Greenhouse users (both Polytunnel and Glasshouse) need to know what the temperature was when they weren’t around to check it. Did it get too hot yesterday afternoon, stressing the plants and stopping the lettuce seeds from germinating? Did it get too cold last night, and should I have closed the vents a bit more? Did I overheat the greenhouse last night? A maximum and minimum recording thermometer will answer those questions. Both digital and traditional analogue types are available. Analogue types are generally cheaper and simpler to use, and need no power source. Instead of a single tube and bulb with mercury or coloured alcohol, they have a U-shaped tube with a bulb at each end and a column of mercury in the middle. A temperature change pushes the mercury down one arm and up the other. The mercury pushes a floating pin along each tube as it moves and leaves it behind when it moves back, showing how far it went since you last reset it. The thermometer is reset by tipping it up on end or pulling the pins back with a small magnet. Digital models are reset by pushing buttons as per the user’s manual. For best results place the thermometer or its digital sensor at the level of the plants and shade it from direct sun, which overheats it and gives a false high reading.
In hot sunny weather opening all available doors and vents may not be enough to cool your greenhouse, especially if there’s no breeze. Plants suffer from stress if the temperature goes above 25-27°C (77-81°F): they wilt if they fail to get water from the soil and up to their leaves as fast as it evaporates out. If they wilt badly enough the leaf edges will die and turn brown (scorch) and growth and yield will be reduced.
Apart from plenty of watering and full ventilation, shading is the answer. It blocks the entry of infra-red light into a greenhouse and reduces the build-up of heat but it also reduces the visible light needed for plant growth, so use only as much shading as the plants need and only when they need it. A recording thermometer can help you decide how much and when.
Shading paint can be applied to glasshouses with aluminium or painted wood frames, but it can stain polythene, polycarbonate and unpainted wood permanently. Paint it on in spring or summer when needed and clean it off in autumn when no longer wanted. It’s inflexible when the weather changes often.
Shade netting or roller blinds can be fixed up to the inside of the frame or over the top of the greenhouse, and small plants can be shaded with larger plants or sheets of newspaper or tinfoil above them. Shading fixed inside the greenhouse is easier to adjust and blocks direct sunlight from the plants, but it still allows heat to build up. External shading controls the heat better, but can be more expensive to fit and interfere with vents. Automated blinds are handy but cost more.
Polytunnels don’t hold in the heat as well as glasshouses unless they have a ‘thermic’ polythene cover. On cold nights it can be better to have some air movement through a polytunnel than to seal it up tight and let frost build up inside. See also http://www.polydome.ie/blog/?tag=polytunnels-frost-protection . Glasshouses can be shut up tight, making sure that there are no gaps or broken panes.
Electric, gas or paraffin heaters can provide extra heat – at a price. Unwanted candle stubs produce more heat than you might think when left burning all night. Horticultural fleece can be wrapped around individual plants and their pots if you take it off when the sun shines, and bubble-wrap sheeting can be used in the same way or fixed inside the greenhouse walls to insulate them (you lose some light). Containers of water absorb heat during the day and radiate it back out at night, like storage heaters.
Watering should be reduced to a minimum to concentrate the plants’ cell sap and lower its freezing point. Don’t stop watering! It can get surprisingly warm in a greenhouse during sunny spells, and plants will be dying for lack of water if not checked regularly. I lost plants that way.
The heat given off by large amounts of fermenting compost or farmyard manure was used in the past to warm crops such as pineapples. It still works, but most modern greenhouses have neither the space nor the cheap labour to do that today. Soil-warming cables can protect plants from quite hard frosts in the same way, or for the occasional really cold night you could warm small pot-plants in an electric propagator.
Whether it’s Bali or Ballybunion, everyone wants to go away for a break. So what happens to your tunnel or your glasshouse while you’re gone?
Automatic watering is ideal. Be sure to set the controls well in advance so you’re sure it’s working well and regularly, and delivering enough water. Ventilation is easier; you can leave the vents wide open in mid-summer without fear of night frost but automatic vent openers are less liable to storm damage. They need no electricity and are easy to fit. Wedge doors nearly closed or screen them with wire mesh to keep out pets and wildlife.
Bribe a neighbour to keep an eye on it with free produce or a promise of looking after theirs later. Automatic watering and ventilators are good but nothing beats the human touch: unexpected problems can crop up (pun deliberate) and the comings and goings of neighbours deter thieves.
Move out pots to a sheltered, shady spot where they can get rain or be watered if needed. Remove more bottom leaves from your tomato plants than usual: the leaves on the top 70cm of the plants contribute most to their growth. This reduces their need for water and lets more fresh air around the plants, which helps control fungus diseases.
Remove flowers and developing fruit from plants to reduce the amount of unwanted and over-mature fruit growing while you’re away. It also further reduces the plants’ need for water.
