Wally Richards – Glasshouse Gardening

Some gardeners may have been lucky enough to receive a glasshouse for Christmas. If so, congratulations, you are in for a lot of fun.

I have been asked several times in the past for basic information about growing in a glasshouse so now is a good time to look at the subject.

If you do not have a glasshouse but are keen on gardening, you should invest in one.

It can be a plastic or glass structure and if you have to choose between the two, go for glass as you only have to replace broken panes, not all of the plastic every few years.
Plastic is cheaper especially if you build your own. It takes only a little bit of building skill to construct an A-frame using 50 x 50 mm wood and covering it with horticulture plastic film. Alternatively cover with a clear light type nova lite.

The bigger the house, the more ventilation is needed. The door is one major vent, and one or more roof vents should be included. In a glasshouse you can always remove panes of glass in summer for extra ventilation. I never paint or use shade cloth on the outside of my glasshouses. The glass is allowed to become dirty from the end of winter but washed clean once a year in autumn. With the right use of the vents and water you can keep the house’s temperature from getting too hot in summer. If you need to shade the house in a hot summer you can make up a slurry of hydrated lime and water and paint this over the outside of the glass.

Plants that cannot stand full hot sun can be placed under benches for shade or string up some shade cloth inside the house over the sun sensitive plants. Many gardeners have an earth floor in their glasshouses so they can grow directly into the earth. I have never liked this idea as soil can harbour diseases which will likely affect plants.

Also the idea of digging out the soil every so often and replacing it with new top soil always seemed too much work. I always opt for a concrete floor and grow all plants in containers. This makes it easy to remove a plant from the house if it becomes diseased and could affect other plants. Concrete has other advantages – it is easy to clean, weeds can’t grow on clean concrete and it is a heat trap that releases the warmth when the sun goes down. On hot sunny days you can wet the concrete and the evaporation of the water will cool the house and keep the temperature down.

If you are unable to lay a concrete floor then place weed mat over the soil and cover with pea metal, a couple of inches thick. Another choice would be weed mat with pavers, cobbles or bricks. In these cases, dirt will build up over time and you likely will need to do a bit of weeding.

Placing of the glasshouse is important. Try and locate it where it’s going to get as much winter sun as possible (it will also get maximum summer sun too unless shaded by deciduous trees). One side of the house should face north, the “sunny side”. This side is where you grow your tomato plants, cucumbers etc. The other side should have a small work bench or a bench running the whole length of that side, depending on the extent of your activity and ideal for seedling trays, cuttings, growing on lines and small container plants. Under the bench can be used for pot storage etc and for plants that need a shaded situation.

Having a glasshouse allows you to have a greater control over the inside environment.

The plants are sheltered from wind which is a great plus in growing. Yet the wind passing over a glasshouse will lower the inside temperature by a number of degrees. Your plants don’t get rained on and if in containers, you are the supplier of their water needs. Don’t put in an overhead sprinkler system to water. It will likely cause damage and disease to plants. If you want to use a irrigation system, then install a dripper type. I prefer to water only with a soft wand late in the day. I keep a bucket of water in the glasshouse and on hot days, during the day, if any plants need some water I just use a cup of water from the bucket (it’s going to be naturally warm water and not give the plants a cold shock on a hot day).

You can water in the morning if you prefer. In very hot areas watering will likely be needed in the morning and late day during summer. Plants in containers that wilt through lack of moisture during the day can have a saucer or tray placed under them for extra water reserves. Wilting can also be caused by too higher temperatures during the day. Try and not let the temperature exceed 30 and best at about 25 degrees C. Good ventilation and water on the concrete can keep the inside temperature lower than the outside. A dripping hose can keep the supply of water for evaporation during the day when you are not present.

As temperatures start to fall in autumn, your watering should also lessen. In winter all plants should be kept on the dry side as wet growing mediums mean a very cold root system and a greater chance for diseases to strike. In fact in winter you should only give small drinks when the plants start to droop through lack of moisture.

As winter approaches spray the plants with Vaporgard over and under foliage. It will protect them from frost and greatly reduce their water needs. The film which lasts for about 3 months will also protect them from pests and diseases to a degree. The product can also be used in summer to reduce the water needs of plants.

If plants wilt during the day and yet the mix is damp, it means that the roots cannot keep up with the moisture loss through the leaves. Vaporgard will help solve that problem.

Tomato plants must have sufficient moisture when setting fruit, if not blossom end rot is likely. For pest control use Neem Tree Granules around near the base of plants. If you get an outbreak of say whitefly, then spray with Neem Tree Oil and Key Pyrethrum at dusk but not when there is high humidity. Diseases can be prevented by sprays of MBL, Mycorrcin and Perkfection. Repeat about every month with Perkfection and 2 weekly with the others. Rok Solid (mineral rock dust) sprinkled around the base of plants will provide many of the extra minerals needed for healthy plants.

