Saturday, September 23, 2017

Sharing our Hothouse Skills no.1

Recently Food Garden Group members who have a hothouse or want to get one met and shared their hothouse knowledge and skills.  This was the first time we specifically talked about hothouses and we all learnt a lot.  This blog post is an overview of what was discussed, with the aim to help others to make the most out of their hothouse.

Building a hothouse

You can either buy a kit and assemble that or build your own.  In both cases you need to have some handyman skills.

A kit is easier as all the design work is done for you.  People at the workshop commented that assembling a kit can be quite tricky and lead to mistakes if instructions are not step-by-step and clearly worded. 

Cheap kits will be a disappointment as they will not be sturdy (for instance no cross braces).  The materials will be thin and UV will limit the lifespan.

In the workshop Sproutwell (see was mentioned as an example of a company that provides sturdy reliable hothouses.  Every year they have a stall at AgFest (agricultural show in NW Tasmania), so you can see their hothouses before buying.  Buying a kit over the internet without having seen it will be a gamble.

Example of a nice hothouse kit (thanks Russell - Acton)
And here is another good example of a kit (thanks Fiona - Mount Nelson)
Some of the better kits come with 'automatic window openers' that open and close windows depending on temperature. You can also retrofit these.

Following our workshop Margaret provided links to two suppliers that provide openers that suit many window types of up to 15 kg:  and 

Consider a poly tunnel if you have a bit more space.  They are easier to install than kits and less prone to be smashed if you make sure the plastic does not flap in high winds.

A poly tunnel hothouse (thanks Belinda - Sandford)
I built my own hothouse because the site in our garden (a formed outside brick BBQ) did not lend itself for a kit, and also because I needed a hothouse that would cope with extreme winds.  Collecting recycled windows and a door, and then designing the whole thing was not easy.  I used new timber for framing as secondhand timber was not straight enough, and new polycarbonate transparent roofing sheets.  I spent around $700 in total.  That is a big saving compared to what a robust kit of the same size would cost.  This hothouse is very robust.  I can put hooks in the timber frame and attach shelves or wires any way I like.

My hothouse at Lindisfarne built around an existing brick BBQ
In the workshop Ross commented that he had seen many hothouses with broken glass windows.  This must be because they are fitted with cheap 3mm. thick glass.  The windows in my hothouse have thicker glass and are fine.  If you are thinking of collecting recycled windows and design your own choose ones that have thicker glass or replace the glass.

The Solar Greenhouse Book (by James McCullagh - Rodale press) is a very good classic manual for building a hothouse.  
An owner-built hothouse based on The Solar Greenhouse Book (thanks Russell - Sandfly)
Many people overlook the importance of night-time temperatures in their hothouse.  A hothouse made out of synthetic materials will at night quickly lose heat accumulated during the day and become quite cool.  There are simple and effective ways of minimising heat loss (see below).

A maximum/minimum thermometer in action
You can use a maximum/minimum thermometer to give you an idea of how temperatures fluctuate over a period of time.

A hothouse should be
  • Built on an absolutely level base
  • Be well outside root zones of trees
  • Be within a wildlife-proof enclosure if wildlife is a problem (windows, open to provide ventilation, will provide easy access for wildlife)
  • Be in the least windy and most sunny spot in your garden, with a tap nearby
  • Fitted with a back-wall and/or floor that are brick or concrete, so heat is stored during the day and used to heat the hothouse at night (passive heating).  If this is not possible, use pavers, pebbles, bricks, concrete, a bottle wall, or containers filled with water for the same purpose
  • Positioned to minimise damage by prevailing winds
  • Made with plenty of windows that can be opened for ventilation
  • Tied down to the ground if it is not heavy so it is not blown away or destroyed in strong winds
At our workshop it was commented that a gravel or pebble floor can be a nuisance as you may forever be pulling out weeds.

As an alternative to a conventional hothouse you could consider a mini greenhouse, a conservatory-style hothouse on the sunny side of your house or movable plastic grow tunnels.
Recycled counter/display box as mini hothouse  (thanks Simone - Geilston Bay)
Hothouse soil

Putting a hothouse where you have good soil in your garden, and then using that soil to grow things in, may seem a good idea, but in my opinion, it is much better to give your hothouse a concrete or paver floor (see above) and use pots.

Especially in Spring, plants in pots will appreciate their slightly warmer soil because the pots are surrounded by the warmer air of the hothouse.  By having pots you take full advantage of the warmer conditions in the hothouse.

However, be aware that plants will not thrive in pots that are too small.  Also, soil in pots can turn into 'concrete' if you use soil straight from your garden or compost heap without making it more porous for water.

At the start of each hothouse season (August) I remove last season's soil from all hothouse pots and put it on the compost heap or garden.  I then make a new mix of mature compost + coir + sand + sheep poo + sugar cane mulch.  I put rocks or weed mat in the bottom of each pot and refill it with the new soil.  

You can also use vermiculite and/or perlite in this mix.  Vermiculite can be expensive.  At our workshop Margaret commented that she buys it at a good price from several Ebay sellers, for instance here .  She added that it comes in different grades (sizes), so you need to check before you buy - the finer stuff is good for seed raising, and the courser grade can be used in hothouse pots.  