Written by horticulturalist Peter Whyte
Never mind how cold it is in your garden; the sun is getting higher every day and even short clear spells will allow it to build up a lot of heat in your glasshouse or polytunnel. You might think this is a good thing, but not always so. Lettuce seed won’t germinate if it’s too warm, and temperatures above about 35°C (95°F) will destroy the red pigment in ripening tomatoes so they never colour up properly. Overheated plants need more water to keep cool. As well as that, high temperatures put plants under stress and promote diseases such as grey mould and pests such as red spider mites. You can’t stand there 24/7, thermometer in hand, to open and close doors and vents with each change in temperature. Ordinary mortals adjust the ventilation once in the morning for the expected daytime temperatures and once in the evening for the night, and that’s near enough most of the time. The weather forecast can help you decide how much ventilation to give in the morning. A recording thermometer can show you how you did today, helping you to fine-tune your settings for tomorrow. End doors alone give enough ventilation for most polytunnels up to about 20 metres long. Side windows, roll-up vents and louvre vents are good but not cheap. Glasshouse vents can be fitted with gas-filled struts that automatically push them further open when they get warmer, without needing electricity. The best ventilation comes from a through flow of air, so open both ends of a polytunnel or more than one window in a glasshouse. Open many vents a little bit instead of opening two vents wide to reduce draughts. Be cool!
Peter Whyte (Lanscape and Garden Consultant) B Agr Sc (Hort), Nat Dip Sc (Apic), Dip Tr & Ed, MI Hort
Traditionally, people sowed vegetable seeds in their gardens on Saint Patrick’s Day. That was because they were off work and free to do it rather than weather and soil conditions being suitable. But, right enough, the soil is often dry enough to dig and warm enough for seed germination about then.
If you have a glasshouse or polytunnel you don’t have to wait for all that. You can sow crop seeds earlier under cover and have plants to put out instead of seeds, gaining several weeks of extra growth. Sow leafy crops like cabbages and lettuce in modules for planting out later, and also a few seeds in the border soil inside to mature early. If you have lots of room, keep one courgette plant growing inside until the outdoor-planted ones start fruiting. You can then chop it out and use the space for something else, but it will have been cropping for weeks by then.
Tap-rooted plants such as carrots can’t be transplanted and don’t take kindly to modules, but you can still sow some inside for early crops. Freshly-dug baby carrots, washed instead of peeled, and eaten raw are fit for a king.
Seedlings are easiest to raise in a heated propagator. If using it inside the house carry the seedlings out to a glasshouse or tunnel in the propagator with the cover closed to protect them from cold winds as the temperature shock would be too much for them.
The sun is getting stronger, so be prepared to open vents or doors on sunny days. But make sure to close them at night! Throw a sheet of bubble-wrap or fleece over plants on frosty nights.
We now have 15 Glasshouses and 4 Polytunnels on display at Polydome’s HQ at Crinkill House in Birr. With Birr Castle Demesne close by any green fingered people looking for a Greenhouse (Glasshouse or Polytunnel) will find the trip well worthwhile. As well as seeing the largest display of Greenhouses and being able to talk to product specialists to sort you out with the best Greenhouse or Greenhouses Accessories for your situation, with good places to eat locally and the renowned Gardens and Science Centre at Birr Castle it will be a great horticultural day out.
Polytunnel’s should be cleaned at least once a year, maybe more if your Polytunnel is nearer trees as it will probably have algae forming on the outside and debris from the trees falling onto it. There are different ways in which to clean your tunnel. Here are a few helpful tips :-
If you get a bucket of warm water and add a small drop of washing up liquid, you will then be able to use a soft brush to clean the tunnel, for the bits you cannot reach get an old bed sheet and with a person each side of the tunnel gently go along the tunnel. Be careful not to do anything that will scratch the cover as this will reduce the light transparency.
If you have plants inside your Polytunnel or Glasshouse in the winter months there are accessories available to help keep the cold at bay and protect from frost damage. The crop protection fleece will work as a duvet for plants and we have a range of thermostatically controlled heaters that can be used. The heaters will have the advantage of making snow melt before it can build up, helping to protect your Greenhouse or Polytunnel against structural damage.
On milder days it would do no harm to open the doors for a few hours to ventilate your Polytunnel . Frost occurs when air stands still. In Q gardens last year the plants inside unheated Polytunnels with open doors were affected less by frost than in the unheated Polytunnels with closed doors.
Its that time of year again, if your anything like me, then its safe to say that you have at least, another 4 days before you even consider starting the christmas shop. However if you are planning ahead, why not cosider one of our Christmas gift cards as an option. It can used on Accessories, Polytunnels, Glasshouses or any stock item. Gift cards are valued for 12 months.
So why not put the spring in someone step this christmas and give them a Polydome Greenhouse Giftcard.