Greenhouse Growing Guide

If you love a supply of leafy greens all year round, and enjoy growing your own veges, it doesn't have to stop now that the cold weather has set in. You just need to switch to going under cover. Usually the word ‘greenhouse’ makes you think of an expert gardener or a professional grower but greenhouses come in all shapes and sizes and are great for any gardener, regardless of experience.

Especially if you like to grow your own food, greenhouses can extend the growing season of plants that would not usually thrive over winter and allows you to sow seeds earlier than usual. For the thrifty gardener, greenhouses provide the ideal environment for propagating plants from seeds or cuttings.

You don’t have to go all out and get a large greenhouse, if you just want to create a warm, sheltered area for some specific plants, cold frames or cloches are a great option too. You can purchase these relatively cheaply from your local garden or hardware store or even DIY your own.


Greenhouses come in all shapes and sizes so think about what you want to grow, how much you want to grow and how permanent you want your greenhouse to be before investing. Talk to your local garden centre or a greenhouse supplier about the different materials available and which will best suit your needs. Glass provides longevity but doesn’t work as well as polycarbonate in frost-prone areas. Polyethylene is a good low cost option but will last only around five years.

If you start your seeds in a greenhouse, you can start them a good six weeks earlier than usual and transplant them to a cold frame or cloche in the garden. Your greenhouse should be in the sunniest spot in your garden and ideally in a sheltered position away from strong winds.


Cut off, recycled clear plastic bottles can create an instant cloche for an individual plant, which can easily be removed during a sunny day and put back when the temperature drops in the afternoon. Tunnel cloches made from wire rings and plastic covers warm the soil for seedlings and protect from frost. They are super versatile as they can be adjusted according to what you are growing and can double as protection from birds and insects. Remember when using the cloche to grow seedlings and plants over winter the cover should always be transparent.

Cloches are the cheapest option and are super easy to use. During the day cloches can be removed, or lift the sides to give the plants a chance to breathe and harden off. Replace or pull down the sides at the end of the day.

Cold Frames

Cold frames are a great option and you can whip up your own DIY cold frame or purchase one. You can create a great looking cold frame with recycled bricks and an old window frame. Or go for a portable cold frame that you can move around the garden as you need to.

Put your cold frame in a nice sheltered position where the cover of your cold frame slopes towards the sun. The cover needs to be able to open to give ventilation to the plants, leave it open during the day and close at night.

Seedlings in pots

Once your seeds have germinated in your greenhouse or the warm sheltered spot you’ve grown them in, transplant them into larger pots, or plant directly in ground in your cold frame. Seedlings grown in a cold frame are less likely to get transplant shock as they are better acclimatised and the cold frame helps with hardening off the seedlings.

If you are planting in ground in your cold frame, ensure the soil is moist as the soil in your cold frame can dry out quickly so check and water the soil regularly. If it is particularly cold when you set up your cold frame, allow 1-2 weeks for the soil in your cold frame to warm up enough to allow planting.


Sowing into seed trays in a greenhouse:

  1. Fill the seed tray with Tui Seed Raising Mix up to 1cm from the top.
  2. Tap the container gently on the ground to settle the mix.
  3. Water lightly to ensure mix is moist.
  4. Sow the seeds as directed on the packet.
  5. Lightly cover with Tui Seed Raising Mix.
  6. Cover your container with glass or a light plastic bag to help retain moisture and warmth. Although it’s in a greenhouse covering is still important to retain enough moisture for germination. Remove the cover as soon as the seed germinates.
  7. Place in a warm spot in the greenhouse. The seeds do not need much natural light until they have germinated.
  8. Water lightly and regularly. Avoid over watering as this can cause the seedlings to rot.

Planting seedlings grown from seed in the greenhouse or purchased for the cold frame:

  1. Once the seedling has two sets of true leaves they are ready to be transplanted into small pots.
  2. If you are planting from seedlings you have bought, gently remove from their container and loosen the root ball.
  3. Partly fill your container with Tui Vegetable Mix.
  4. Position the plant in the container. If planting multiple plants, space accordingly in the container.
  5. Fill your container with Tui Vegetable Mix up to 3cm from the top.
  6. Press soil gently around the base of seedling.
  7. Soak planted pots in a bucket of Seasol plant tonic to help reduce transplant shock.
  8. Once your cold frame is set up place the seedlings in their new containers inside the cold frame.
  9. Ensure that the seedlings have adequate ventilation during the day but the cold frame is closed overnight.
  10. Water regularly.
  11. Once your seedlings have got good growth on you can now move them into the garden as they will be hardened off.

Tui Tips

  • Plants should only be covered when they need to be protected as the covers prevent fresh air, rain, pollination and often sun which are all essential to healthy plants.
  • While covers are on try to water in the morning so that the plants are not damp at night, and try to water the soil, not the leaves to reduce the chance of diseases.
  • Don’t leave your plants under the cold frame indefinitely as they need to be exposed to air, rain, pollination and sun.
  • Once plants are hardened off, remove them from the cloche or cold frame.

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Greenhouse Growing Guide Comments

  • I'd placed several sowed seeds in jiffy pots in the green house but less than half of them germinated. Of the half that germinated, most of them died within a week before any true leaves appear. So I think the greenhouse is not suitable for housing seed trays or jiffy pots?

    Janet Jin

    • Hi Janet, the greenhouse might be too hot to raise seedlings in at the moment, especially if there is not shade or protection on the roof.

      Seed needs to be fresh before sowing, and need to be stored in a cool dry place where no moisture can come into contact with the seeds, this will keep them viable. The older the seed is and as each year goes by the viability of the seed reduces depending upon how it has been stored.

      Try some shade over the glass house and make sure there is good ventilation so that the seedlings don’t “cook”.

      Tui Team

  • I have a greenhouse that was converted from a garage we never used. It has the original roof but the sides have been replaced with corrugated polycarbonate. It's in the full sun and gets warm even on cold days. I have had no luck with any of the green leaf vegetables I've planted. Do you think it is too warm for them? I live In Christchurch.

    Lesley Smith

    • Hi Lesley, it could be getting too hot especially if the polycarbonate is clear. It may need a shade cloth put up in summer or paint the polycarbonate with a very watered down white paint to filter the bright light. You haven't said what is happening to the vegetables  - are they not growing? Are they spindly? Have they been planted in a quality potting mix? These are some factors to consider. 

      Tui Team

    • Thanks for your reply. Nothing really happens to the vegetables: they just sit there, looking healthy, not spindly, but not growing or developing into whatever they are supposed to be: broccoli, kale, lettuces, etc. The broad bean plants have grown (they are tied up as recommended), but there are no flowers and no beans. I use good quality potting mix and water the plants regularly. I have stated leaving the doors to the greenhouse open all the time to encourage insects to go in and to stop it getting to hot. I am a keen gardener but mostly with shrubs and flowers, this is the first time I have ever had a greenhouse it is turning out to be a steep learning curve for me.

      Lesley Smith

    • If you have space try planting half outside in the garden and half in the greenhouse, to see if it is the conditions in the greenhouse. Also try using Tui Organic Seaweed Plant Tonic. It is great to use to get plants moving and stimulate root growth and improve overall plant health. Use this weekly at a rate of 70ml per 9L watering can. Tui Mini Sheep Pellets would also be ideal for your greenhouse, they are a smaller pellet and easy to apply.

      Tui Team