Glasshouse: what to grow, how to care and essential tools

Dianne Bush's glasshouse in Hamilton has a lovely story behind it.

In cold climates, growing under cover allows you to extend the growing season by starting your crops early and giving them an almost endless summer. New glasshouses are costly but glass has advantages over plastic, longevity being one of them. Ultraviolet light won't cause your panes to fall to bits, as plastic does over time, but broken glass panes can be expensive to replace. Plastic tunnelhouses generally cost less and are easier to transport and install. Or build your own glasshouse (or a cold frame – pictured – for smaller gardens) from old windows. 

Big isn't always better, but a too-small glasshouse is a source of frustration. You don't want to have to stoop because of a lack of headroom. When planning a new garden – or looking for space to squeeze a glasshouse into your existing one – find an open spot that gets all-day sun. 

What can I plant in my glasshouse?
The best thing to plant at this late date will be fast-producing greens that can be picked leaf by leaf and added to your winter dishes. Leave the door open: these vegetables are not tropical flowers, but they'll benefit from the shelter and warmth of the glasshouse. If you like the idea of growing something over winter but don't want to fuss over it at all, sow a green crop. Lupins are hard to beat for greening up the indoor space and are easy to sow and grow. They'll enliven the soil and add nitrogen to give your spring-sown vegetables a great foundation for growth. 

If your plants end up looking over-exposed – burned leaf margins are a giveaway – whiten a few window panes with paint.

If you plan to leave your glasshouse bare, rake it to clear vegetation. Use a brush to whisk away webs from the walls, roof and framework of the structure. And wash everything down to rid the space of hiding places for mites, slugs and other creatures that might like to overwinter indoors. Give the soil a good long soak, preferably with water that hasn't been treated with chlorine. This will wash out salts that have accumulated during spring and summer and leave your soil ready for another round of supporting healthy crops. Doing this task in winter can leave the soil soggy for too long.

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A glasshouse allows you to extend the growing season by giving them an almost endless summer.
PAUL MCCREDIE

A glasshouse allows you to extend the growing season by giving them an almost endless summer.

What else do I need in my greenhouse?

1. Irrigation. The downside to growing under cover is that it can become deadly dry. The safest way to ensure your plants don't die of thirst is to install automated irrigation. 

2. Revitalise the soil, as it can go stale if it isn't carefully monitored. Add revitalised soil at the start of spring and keep it ticking over naturally by inviting the outside world in.

Collect rain water for irrigating, build humus-rich soil, and sow winter cover crops like broad beans.

If you don't want to plant directly into the soil floor of your glasshouse, grow vegetables in pots or bags of potting mix instead.

Tomatoes and other edibles can be grown in a glasshouse beside an outdoor vegetable garden.
PAUL MCCREDIE

Tomatoes and other edibles can be grown in a glasshouse beside an outdoor vegetable garden.

3. Good ventilation. Heat and protection from rough weather means we get to eat various exotic plants, but a hothouse can become an oven if it's too well sealed.

Ventilation is the key to success so get into the habit of opening the door or flipping open roof vents. Heat, plus moisture, attracts moulds and fungi. Let the air flow freely through your glasshouse to move excess moisture and unwanted fungal spores.

But at the same time.... 

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4. Store up heat. Install a heat storage unit such as a barrel of water or a stack of bricks that will hold the heat of the day and radiate it out again through the night. This will help keep the cold at bay.

You could also cover your plants with sheets of newspaper or bags when frosty weather threatens. 

Edible plants often do well in a glasshouse.
Paul McCredie

Edible plants often do well in a glasshouse.

5. Build bench-height raised bed for veges. Bench gardens under glass are far from common these days, but why let that stop you from having one? They are great for gardeners who don't bend easily in the middle. Assess the best height for your reach and flexibility, fill it up with good quality soil and start sowing seeds.

 

Egyptian walking onions grow under an old grapevine in a glasshouse.
SALLY TAGG

Egyptian walking onions grow under an old grapevine in a glasshouse.

 - NZ Gardener

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