Hi all, this site has been of great use to me over the last couple of years for research etc and I thought I might be able to contribute by starting a thread on greenhouse good practice, pests and pestilence. I’m not claiming to be an expert so hopefully some others can weigh in with the benefit of their experience too. My methods are organic.
NB – I am in the UK which equates to US zone 9 Cool Temperate Climate.
1. Get the Right Genetics! - Don’t be thinking that a greenhouse is like growing indoors under lights – you will be at the mercy of god’s great bulb and natural day length unless you have a method for blacking out your plants every day. Early finishing strains are very advisable, as are mould resistant ones. Finishing times in august are great, September is ok, October can be pushing your luck and November, even in a good year is on the edge of possibility.
I found the genetics I was looking for by research which led me to Canada. Danish genetics sound promising too. A few dutch strains like Passion #1 etc are reasonably early but not very potent. Sativas may do well as the buds are smaller but you may need a warmer climate than I have for them as they are late finishers usually.
2. Ventilation – this is very important in a greenhouse to stop it turning into an oven during the day and a mould factory during the night. The best way is to have as many roof vents as possible – ideally a quarter of the roof are should be vents and the very best way to control them is with an automatic opener – these are about 20 pounds each but well worth it as they may save your harvest. Also low-level vents are important too – then the cool air is drawn into these as the hot air rises out through the roof. Obviously the door can be used to ventilate the house and if you are buying a polytunnel – try and get one with a door at each end as this vastly improves ventilation.
3. Heat and Fans – the best way to heat a greenhouse is with electricity if you can get the power there, meaning you can also have fans too which really help. Any form of heat which involves burning paraffin or gas will create water vapour which will raise the humidity and encourage mould. Computer/12v fans can be used in conjunction with a car battery and one of those trickle charge solar panels that you put in the car to stop your battery going flat.
Other ways to heat the greenhouse without power are by putting anything in there that will store the day heat and keep the temps up during the night as the heat dissipates. Water butts are good and handy for watering too! Piles of old concrete slabs or bricks work too but make sure you are not creating a habitat for slugs and snails (more about them later).
4. Pots. Pot size is a good way to control plant size so bear in mind that a plant in a huge pot will grow huge and be ready to do some serious training unless you have a 3 story greenhouse! Also plants that are a little on the pot-bound side will stretch less and flower a tiny bit earlier. Pots are generally black and this can cause them to cook your roots in the sun unless you take precautions. Wrap the pots in something white or double pot them with some insulation inbetween. One useful dodge is to dig a hole and bury the pot in the soil if you have soil in your greenhouse as this also gives you more height. If there is any danger of the water table rising near to the surface of the ground then put a big pot or bowl with no holes into the ground and stand your pot in this (watch you don’t water too much as well and be prepared to remove the run-off somehow)
5. Pests. Too many to list here but I find many are kept under control by the ecosystem in an organic garden . I would say that enemy number 1 is SLUGS and SNAILS. There are many ways to control these but I find many of them such as beer traps may help but not solve the problem. Here are the 2 I find most effective.
Copper – slugs and snails will not crawl (for want of a better word!) over copper as it gives them an electric shock – it’s the molluscian equivalent of sucking a bettery ☺. Remember that this wont work if your plant is touching the glass or anything else that will provide the critters with a way round your copper barrier. i ripped up a hot water tank and made copper collars but you can buy copper tape from a garden centre.
Sacrificial Crops – grow some lettuce in your greenhouse – the slugs will eat this in preference to your special plants – if they leave any for you, go mad and make yourself a salad! (note - lettuce seeds do not germinate in hot temperatures so sow your seeds in cool weather and don’t keep them in the greenhouse till they are up and growing!) slugs also love basil. Another effective method is to go out after dark and catch them in the act but remember this may draw attention and that a torch = light = broken dark period.
SPIDER MITES may be a problem but neem oil will sort that.
6. Pestilence – MOULD/BOTRYITIS is your major concern in any cool temperate or humid climate. I reckon More outdoor buds are lost to mould every year than anything else (maybe rippers or 5-0 but lets not go there). Things that reduce the risk of mould are air -
circulation and ventilation, reducing humidity – ventilate during the day and try and stop the temps dropping too much and too quickly during the night whilst keeping a little ventilation too. Close the door before temps have dropped. This is where a thermostatic fan heater is your friend because as temperature drops, relative humidity rises and you get condensation which leads to mould. Fans on their own will help too – mould spores have to settle on your plants before they can grow.
There is some evidence that spraying/foliar feeding with seaweed stops mould spores from germinating.
Cleanliness is also important – remove any damaged or dead vegetation from the plants and floor.
Bud size and density also affect the chances of mould – the bigger and more dense the more chance of mould. Some strains are naturally more resistant than others too.