I do not add any fertilisers to my potting soil because I mostly grow tomatoes in my hothouse, but if you intend to grow plants that 'eat a lot', you could add Complete Organic Fertiliser (for more on COF see here) to this mix. 

Not using pots, but using the ground in your hothouse, year after year, for crops of the same plant families, is asking for trouble, especially because diseases will be more active in the warmer conditions of the hothouse.  If you decide to plant in soil instead of in pots, rotate your crops (for more info see here) or replace the top layer of soil once a year.

It is a good idea to mulch hothouse soil throughout the season to limit evaporation.

Hothouse ‘seasons’

In Tasmania you could say that there are four 'hothouse seasons':

The first 'season' (on average May - August) is the period with low daytime temperatures and frosts.

Ideally the hothouse is empty and you clean it thoroughly. I did my winter cleanup a few months ago.  I found eight cocoons!  It is amazing how many overwintering insects you find hidden in nooks and crannies and folded leaves of plants.  By removing everything and cleaning thoroughly you get rid of a lot of pests that will become active in Spring.  By not having anything to eat for the pests that you overlooked you force them to leave the hothouse.  For persistent pests like whitefly it can be the only way to end the infestation if you don't want to use chemicals.

In reality the hothouse may not be quite empty at this time because you may have some overwintering plants there because no where else is it sunny enough and warm enough for them.  If that is the case check them very thoroughly, because they are the most likely spots for overwintering pests.

The second 'season' (on average September - October) is the period in which daytime temperatures are a bit higher, but still under 20 degrees C, and there no longer are any frosts.

During this time you will gradually fill the hothouse with plants. 

Ventilate the hothouse during the day if temperatures are forecasted to raise above 15 degrees C.

In the first year I had my hothouse I made the mistake to conscientiously water pots every second day and some of my tomato plants died because wet soil in only moderate temperatures made their root systemts too cold.  

During this period I water once a week, and only moderately, adding Seasol or PowerFeed to the water. 

In the third 'season' (on average November - March) temperatures in the hothouse are over 20 degrees C most of the time.

Ventilate every day, closing windows at night, or not at all, if it is warm.  Be on the lookout for mildew and apply water/milk mixture as soon as you detect any.  Prune plants to improve ventilation.

If time permits I water early in the day, rather than in the evening, so surplus water quickly evaporates and does not hang around.  Mildew loves wet humid conditions, so this is the time when  it can be at its most rampant, if you do not ventilate enough.

Watering is needed every second day, or every day, when it is really warm. I installed an automatic watering system in my hothouse and plants died because they were over or under watered, so I learnt that it is best to check each pot every day with a finger, and water if needed.  Some people like to use  a soil moisture sensor instead of a finger.

Feed Seasol or PowerFeed or similar products once a week.

The forth 'season' (on average April - May) is the one where daytime temperatures gradually go down and plants that have been there since the start of the season are beginning to look a bit tired.

This time of year is about prolonging the life and wellbeing of your plants.  Decrease water as day temperatures decrease.  Ventilate when days are warm.  Feed PowerFeed or Seasol or similar once a week to invigorate plants.

Last season I picked my last hothouse tomatoes at the end of May.
Instead of stakes I use flexible fabric ties that come down from above
You can expect crops from your hothouse to start a lot earlier and finished a lot later than those grown outside in the garden.

Crops to consider for your hothouse
  • tomatoes
  • capsicums, peppers
  • cucumbers
  • aubergines
  • lemongrass
  • bananas (see here)
  • melons
  • use the hothouse to sow seeds in trays (see photo of Belinda's poly tunnel above), and plant them outside after hardening off
  • carrots – so you get a nice early crop (see below)
Food Garden Group member Max K sows Chantenay carrots (short thick variety up to 15 cm long).  He sows them around 1 August (sowing before the winter solstice would lead to them going to seed early), and they are ready around mid October.  He sows them in 25 cm deep Styrofoam boxes with holes in the bottom in potting mix with nothing added.  He foliar-feeds regularly with fish emulsion.  They don’t need a lot of water. Just water once a week to keep soil moist.

Pest Control in hothouses

Hothouse conditions can be very attractive for pests (heat, humidity, high concentration of plants, nooks and crannies to hide).  Hothouses are ideal places for pests to overwinter, so do a thorough winter cleanup (see the first period above).

Ideally have no plants in your hothouse during winter so there is nothing to eat.

Best pest prevention is to make your crops as healthy as possible.

Glue strips are useful to catch insects and to understand what insects you have in your hothouse, but they are broad-spectrum, so you may also catch bees and other beneficial insects.

Milk spray (1:9 milk – water) is totally effective against mildew.  Spray all parts of plants during daylight hours.  

Spraying soapy water (dishwash liquid + water) is a good and non-poisonous remedy against aphids.

Hunt for slugs and snails at night.

Spray Dipel against caterpillars.

Here ends the summary of our hothouse workshop.  A hothouse can really extend your season.  In Tasmania with its unpredictable often cool climate that is a real bonus.


1 comment:

  1. Hi Food Garden Groupies, one other option for pest control to consider is the use of beneficial organisms (good bugs). I've been using them for many years now with good results. We get ours from Biological Services in South Australia.