Sharing our Hothouse Skills no.2

A meeting of members interested in hothouses last winter was followed by a second session on the subject in the middle of summer. We looked at the host’s hothouse, and everyone had the opportunity to ask hothouse-related questions or discuss successes and failures in their hothouse. This blog post focuses on three topics that were discussed and that might be of interest.

This second blog post follows blog post Sharing our Hothouse Skills no.1

What we grew
Many of us grow tomatoes and capsicums in our hothouse, but others reported growing cucumbers, aubergines, lemons, watermelon, and bananas (see blog post Growing Bananas in Hobart )

Margaret reported that she had taken up Max K’s idea (mentioned in the first session) to grow early carrots in her hothouse. She used an old recycling bin/tray. The very early carrots were really delicious. When the weather improved she moved the tray outside, but the aim, to have carrots much earlier than you could have outside, had been achieved. She will do this again next season.

We talked about trying to have ripe tomatoes in our hothouse as early as possible. Various hardware stores sell small quantities of well-developed tomato seedlings in mid August. I bought 2 Sweet Bites and 2 Tommy Toes and put them in pots in my hothouse in the last week of August. They began to produce in the first week of December, and have been absolutely prolific. Fingers crossed, with a weekly foliar feed of seaweed extract (Seasol or similar) or fish emulsion (PowerFeed or similar) they will hopefully continue producing until the end of May.

Margaret planted turmeric and ginger tubers (both bought at Eumarrah) in large pots in her hothouse. After a slow start, they performed well, and she will continue to have them.

Pests
I reported having no aphids, and almost no whitefly this season, probably for two reasons: I cleaned the hothouse rigorously mid-winter, leaving nothing in it for a while, and I ventilated much more this season.

The least chemical way of removing whitefly is to hang yellow sticky glue traps (see photo below) in your hothouse. However, you will also catch bees and other beneficial insects. David and Helen adopted two novel approaches: with a vacuum-cleaner on and ready, Helen shook plants, and David sucked them up the vacuum-cleaner nozzle as they fluttered up. They also caught many by shaking plants and holding up a yellow glue strip.

Ross mentioned the benefits of decoy plants. The best pest management is to make sure your plants are as healthy as possible. Healthy plants have ways of resisting pests. Unhealthy plants are easier targets for pests. Ross reported that one professional grower deliberately neglects a few plants at the end of rows. The unhealthy plants attract the pests, and leave the healthy plants alone. The grower than kills the pests in the unhealthy plants.


David and Helen’s hothouse
David decided to get a hothouse so he could have produce earlier in Spring, and continue to have produce later in Autumn. He decided to build his own hothouse, not because this would be cheaper than buying a kit (he spent around $1600), but because this allowed him to build exactly what he felt was best for the site and the local climate.

David and Helen’s property is in a semi-rural area on Hobart’s eastern shore near the airport. This is an area with many more hours of sunshine and far less rain than areas west of the Derwent. The challenge therefore was to build a hothouse that would store heat and keep temperatures as high as possible in winter and spring, and keep temperatures down in summer and autumn.

David decided to use polycarbonate sheeting on a timber frame. The thickness, durability, and price of polycarbonate sheets varies enormously. Thickness can vary between 3mm and 10mm.  Karen, who works for a company that sells these sheets, replaced all the glass in her 40-year old hothouse with polycarbonate sheets.  It can be cut with a stanley knife or a small electric saw for large quantities.  For more information she recommended you look at LaserLite Multiwall .

The resulting design has a door on one side and horizontally-opening windows on the opposite side. The hothouse has been positioned, so prevailing winds can go right through, when this is needed to lower temperatures inside.

The hothouse has a concrete floor that was painted black to store as much heat as possible.

David decided to use two types of shade cloth. Green shade cloth that blocks out quite a bit of light, and a cream shade cloth that does less so.

Cream shade cloth inside the hothouse, on the north and east side, can be rolled up or let down, to filter sun.

On the roof David installed two rolls of shade cloth: one made of cream-coloured shade cloth, and one of green shade cloth. In mid-winter both sheets are rolled up onto pieces of plumbing pipe (see roof in first photo below). As temperatures go up, David rolls out the role of cream shade cloth, so it covers the whole roof. Then, when the temperature inside goes above 28 degrees, he rolls out the second roll of green shade cloth on top of the cream sheet.

Shade cloth on the outside of the north-facing side of the hothouse can be rolled up when not needed (first photo below), or completely cover the front when temperatures need to be kept down (second photo below).

The polycarbonate sheets on either side of the door are non-transparent to diffuse the sun from that direction that can be particularly fierce.

Plants inside the hothouse are all in large pots that sit in trays. The trays are filled with water when Helen and David go away and keep soils wet. David commented that the blue trays (first photo below) are really too high, and when they put too much water in them, plants did not respond well. The much lower black trays (second photo below) are much better. In mid-summer either system allows Helen and David to be away for up to 4 days.

David installed in his hothouse a clever little battery-powered box that measures and documents temperature and humidity over a long period of time. With this data logger he was able to measure whether the various modifications he made to his hothouse helped avoid extreme low and high temperatures. If you would like to know more about this device, have a look at Hobo Data Loggers .

Thanks everyone who was present at the session and contributed to the discussion.

Thank you David and Helen for hosting the session, and showing us your interesting hothouse.