Overwatering and keeping the plants too wet will not help and neither will too much fertilizer. Water early in the day so you are not raising humidity in the evening. If you get mould on stems you can keep it at bay by wiping it off with a baby wipe every few days – try not to knock the stem and release the spores!.
Don’t let the plants get too congested and remove smaller lower branches to keep good air flow below the plant- better a greenhouse half full of mature buds than one crammed with mouldy ones!.
The later in the year you are trying to grow, the more the chances of mould increase. Don’t be scared to take bigger buds as they start to ripen. As soon as you start to get amber trichomes the chances of mould increase exponentially so get over the fact you may have to harvest early especially after the nights start drawing in. When you get that autumnal twang in the air your time is getting short!
Be vigilant and remove any buds that start looking icky. This vigilence must also extend to any other crops in your greenhouse – especially tomatoes!
7. When to plant? If you don’t want to be wrestling with your plants all year then don’t bother cracking the seeds till may or even june. – even then that equates to a 60-90+ day veg period. Tie the plants down or top them (bear in mind that topping as the days shorten will delay flowering as you are removing hormones so it is better to top early or bend them if it’s getting late in the season) if you started your plants in march and have worked out that they are going to get too big them chop them back a few times now and get a nice bush going
8. Stealth – this is very much down to your own circumstances. I like to use the ‘hidden in plain site’ method and grow tomatoes and other crops in my greenhouse – imo people expect to see tomatoes in a greenhouse. Their colour is very close to indica green. Start some tomatoes at the same time as your plants so as they are not over before you need them. Also, if you have space outside and you don’t already then start to grow some veg and make yourself a nice garden – this will both do you good and people will see you in your nice garden and not wonder why you have suddenly started taking a huge interest in the new greenhouse in your otherwise unkempt garden! Also the biodiversity you encourage will help create the ecosystem equilibrium needed to keep pests at bay.
9. Odour. Whatever anyone says about low odour, ripening plants will smell! One way to mask this is to grow other smelly plants in your garden – eg. Mint or chives or lavender at the door to your greenhouse is good because you can tread on it and release the smell as you go in the door. Sweat peas are good and passing a bunch of them over the fence to keep your neighbor happy wont do any harm. Plant them late so as they coincide with your flowering period. Repeat flowering scented roses art good.
The problem with most scented flowers is that they have been and gone before you need them . some that flower late and are scented would be Lonicera Serotina (I think its called florida honeysuckle) another is Osmanthus Heterophyllus or false holly
Im sure ive missed plenty so please contribute. I would love to see a post from a greenhouse grower from a warmer, more arid climate as I’m sure different rules apply there.
Good luck and stay safe
VerdantGreen.
NB – I am in the UK which equates to US zone 9 Cool Temperate Climate.
1. Get the Right Genetics! - Don’t be thinking that a greenhouse is like growing indoors under lights – you will be at the mercy of god’s great bulb and natural day length unless you have a method for blacking out your plants every day. Early finishing strains are very advisable, as are mould resistant ones. Finishing times in august are great, September is ok, October can be pushing your luck and November, even in a good year is on the edge of possibility.
I found the genetics I was looking for by research which led me to Canada. Danish genetics sound promising too. A few dutch strains like Passion #1 etc are reasonably early but not very potent. Sativas may do well as the buds are smaller but you may need a warmer climate than I have for them as they are late finishers usually.
2. Ventilation – this is very important in a greenhouse to stop it turning into an oven during the day and a mould factory during the night. The best way is to have as many roof vents as possible – ideally a quarter of the roof are should be vents and the very best way to control them is with an automatic opener – these are about 20 pounds each but well worth it as they may save your harvest. Also low-level vents are important too – then the cool air is drawn into these as the hot air rises out through the roof. Obviously the door can be used to ventilate the house and if you are buying a polytunnel – try and get one with a door at each end as this vastly improves ventilation.
3. Heat and Fans – the best way to heat a greenhouse is with electricity if you can get the power there, meaning you can also have fans too which really help. Any form of heat which involves burning paraffin or gas will create water vapour which will raise the humidity and encourage mould. Computer/12v fans can be used in conjunction with a car battery and one of those trickle charge solar panels that you put in the car to stop your battery going flat.
Other ways to heat the greenhouse without power are by putting anything in there that will store the day heat and keep the temps up during the night as the heat dissipates. Water butts are good and handy for watering too! Piles of old concrete slabs or bricks work too but make sure you are not creating a habitat for slugs and snails (more about them later).
4. Pots. Pot size is a good way to control plant size so bear in mind that a plant in a huge pot will grow huge and be ready to do some serious training unless you have a 3 story greenhouse! Also plants that are a little on the pot-bound side will stretch less and flower a tiny bit earlier. Pots are generally black and this can cause them to cook your roots in the sun unless you take precautions. Wrap the pots in something white or double pot them with some insulation inbetween. One useful dodge is to dig a hole and bury the pot in the soil if you have soil in your greenhouse as this also gives you more height. If there is any danger of the water table rising near to the surface of the ground then put a big pot or bowl with no holes into the ground and stand your pot in this (watch you don’t water too much as well and be prepared to remove the run-off somehow)
5. Pests. Too many to list here but I find many are kept under control by the ecosystem in an organic garden . I would say that enemy number 1 is SLUGS and SNAILS. There are many ways to control these but I find many of them such as beer traps may help but not solve the problem. Here are the 2 I find most effective.
