A beginner’s guide to growing under cover

What you need to know if you’re planting in a greenhouse for the first time.

1 Plan your planting

You probably have a list of dream plants that you want to grow. A common issue for many beginners is realising your dream list is too big for your greenhouse. Too many plants and you’ll have:

■ too much shade;

■ too much humidity;

■ perfect conditions for pests, diseases, mould, and mildew.

It’s a good idea to map out space for plants (horizontally and vertically), and think about placement, eg tall plants on the southern side.

2 Create space

If you want an area for germinating and growing seedlings in late winter, you’ll need to include a workbench and shelving.

Tip: build a bench heater

This space-saving idea gives you a workbench and helps to keep the greenhouse warm. Fill a line of 200-litre black drums with water, then top with plywood to act as a bench. The drums absorb heat during the day and release it overnight.

3 Measure

You need at least a basic thermometer to measure the temperature. Better is a thermometer with a hygrometer to check humidity levels. You can also get digital climate stations that track temperature and humidity 24-7.

Tip: how to create the perfect climate If a greenhouse gets too hot and humid, open the vents, prop open the door, and cover the roof with shade cloth. If it’s awful, use an exhaust fan to create airflow.

If humidity is too low, wet down the flooring (eg, gravel), any other bricks and stones, and shelving. Misting is another way to increase humidity levels.

4 Strategies to keep diseases & pests at bay

■ Don’t bring tender or sickly plants into a greenhouse over winter – this is a common entryway for pests and diseases.

■ Only start seeds inside – don’t bring in cuttings or seedlings you’ve bought as they may carry the nearly invisible whitefly and its eggs.

■ Disinfect seed trays and pots between batches, and only use a sterile seed raising or potting mix.

5 7 tips for sowing

The beauty of a greenhouse is you can start your spring garden 6-8 weeks before your last frost.

■ Invest in commercial propagation trays. A 60-cell plastic tray is 505mm long by 305mm wide, and the cells are large enough to start a seed and raise it right through to transplant size. Larger, sturdy trays are much easier to move than lots of little punnets and will last forever. Individual cells also hold more moisture and keep roots warmer.

■ If you’re re-using trays, wash them out with the hose, then disinfect them in a mix of 9 parts water to 1 part bleach.

■ Always use fresh seed raising mix to avoid introducing disease.

■ Fill trays to the top, and brush away extra to get them level. Press a seed into the top of each cell, lightly but firmly, so there’s an indentation from your fingertip – you may still see the seed.

■ Spray seeds with a fine mist – do it at least three times every five minutes or so to ensure the soil is moist right through.

■ Cover. Some propagation trays have ready-made lids. Alternatives are plastic wrap from your kitchen, clear plastic sheeting, or glass.

■ Keep trays in a warm, dark place until the seeds germinate. Even in a greenhouse, the night-time temperature can fall to only a few degrees above what it is outside – a heat mat can help keep the temperature in the ideal range (20-25°C), and is also helpful if you want to grow microgreens.

6 4 tips for growing seedlings Once seeds have germinated, it’s critical to give them the best conditions.

■ Keep them in full sunlight from dawn until dusk. If they’re not getting enough light, seedlings will be pale, spindly, and fall over. Healthy seedlings have dense tissue, good colour, and will look ‘stocky.’

■ Keep them warm. You may want to keep them on heating mats and/or cover them with frost cloth on cold nights to keep the temperature a few degrees higher.

■ Watch your watering. As temperatures get warmer, seedlings may need multiple waterings a day as propagation cells can dry out quickly. The soil should feel slightly damp. Too wet, and there’s not enough oxygen for growing roots. Too dry and plants can quickly die.

■ Feed little and often. Once seedlings have their first true leaves, they need regular food. It may be provided by slow-release fertiliser in your seed raising mix, or you may need to supply a liquid fertiliser.

7 Learn how to fight whitefly

One of the most annoying pests in a greenhouse is the whitefly. They suck sap from plants which turn leaves yellow and sickly, and spread viruses. They also produce honeydew, which then becomes infested with sooty mould.

■ Practice good biosecurity (see tip 4), and control weeds.

■ Monitor with sticky yellow traps at the height of new growth, so you know there’s a problem as soon as possible – have at least one right by the door.

■ Quickly repair any rips or holes in a wall.

■ Use netting over the door and vents.

■ At the end of the growing season, remove the remains of crops and leave the greenhouse locked up for at least a week in hot weather, 2-3 weeks in cooler weather, to kill any adults that may be inside.

■ You can use chemicals to control whitefly, but they’ll develop resistance to them if used too much. Alternatively, you can buy parasitic wasps (Encarsia formosa) which eat the larva.

Read more

What to think about if you’re buying a greenhouse for the first time thisnzlife.co.nz

Keep a greenhouse diary. Make notes of everything you do, so you can change strategies from year to year, and keep track of what plants have been in which areas, which varieties have done well, etc.