Preparing Your Tunnel House for Tomatoes and Other Heat Lovers

There's something wonderfully optimistic about preparing the tunnel house in late winter and early spring. A task to complete in the Barren Phase of the Moon in August or September.

Outside, the garden may still be dealing with frosts and cold soil, but inside the tunnel house we're already thinking about tomatoes, cucumbers, capsicums, chillies and eggplants.

And this is exactly the right time to get organised.

The temptation, of course, is to rush out and buy the first tomato plants we see. But before we plant anything, there is some important groundwork to do.

Start with a Good Clean

First, clear out anything left from last season.

Remove old tomato, cucumber, capsicum, and aubergine plants, fallen fruit, and weeds. Anything that showed signs of disease is best removed from the garden rather than added to your compost.

Then give the tunnel house itself a clean.

Wash down the inside of the plastic or glass, clean the framing, benches, and trays, and give your pots, stakes, clips, and tomato supports a clean before you use them again.

It's also a good opportunity to check irrigation lines and drippers. Far better to discover a blockage now than when your tomatoes are relying on them.

What Grew Here Last Year?

Before adding anything to the soil, think back to last season.

• What was growing in this bed?

• Did you have problems with disease?

• Were the plants vigorous but produced very little fruit?

• Did they struggle?

• Have tomatoes been growing in exactly the same soil for several years?

Crop rotation can be difficult in a tunnel house because, for many of us, tomatoes are one of the main reasons we have one.

If you can't rotate your crops, don't panic. It simply means that soil health, hygiene, and good growing practices become even more important.

Feed the Soil, Rather Than Digging It

I'm a great believer in disturbing the soil as little as possible.

There's no need to dig the entire tunnel-house bed over each spring.

Remove persistent weeds and leave healthy old roots in the ground where practical. They will gradually decompose, feeding soil organisms and leaving tiny channels through the soil.

If an area has become badly compacted, gently loosen it with a garden fork without turning the soil over.

Then add around 2–5 cm of good mature compost to the surface.

One thing we often forget about tunnel-house soil is that it hasn't been receiving winter rain.

Put your hand into the soil and check.

You may discover that while the surface looks beautifully dark and moist, just a few centimetres underneath it is surprisingly dry.

If it is, give the beds a really good, slow watering before you start preparing them for planting.

Tomatoes Are Hungry — But More Isn't Always Better

Tomatoes are hungry plants, but that doesn't mean we need to throw everything we've got at the soil before planting.

Too much nitrogen can produce magnificent, lush, green tomato plants with lots of leaves and surprisingly little fruit.

Good additions can include:

• mature homemade compost

• well-rotted manure in moderate quantities

• worm castings

• a balanced organic fertiliser, if your soil actually needs it

I'd be much more cautious about adding large quantities of fresh manure, poultry manure, lime,, or wood ash simply because it's spring.

If possible, test your soil first.

Tomatoes generally prefer a slightly acidic soil, somewhere around pH 6–6.8.

Rather than feeding by habit, let's start learning what our soil actually needs.

What About Calcium?

Blossom-end rot inevitably brings up the subject of calcium.

Those horrible dark patches on the bottom of otherwise beautiful tomatoes are often described simply as a calcium deficiency.

But that doesn't necessarily mean there isn't enough calcium in your soil.

Quite often, the problem is that the plant hasn't been able to take calcium up consistently, particularly when the soil has gone from very dry to very wet and back again.

One of the best things we can do is maintain consistent soil moisture.

Improve the soil with organic matter, water deeply and evenly, and once the soil is warm, mulch around the plants to help prevent those dramatic fluctuations in moisture.

Don't automatically add lime or other calcium products unless you know your soil needs them.

Get the Infrastructure Ready Now

An empty tunnel house is a wonderful place to work.

Use this time to get everything ready before the plants arrive.

Put up your tomato strings or supports. Install cucumber frames. Clean your stakes. Check the irrigation. Tidy the paths. Make sure doors and vents are working properly.

And if you haven't already got one, buy yourself a soil thermometer.

It is one of the simplest and most useful tools for a spring gardener.

Don't Be Fooled by Warm Air

This is particularly important for those of us gardening in cooler parts of New Zealand.

Walk into a tunnel house on a beautiful sunny spring afternoon, and it can feel like summer.

But put your hand into the soil and it may tell a completely different story.

Heat-loving plants need warm soil as well as warm air.

As a useful guide:

• Tomatoes: wait until soil is consistently around 15°C or warmer

• Capsicums and chillies: happier around 18°C+

• Eggplants: ideally around 18°C+

• Cucumbers: preferably around 18°C+

Take your soil temperature in the morning rather than relying on the temperature during a sunny afternoon.

What we're interested in is consistent warmth, not one unusually warm day.

Don't Mulch Cold Soil Too Soon

I love mulch, but there is a time and place for it.

In summer, mulch is invaluable. It helps conserve moisture, protects soil life, suppresses weeds and reduces fluctuations in soil moisture.

But in early spring, a thick blanket of mulch can actually prevent the sun from warming the soil.

So I prefer to let the beds warm first.

Once the soil is warm, the plants are growing strongly,

and temperatures are rising, that's the time to bring the mulch back.

Ventilate, Even When It's Still Spring

It is extraordinary how quickly a tunnel house can heat up on a sunny spring day.

Good ventilation helps reduce condensation and lowers the risk of botrytis and other fungal diseases. It also helps prevent plants from producing excessively soft growth.

Open doors and vents when conditions allow, but remember to close them again as temperatures fall.

Here in Central Otago, a gloriously warm afternoon can still be followed by a very cold night.

And Finally — Don't Rush the Tomatoes

This is probably the hardest advice of all.

The garden centres start filling with beautiful tomato plants and suddenly it feels as though we're already behind.

We're not.

A tomato planted into cold soil doesn't necessarily get a head start. It may simply sit there, stressed and unhappy, waiting for conditions to improve.

A tomato planted a little later into warm, biologically active soil can establish quickly and often catch up remarkably fast.

So rather than planting according to a date on the calendar, watch your own garden.

This is where our garden diaries become incredibly valuable.

Record:

• when you cleaned the tunnel house

• when you added compost

• your soil pH

• morning soil temperatures

• when the soil first consistently reached 15°C

• your last significant frost

• tomato planting date

• first flowers

• first fruit

• first ripe tomato

After a few seasons, you'll begin to build up a picture that no seed packet or gardening book can give you.

You'll know when your tunnel house, in your garden, in your particular microclimate, is ready for tomatoes.

And that is really what gardening with the seasons is all about.