Copper – slugs and snails will not crawl (for want of a better word!) over copper as it gives them an electric shock – it’s the molluscian equivalent of sucking a bettery ☺. Remember that this wont work if your plant is touching the glass or anything else that will provide the critters with a way round your copper barrier. i ripped up a hot water tank and made copper collars but you can buy copper tape from a garden centre.
Sacrificial Crops – grow some lettuce in your greenhouse – the slugs will eat this in preference to your special plants – if they leave any for you, go mad and make yourself a salad! (note - lettuce seeds do not germinate in hot temperatures so sow your seeds in cool weather and don’t keep them in the greenhouse till they are up and growing!) slugs also love basil. Another effective method is to go out after dark and catch them in the act but remember this may draw attention and that a torch = light = broken dark period.
SPIDER MITES may be a problem but neem oil will sort that.
6. Pestilence – MOULD/BOTRYITIS is your major concern in any cool temperate or humid climate. I reckon More outdoor buds are lost to mould every year than anything else (maybe rippers or 5-0 but lets not go there). Things that reduce the risk of mould are air -
circulation and ventilation, reducing humidity – ventilate during the day and try and stop the temps dropping too much and too quickly during the night whilst keeping a little ventilation too. Close the door before temps have dropped. This is where a thermostatic fan heater is your friend because as temperature drops, relative humidity rises and you get condensation which leads to mould. Fans on their own will help too – mould spores have to settle on your plants before they can grow.
There is some evidence that spraying/foliar feeding with seaweed stops mould spores from germinating.
Cleanliness is also important – remove any damaged or dead vegetation from the plants and floor.
Bud size and density also affect the chances of mould – the bigger and more dense the more chance of mould. Some strains are naturally more resistant than others too.
Overwatering and keeping the plants too wet will not help and neither will too much fertilizer. Water early in the day so you are not raising humidity in the evening. If you get mould on stems you can keep it at bay by wiping it off with a baby wipe every few days – try not to knock the stem and release the spores!.
Don’t let the plants get too congested and remove smaller lower branches to keep good air flow below the plant- better a greenhouse half full of mature buds than one crammed with mouldy ones!.
The later in the year you are trying to grow, the more the chances of mould increase. Don’t be scared to take bigger buds as they start to ripen. As soon as you start to get amber trichomes the chances of mould increase exponentially so get over the fact you may have to harvest early especially after the nights start drawing in. When you get that autumnal twang in the air your time is getting short!
Be vigilant and remove any buds that start looking icky. This vigilence must also extend to any other crops in your greenhouse – especially tomatoes!
7. When to plant? If you don’t want to be wrestling with your plants all year then don’t bother cracking the seeds till may or even june. – even then that equates to a 60-90+ day veg period. Tie the plants down or top them (bear in mind that topping as the days shorten will delay flowering as you are removing hormones so it is better to top early or bend them if it’s getting late in the season) if you started your plants in march and have worked out that they are going to get too big them chop them back a few times now and get a nice bush going
8. Stealth – this is very much down to your own circumstances. I like to use the ‘hidden in plain site’ method and grow tomatoes and other crops in my greenhouse – imo people expect to see tomatoes in a greenhouse. Their colour is very close to indica green. Start some tomatoes at the same time as your plants so as they are not over before you need them. Also, if you have space outside and you don’t already then start to grow some veg and make yourself a nice garden – this will both do you good and people will see you in your nice garden and not wonder why you have suddenly started taking a huge interest in the new greenhouse in your otherwise unkempt garden! Also the biodiversity you encourage will help create the ecosystem equilibrium needed to keep pests at bay.
9. Odour. Whatever anyone says about low odour, ripening plants will smell! One way to mask this is to grow other smelly plants in your garden – eg. Mint or chives or lavender at the door to your greenhouse is good because you can tread on it and release the smell as you go in the door. Sweat peas are good and passing a bunch of them over the fence to keep your neighbor happy wont do any harm. Plant them late so as they coincide with your flowering period. Repeat flowering scented roses art good.
The problem with most scented flowers is that they have been and gone before you need them . some that flower late and are scented would be Lonicera Serotina (I think its called florida honeysuckle) another is Osmanthus Heterophyllus or false holly
Im sure ive missed plenty so please contribute. I would love to see a post from a greenhouse grower from a warmer, more arid climate as I’m sure different rules apply there.
Good luck and stay safe
VerdantGreen.
