getting started."
Pam Jessup on Monday 28 February 2011
"Peter - most non-glass greenhouses use Polycarbonate and that may have a UV coating on the outside to extend it's life. You have to be very careful when cleaning this - generally soap or mild washing up liquid mixed in water (something plant derived like Ecover is probably a good idea). There's some more tips here although the products are for the US:"
Jeremy Dore on Tuesday 1 March 2011
"Thanks for this fine article on cleaning the greenhouse it answered all my questions on one page keep up the good work it realy is helpful for the new gardener"
Thomas Dolan on Monday 7 November 2011
"Verily, Ecover is a good cleaner, but I have found that vinegar, a 1/2 cupful in a bucket of luke-warm water, with the aid of a dish washer brush and a sponge type soft scourererer (!) is a pleasant way to spend a Saturday or Sunday mid day at this time of the year. One needs a convenient place to put the panes to drain dry, have all the 'W' clips ready, a safe step ladder, and a still day. A good wife or even a friend is also an acceptable accessory !"
Pete Ballan on Monday 7 November 2011
"My father had a severe White fly infestation on his tomatoes last year. He would like to disinfect his greenhouse with a smoke bomb of some sort, any suggestions? Can you get hold of them, and are they safe?"
Christine Montrose on Sunday 22 January 2012
"I have had my glass greenhouse 12 months I am a widow living on my own who is scard of spiders I would find it difficult to reach the higher part of the greenhouse and getting between the glass could I use a hosepipe or is there any other way to get to the high parts I just seem to be neglecting my greenhouse because of spiders my husband used to get rid of them for me"
Lynn Salisbury on Thursday 1 March 2012
"I too am a 80 yr old widow and just got my greenhouse. My father used to use sulfur to really, really clean our greenhouse out. I remember one has to be careful for humans and animals regarding the use of this. It is the best and easiest way to clean these things if one is careful. I never have much luck in lighting the wicks and think I might not have the correct paper to use. Any help with the kind of paper to use? Thanks. "
Dorothy on Tuesday 6 March 2012
"This winter has been bad for moss recovering on the greenhouse glass, and I shall have to wash it all down again. Perhaps some of this moss killer stuff in a dilute form will help keep it at bay. I shall find out this afternoon, as it is a beautiful sun-shiny day. By and large, the greenhouse is one of those jobs that it really is only necessary to do this chore once a year if yer lucky. The 'Wise One', my wife, thinks the use of a sulphur thingy might be good for the pests. I think it will be best to listen to her counsel. "
Pete Ballan on Wednesday 7 March 2012
"I know spiders are essential and I know that gardens are for creepy crawlies. Everyday I clean my greenhouse and everyday I have to hose it out all the webs are back. I spend all of my time hosing it down from the doorway as these spiders are super fast web spinners. So I just made a natural spider deterrent adding 5 tablespoons of lavender oil to one quart of water. Lavender is a great natural spider repellent, which is nontoxic to plants. Know more on houseplant care articles"
Shirley Cox on Wednesday 2 May 2012
"The using of the sulphur candles would only produce an eventual acid etching on the glass, I reckon. Washing down the glass panes with vinegar, or very dilute water and moss killer, carefully, so as not to splash the mix anywhere for growing seeds 'n' wotnot, is ok. I have come to a ritual of cleaning the glass at ANY time they look as tho' they need it. A little but often, as they say. She who must be obeyed, has bowed to my will, and says, 'Be it on my own head' ! For every gardener, and there are millions of us, there is a method that they use, and if it works for them, it is the right method."
Pete Ballan on Wednesday 2 May 2012
"I must try the idea by Shirley Cox I did not know lavender oil was a spider deterrent would you put it in a spray bottle and spray the glass"
Lynn Salisbury on Wednesday 2 May 2012
"To prevent infestations of white fly and other aphids I have very successfully grown a few Tagetes Lemon Gem plants in the greenhouse. This has proved a wholly successful deterrent over several years."
Brian McDowell on Monday 14 May 2012
"My greenhouse seems to be harbouring cucumber mosaic virus or at least something that makes the leaves of anything I put in there fern-like. I get it every year despite cleaning and scrubbing the whole thing. I've tried disinfectants and just soap and water and nothing seems to get rid of it. I don't have soil, the greenhouse is on slabs and everything is in pots with new compost every year. Every year my tomatoes are a wash-out. This year I bought some plants rather than growing from seed but after a couple of weeks in the greenhouse they are ferny. Any tips?"
Beccy on Sunday 10 June 2012
"Funny things 'green'houses. Why are they called 'green' houses, when they are clear glass ? ? They're certainly green , with slime and moss at certain times of the year ! I have just cleaned the panes yet again, of said slime, cleared moss out of the recesses for the glass, and all looks pretty good. This was done with vinegar and warmish water, a plastic label for the house frames, and a good stiff scrubbing brush to give a good finish. Monty Don would be proud of me ! So, leaving out a top pane from the end of the hoosie, for cooling purposes and insect in and eg-ress, I look forward to another season of rotting seeds, non- appearance of seeds, curly tomato plants, unexpected visitors in the seed trays, like carrots with toms, cabbages and broccoli, PANSIES (Where the hell did THAT one come from ?) and unidentifiable bean family wayfarers. It's like a boarding house for homeless seedlings here ! Nothing like a lucky dip lotty, and the Wise Woman, coming up behind and asking awkward questions. Read on and weep, ye great and good of Kew and Chelsea gardens. "
