Nina Forrester used to grow calla lillies commercially on the very windswept landscape south of Dargaville.

Nina adjusting the GROWhome. Photo supplied.

As the many local Kumara growers in the area will tell you, the soils are good, but the wind can be a real drawback. Growing within the confines of a greenhouse provided the necessary shelter for the more delicate callas’ but inevitably added much extra cost.

Glass is a wonderful material but it is easily broken and expensive. Horticultural plastic, however, is far more forgiving and flexible. It will fit any shape, and is very easily repaired or replaced if it is damaged and at a fraction of the cost. 

Nina, who now operates from the more sheltered environs near Whangarei said, “it was the wind which really set me thinking. The answer had to be something capable of withstanding the battering and protecting the crop, but moveable each season. If it could be easily handled as well, then it would be even better.”

After some experimentation, Nina realised the shape she sought was one which avoided the natural “lift” of a gently curved top.

It is best explained by imagining the upper surface of an aircraft’s wing. The curved shape causes acceleration of the air flowing over the top of the wing. This causes the lift that allows the aircraft to fly.

So, a smooth rounded surface will cause any body to be raised upwards. If that airflow is interrupted as the air travels across the surface, then the lift is very much reduced. A deeper or sharper profile will cause turbulence on the downwind side and far less lift as a result.

The ideal shape Nina needed was a variation on the polytunnel. Her time raising calla lillies allowed her to play with the shape and iron out the potential problems.

The end result is the GROWhome, looking not unlike one of those cocoon type tents beloved of the mountaineering brigade, and designed very much upon the same principles.

But whereas mountaineers prefer to be as low to the ground as possible, any self-respecting greenhouse needs headroom. The massive heads of sweet corn Nina had in her own GROWhome, were more than double the size of the ones in her veggie patch and already, in early December, touching the top of the two metre roof.

There are two main materials in the GROWhome, – galvanized steel and long life plastic. Galvanized steel tube provides the best strength-for-space framework upon which to spread the plastic sheeting. Access is provided by a galvanized steel-framed double-zip door wide enough for a wheelbarrow.

 Tension on the plastic cover is maintained by a winch set at the back of the GROWhome, and designed to draw the entire sheet together on a reel.

The steel pipe frames are secured into the ground with screw anchors that also act to hold the webbing over the whole structure. Everything is therefore flexible and adjustable according to prevailing conditions.

To allow ventilation on hot days the sides are easily rolled up and held in place.

When Nina developed GROWhomes they were designed to be taken down at the end of each winter and moved to fresh soil. 

“It was much easier than changing the soil by moving barrow-loads into the greenhouse! We have found that is the great advantage of a GROWhome – we can set them up for a customer within a couple of hours.

“They are actually very simple to erect as it only involves assembling the frames, which come in three parts for ease of shipping. Just a couple of screws are needed at each joint. Then you put in the screw anchors, using the pattern supplied to locate them.”

Nina says the last step is to put on the cover (which comes with door and winch factory-fitted), tighten the winch and the webbing/ventilation system and, hey presto, the GROWhome is ready for plants.”