Germinating seeds or doing cuttings will likely require a bit of shade cloth over the trays in summer, or pop them under the bench. Use Vaporgard on the cuttings. In the cooler months a heat pad will help germination and striking. When growing in winter, extend the light hours with artificial light. If growing tomatoes in winter chose types that set in colder conditions.

Glasshouses or tunnel houses are a lot of fun and save you lots of dollars over the years.

Any problems ring me at 0800 466464 (Palmerston North 3570606)
Email Website

35 comments on “Wally Richards – Glasshouse Gardening

  1. Hi there i was intending to install a green house using polycarbonate instead of glass I would be interested to know what you think. I have been told the polycarbonate is warmer.
    I am also wanting to know why you prefer pots on concrete instead of a dirt floor. Apart from the extra work I assume the advantage is you can get more stuff in the Greenhouse on a dirt floor. I had intended to dig in sea weed to fertilise and just accept that I dig the dirt over every few months as required. I think I am rambling on it would be better to talk to you, I will try to call on Friday

  2. Hi,
    I am a newbie to glasshousing. I have a concrete floor and as a consequence use containers as you recommend. What sort of soil recipe should I be using for the containers.

  3. Mark
    Use a mix of mainly compost (purchased as its weed free) add a little clean top soil or even better worm casts.
    Additional food can be added also such as sheep manure pellets, blood & bone etc.

  4. myrtle on said:

    Hi Wally
    Can you please recommend a few varietys of tomato for me to plant in the glass house for winter. Would sweet 100’s be ok as well?

  5. Hi
    You can try sweet 100 but really you need cold setting types such as the Russian ones, Silvery Fern Tree is a good one also try Russian Red.
    Wally Richards

  6. Mike Batchelor on said:

    It is autumn and there is some green algae on the inside of my glasshouse. how do I get rid of it?

  7. Hi mike,

    Try some wet & forget sprayed and then hose off.



  8. Is Wet and forget harmful to plants

  9. Hi Mike,

    Have a look at their website or give them a call – i don’t think it is very harmful to most plants, but always exceptions to the rule.



  10. William Ingley on said:

    Hi, can you please tell me if I can use raw macrocapa sawdust as a mulch? I have been offered as much of the sawdust as I want for free and wondered if I could use it and mix it in with the existing soil, or perhaps mixed with a commercial compost?

  11. Wally Richards on said:

    You can do either, add some blood & bone to soil before spreading.
    Do not place around plants that hate wet feet if the drainage is not very good.
    Wally Richards

  12. Mike was wanting something to clean his Algae off his glasshouse. I use Caskade Lichen and Algae Remover as a far more economical product than which what was suggested as the dilution rate 50 or 100 to 1 so It works out far cheaper and also cost $40 to buy 5 ltrs about 1 year ago so try giving Jasol in Auckland a ring on 0800 429 628 they are in Ellerslie Auckland
    There web address is
    I bought my container in Tauranga but I don’t get any commission from them but save yourself a heap of money and go there.
    You might have to get it off one of there agent so make some enquiries

  13. Amanda Wilson on said:

    Hi Wally,
    My 12 year old daughter and I have recently gotten the gardening bug. Our inexperience has led us to sow a container of Dill seeds (can’t have scrambled eggs without them 🙂 but have been told we were far to late in the season to have much success. So far though, all the little seedlings have popped up in the terracotta bowl and actually look quite healthy. I had a flash of inspiration the other day and resurrected our old glass fish tank with the idea of using it as a miniature glass house. My problem is that the tank is wide rather than high and if the Dill keeps growing it will eventually hit the glass at the top. (About 20 cm away) Am I likely to loose the plants at this stage if I take the lid off and expose them to much colder temperatures, or should I just let them sit in a warm spot in the open now and see what happens. Many thanks for your time. Cheers … Amanda

  14. Wally Richards on said:

    Hi Amanda
    You could place the container and tank out in a sunny spot with the glass top on till they reach the top then remove the glass.
    Seeing its mobile you can move to a frost free spot like a porch on anytime it is cold and frost is likely.
    Place in full light next day.
    All the best
    Wally Richards

  15. peter jamieson on said:

    Can you suggeast where I can purchase a heat mat from please? To germinate my seeds.