Pete Ballan on Sunday 10 June 2012
"A smoke bomb like Christine suggested sounds good to me. Can't think about washing slime and green stuff off windows. Besides, at my age Arthur-ritis in hands and back shrinks from the thought. Can't someone suggest a quick smoke or sulfur bomb helper with cleaning my greenhouse when the time comes this fall???? I finally bought some sulfur but can't find a proper wick. "
dottie on Monday 11 June 2012
"Do I need to sterelize the soil as well as deep clean this year was my worst ever with ants and bugs in the soil that seemed to eat the roots of my plants"
Linda Crane on Wednesday 3 October 2012
"what about field mice? they had all gone at the beginning of December having used humane traps. now I find evidence that they have returned. bubble wrap paper chewed and big holes in the soil surrounding fuchsia plants. Any ideas please?"
Sheila on Saturday 12 January 2013
"I'm trying a new method of growing the years garlic, in wee potties to start them orft. The garlic cloves from last year were small and wretched looking, butr starting to sprout on the open soil! We'll see what a glass house beginning is like, sheltered and reasonably clean paned. I find, on the whole, that greenhouse seeds are not much better than sowing seeds in the open, in situ where they are to be harvested ! Mind you, being on a North facing slope is a challenge to ANY 'lotmenteer. I keep reminding myself in times of stress and worry that, patience is the name of the game, and the participation is as much value as the result. One year is much the same as any other, with different results ! Whatever, the greenhouse is a good storage place for the pots, trays, 1/2 empty seed packets and odds 'n' sods. "
Peat Ballan on Monday 14 January 2013
"Says somebody above : "The using of the sulphur candles would only produce an eventual acid etching on the glass, I reckon. " Best go back to school then! Glass is etched with alkali, or hydroflourc acid (v v nasty stuff). Sulphuric acid won't touch it. "
connor on Thursday 1 August 2013
"Thank you Connor. Ah niver went to skool, loike. I have niver managed to get mesel' gassed or blown uppy like, yet, but sulphur is sulphur, and mixed with charcoal and potassium nitrate, kills the weeds when piled in small patches aroond the stems of weeds, and created a sterile crater where the once were an' geet columns of acrid smoke! Ah wiz gannin on aboot acid rain type effect, like, y'naw ?"
Peat on Saturday 3 August 2013
"Thank you for all the wonderful comments on greenhouse care. I am concerned about the soil on the floor of my greenhouse what can I use to "purify" the soil and get rid of the effects of blight(which I had on my tomatoes this year)should I dig out this soil and put in fresh soil or is there a treatment I can use?"
Terry Dixon on Friday 25 October 2013
"Thank you again for you comments and wisdom on cleaning and getting your greenhouse ready for the new season. My husband built me a greenhouse last summer, it has a straw floor and I will be cleaning that out and putting in fresh."
Pamela Jessup on Sunday 27 October 2013
"hi I have recently planted peas into a bed of compost in a poly tunnel, I have got white mould on the top surface. could you please help? cheers ron"
ron devere on Thursday 12 March 2015
"Sulfur candles produce sulfur dioxide gas , this gas when mixed with water molecules in the air will not form sulfuric acid as stated . Sulfurous acid is formed which is a disinfectant solution with a half life of a day before the water molecules in the air degrade the acid and neutralise it . Sulfuric acid is a different beast altogether and you do not want to be messing with that in a greenhouse - but sulfurous acid is ok as a disinfectant "
A Holmes on Monday 12 October 2015
"Why buy Citrox? Serious question. Check out the MSDS: "2. COMPOSITION AND INFORMATION ON COMPONENTS Synergised blend of Citric Acid, bioflavonoids, solubilized in a mixture of Glycerine and water. Active biocidal ingredient Citric Acid - CAS Number: 77-92-9" - see that? "Active biocidal ingredient Citric Acid" Just get some citric acid instead. Personally use sulphur candles made from flowers of sulphur (just common sulphur powder) thou lighting it reliably requires knowledge and effort. Most people haven't a clue how candles work however, so will never manage it. Moreover, they have a backward view of chemicals so wouldn't want to. They think sulphuric acid etches glass for instance (see above). And they'd sooner add to the particulate pollution with their 'natural' wood burning stoves - pollution that kills 50,000 people per year in the uk alone. Incidentally, another 'natural' product is the cigarette."
ranter on Saturday 21 October 2017

Add a Comment

Add your own thoughts on the subject of this article:
(If you have difficulty using this form, please use our Contact Form to send us your comment, along with the title of this article.)

(We won't display this on the website or use it for marketing)


(Please enter the code above to help prevent spam on this article)

By clicking 'Add Comment' you agree to our Terms and Conditions