A TUNNEL HOUSE IS NOT A GLASSHOUSE
If you’re purchasing a tunnel house for the first time, it pays to swot up on how to grow in this unique environment.
When Keith Olsen purchased a Morrifields tunnel house in the winter of 2020, he thought all his under-cover growing Christmases had come at once. For years, he’d been growing in two traditional glasshouses, and although they did produce a reasonable harvest in a good South Otago summer, it was nowhere near the bucketloads of tomatoes, cucumbers, chillies and capsicums that his tunnel house-growing neighbours were boasting.
Having made the purchase, he erected the tunnel house over the winter, carefully levelling the earth floor in order to avoid the risk of pooling water. He filled the beds inside with rich compost sourced from his local freezing works, and started off his seedlings indoors in sterilised seed-raising mix to avoid bringing any disease or pests into the structure. It should have been a recipe for success, but difficult days lay ahead.
Having previously experienced the curse of fungal disease in his glasshouse plants, Keith was sure to space his tunnel house seedlings carefully to allow for plenty of airflow. Once they were in place, he mulched around the base of the plants to avoid moisture evaporating from the soil.
A SLOW SUMMER
As often happens in the deep south, the summer was very slow to arrive – in fact, temperatures were downright winter-like. There was little in the way of growth in the tunnel house, especially among the heat-loving capsicums and eggplants, and there was no sign at all of fruit setting on the few flowers developing on the tomatoes.
Knowing that temperature is all-important for fruit-setting in tomatoes, Keith decided to tackle the problem of upping the heat in two different ways. Firstly, he removed the mulch from the floor of the tunnel house to allow the sun to heat up the soil as much as possible, and secondly, he dealt with the ventilation.
Up until this point, Keith had been leaving all the tunnel house vents (door, roof and window) open during the day. At night, he’d been closing the door and window, and leaving the roof vent open. Now, he closed everything down after dark. During the day, he opened the window just a few centimetres, half-opened the roof vent, and kept the door closed. The change to this plan certainly increased the warmth inside the tunnel house, but the results led to near disaster.
MOISTURE MADNESS
Keith was initially pleased with the results of his heat-retaining efforts. In the morning, the interior of the tunnel house was much warmer than it was outdoors. When he stepped inside the structure during the day, it smelled deliciously jungle-like and was so warm that the environment seemed almost steamy. But there were also problems: the tomatoes still weren’t setting fruit, and, apart from the cucumber plants, nothing else was thriving. On top of that, fungal fruiting bodies were beginning to pop up here and there in the soil, and the inside of the tunnel house was dripping in moisture in the mornings and through a large part of the day.
Keith dealt with the moisture by giving the inside of the plastic skin a good tap all over, something that sent a shower of drops onto the already wet plants below. He then went around each of the plants in turn and shook them to remove any droplets from their foliage. Unfortunately, this shaking backfired as several tomato plants developed cracks in their succulent stems from the movement, and fungal disease quickly set in, putting paid to the vines.
STRANGE SYMPTOMS
It wasn’t long until strange and worrying symptoms began to appear in the tunnel house. The first of these was very confusing. Peculiar bumps began appearing along the stems of the tomato vines. over a period of two weeks, these bumps erupted into aerial roots that grew in tight clusters to a length of 1–1.5cm. As well as this, more fungi were now growing in the tunnel house soil, and a grey mould had invaded not only the foliage of the tomato plants, but their flowers and the few fruit that were beginning to develop. Dozens of flowers had damped off and were showing signs of mould damage.
Keith quickly realised that the fungi and mould were caused by high humidity, and that by removing the mulch from around the plants, he had almost certainly allowed excess moisture to escape from the soil into the interior of the greenhouse. He knew more ventilation was important, but it was still so cold outside. He felt that if he were to open the vents wide and leave the door open day and night, fruit wouldn’t set because of the low temperatures. It was a quandary.
Then a friend arrived and identified the strange stem root growth problem – and it wasn’t what he wanted to hear.
ACTION STATIONS
Although it was too late to save the day entirely, Keith took remedial action. Even though outside temperatures were still cool in January (days were feeling more like early spring than summer), he left the tunnel house window, roof vent and door open day and night. He clipped off all foliage and flowers affected by grey mould, trimmed foliage back by a third to create extra airflow, and trained plants off and away from the plastic skin of the tunnel house. He returned mulch to the floor of the tunnel house, and kept removing any flowers and leaves affected with grey mould as soon as he noticed its presence. Slowly, as summer temperatures finally arrived in mid-february, the greenhouse tomatoes began to hold their own. By March and April, the few that were mature began to ripen.
LESSONS LEARNED
Although it has been a steep learning curve, Keith has some valuable take-homes from the season – ones he’ll put into practise this spring. They are:
• A tunnel house is not a glasshouse. It doesn’t have the same small gaps in its walls and roof that allow for ventilation, and so it gathers humidity more readily.
• Heat radiates back out of a glasshouse faster than it does through the opaque-plastic of a tunnel house, so expect a tunnel house to require more humidity control.
• If outside temperatures are still cool, even in summer, you may need to leaves doors and windows closed at night and during the coldest parts of the day (but leave the roof vent open). If this is the case, use a small fan heater (safely wired in by a qualified electrician) to promote airflow around plants, especially in the evening and early morning. This will help reduce humidity to a safe level.
• Train all vines away from the sides and roof of the tunnel house (where they remain wet if they come into contact with the plastic skin).
• Mulching prevents evaporation from the soil and reduces humidity – but it also prevents the soil from heating up. Be guided by outside temperatures as to whether or not to mulch.
• If you feel you need to remove moisture from a tunnel house plant’s foliage, gently tap the leaves. Never shake the stems.
• Purchase and use an inexpensive moisture probe so that you are not unnecessarily watering the beds and thus introducing more moisture than is required to your tunnel house.