  16. Hi
    Some times garden centres have them, alternatively Brew Shops have them for heating fertmentors and Piggy Pads from stock and station agents is another possablity.
    Wally Richards

  17. I have a small glasshouse that I have always grown tomatoes in but thought I would like to try a telegraph cucumber as well but heard somewhere that the 2 are not compatible, is this true. Would be grateful for your comments

  18. not sure what you mean by moderation

  19. Ross Orr on said:

    Good morning Wally,

    Can you tell me if there is a Flower setting spray for tomatoes in glasshouses & if there is, where it is obtainable.
    Thank you
    Ross Orr

  20. Linda Swanson on said:

    Hi Wally, My husband built me a small hot house, made of fibre glass roofing and glass, what temperature should it be, and also my tom,s have either blight or mildew looking leaf should I spray with copper, thanks for your time, cheers Linda

  21. Carol Green on said:

    Hi Wally
    I am just about to enter the glasshouse land. I am wondering if you could recommend anywhere to buy one in Christchurch please. I am after a glass one and reasonably tall as I am tall. Thanks. I also intend for it to be somewhere I can sit with a cuppa. Thanks Wally

  22. paul ong on said:


    If I plant direct in the soil of the glasshouse , how can I ensure it is diseases free or if infected what action do I take ?


  23. hayden on said:

    hi there, i am looking at building a hot house with horticultural plastic, an a frame is a walk in the park for me with what i have available, but im not planing on growing in it, i plan on drying firewood, i have read up about what they call a “solar kiln” and know and understand the ventilation needs for humidity reasons, (particularly to dry wood)

    but my question is… can anyone tell me the average daytime and night time if uve got it, temperatures of a glass house during the winter in canterbury, i can put this in a place for basicly all day winter sun, and im not worried about the summer, (due to supply and demand reasons)
    any inbfo gratefully accepted, there are many options on the table and this is one of the cheaper ones, there is cheaper but perhaps more crude,

    and before any negativs come in,,, i plan on keeping the wood off the ground either buy metal cages or a “forklift pallet style floor” allowing airflow with poly on the ground under, then also leaving a gap around any sides and ends of glasshouse/solar kiln/box kiln/kiln, (also for airflow) and more than likely, (espicialy with cage idea) there will be gaps in the pile about every 2 meters, for best flow once again, and in flow/ outflow points will be positioned to alow this best

    cheers hayden

  24. Hi Hayden, this sounds like a great idea. I suggest contacting your nearest botanic garden – the head gardener would be able to answer this question.



  25. joy evans on said:

    Hi Wally
    At last I know how to e-mail you direct.

    I have been given several good wooden framed windows and was looking for info on planning, ventilation, etc. Can you help with basics, or another web site please? This will be for plants only; I have a covered area for potting and storage.

    I will take your advice and have a concrete floor rather than beds of soil.

    Thank you for previous very helpful info in e-mails and your book.

  26. Hi, I am a single mum and wanting to build a tunnel house as cheap as i can: I am wondering if it is ok to use timber from a demolished house or shed? also if building a wooden tunnel house is there a law for how close to a fence it needs to be built, or could you use your fence as a part of it. Thanks

  27. Hi

    Yes that would be fine althoe it may only last a few years if it is not treated. you could build an A frame shape out of manuka logs or something similar and cover with clear plastic and perhaps shade cloth depending on mid summer sun – keep it simple cheap and cheerful.



  28. Hi, Im new to a glasshouse, would like to know how to grow eggplant, on some chicken wire up the wall? Also rock melon,

  29. Julie Allan on said:

    Hi I’m new to glasshouse growing and want to know when to plant egg plant capsicum and tomatoes Thanks J

  30. Rayjones on said:

    G’Day Wally,

    Enjoyed the info on tunnel houses, all good practical information. I purchased an 8 metre house 10 years ago & have had great success with tomatoes & winter veg. For the last 2 years tomato & cucumber have been poor with leaves shriviling up. I have a door, 2 roof vents & a window but temperatures still reach the 30 & some days 40’s which I thinl is the problem. My question is would side vents cure the problem, if so has anyone come up with a  practical design. If anyone can suggest another methode I’m all ears.

  31. Steve on said:

    What type of spray would you use to kill weeds in a glass house?

  32. Rose on said:

    My husband covered inside of a large glasshouse with plastic for extra winter protection he planted quite a few vegetables, herbs and transplanted  outside lettuces thinking they would be fine but alas the first frost destroyed most of them. Outside in the vege garden however the lettuces are still going strong and they have been covered in frost. Can anybody tell us why. Do we need to put some heating in the glass house?

  33. Hi Rose, Did the glasshouse get frost inside. It is possible that the glasshouse had more moisture init and the leaves have completely frozen, you can get glasshouse heaters and this would prevent thsi happening. 

  34. Rose on said:

    thanks Tim we are probably going to get a heater, at the moment  trying a few cheaper options such as something solar powered.

  35. Yes solar power is the way to go. let me know how you get on.

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