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Recording Thermometers Horticulturalist Peter Whyte gives some expert advice

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.

Horticulturalist Peter Whyte gives some advice on shading

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.IMG_0616

Shade netting or roller blinds can be fixed up to the inside of the frame or over the top of the greenhouse, and small SHADINGplants 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.


Frost protection ideas from Peter Whyte

PhoenixDecember usually brings the first hard frosts of winter.  Tender plants can be killed as their sap freezes, expands and bursts their cells.

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 .  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.

Holiday Tips

Holiday tips

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.

Tidy up and clear out any weeds; also dead and dying leaves, dropped fruit and other plant remains thaVENTOMATIC UNIT COPYt could host diseases.  Bon voyage!Polytunnel ventilation for blog


Written by horticulturalist Peter Whyte

Growing tips for Tomatoes

TomatoEveryone with a polytunnel or glasshouse is tempted to grow tomatoes – and why not? They taste so much better when picked just before eating.  Here are a few tips for the best crops.

Choose well-flavoured varieties like Shirley or Alicante.  Some traditional varieties like Moneymaker are insipid.  If you are buying plants, look for healthy ones about 20cm tall.  Yellow leaves indicate poor feeding or cultivation and bluish or purplish leaves indicate chilling: such plants will take time to recover and crop later.  Drawn, leggy plants will be the same.

If planting in the soil try to have the plants in slight hollows rather than on top of mounds, so water will soak in rather than run away.  Water them in well, and let the ground surface dry off between waterings.  Vine tomatoes need support: if using canes put them in before the plants to avoid root damage.  Strings are better than canes for plants in grow-bags.  Cherry tomatoes are wide and bushy plants, so give them plenty of room to spread.  

Feed the plants with high-potash feed as per the instructions, starting when the first fruits appear.  Tie in the growing vines to canes or wind the support strings around them regularly, as stems are hard to train when they thicken up.  Hook very long trusses up on themselves or higher leaf-stems to keep them up from mud and slugs.

Break out sideways any side-shoots growing from the angles between leaf-stems and the main stem.  Snap off upwards any dying bottom leaves to let light and air around the fruit.  Bush tomatoes need no training.  Pick a tomato by thumbing down on the knuckle just above it while twisting the fruit upwards.  The green oil on tomato plants is irritant; wash your hands with soap and water afterwards.

Peter Whyte  B Agr Sc (Hort), Nat Dip Sc (Apic), Dip Tr & Ed, MI Hort

Garden Festival at Hillsborough Castle

Looking forward to the Airtricity Garden Festival at Hillsborough Castle next weekend (17th, 18th and 19th of May).  We will have a Janssens Victorian SL Greenhouse up and Jonathan Pyle (a Director of Polydome) will be there to meet and greet visitors to our stand.  Monty Don (pictured) will be appearing on Friday 17th.  Hillsborough Castle is a beautiful location for the festival which has been running for some years and is one of the main events in the Gardening Calendar in Ireland.monty-don3  Further information on the Garden Festival can be obtained at the organisers website:


A distinguished visitor

Had a distinguished visitor in last week, the former RTE reporter Mr Charlie Bird. We checked to make sure he didnt mind us blogging about his visit. Charlie is looking at one of our Western Red Cedar ‘Growhouse’ Greenhouses and we had a pleasant chat over a coffee. A pleasure to meet him. We all including Zara had a little laugh because she didnt know who he was (she being from across Irish Sea).Growhouse5

‘Cool’ ventilation tips from Peter Whyte

Bayliss XLNever 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

Ready, steady, SOW

GT-DP-H 02Traditionally, 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.