Greenhouse growing guide

Come rain or shine, whether you are a seasoned grower, or a complete novice, here is an idea of what you can get up to in your greenhouse all year round. We have advice on what vegetables and fruits you can grow, and the ideal temperatures for your seedlings to thrive.

January

The gardening calendar starts off quietly and the beginning of the year is a good time to snuggle down in your favourite armchair with a selection of gardening books and seed catalogues to gain inspiration as to what you want to grow. No doubt you will discover many new varieties that you will want to experiment with but do remember that the plants you decide to grow must be able to happily co-exist side by side – a multitude of plants all requiring widely different temperature, light and humidity levels are unlikely to thrive, unless the greenhouse has been designed from the outset to have separate partitioned zones in which different growing environments can be maintained. 

This quiet time ahead of the main growing season provides the perfect opportunity to check over the aluminium greenhouse and make sure that all is functioning as it should, particularly once you have had your greenhouse for a while. Our ‘Clean & Care’ service is the ideal way to ensure your greenhouse is in tip-top condition and looking at her best for the spring – please contact us on 01730 826900 for further information.

Frost-sensitive plants do not need a lot of heat in the winter months. A good rule of thumb is to aim for a minimum temperature of 5 - 7°C (42 - 45°F) – certainly there is no need to maintain temperatures higher than 12 - 15°C (55 - 60°F). During the night you may well find it necessary to cover frost-sensitive specimens with a horticultural fleece or even straw or newspaper if night frosts are forecast. Installing a min-max thermometer will help you keep track of temperature and ensure that any heaters are working as they should.

To protect plants from disease inside the greenhouse, keep the atmosphere dry and water sparingly unless plants are in flower. It is good practice to water early in the day and keep water directed to the roots where it is needed – don’t splash the floor, benching or foliage with excess water as this will only increase humidity inside the greenhouse and the risk of pests and mould. Now is the best time to spray to combat grey mould or whitefly should either of these be present. Keep an eye on plants and remove any dead flowers, as well as yellowing or diseased leaves.

Despite the colder weather, the greenhouse will still need to be ventilated but do take care – keep the side vents closed on damp, foggy days and use them only on dry and sunny days. Open them in the morning and close mid-afternoon so as to conserve the heat of the sun before nightfall.

If you have early sowings of sweet peas or onions, now is a good time to pinch them out and plant them into larger pots. This is also a good month to take cuttings of tender plants. Make sure any compost used for sowing or potting is brought into the greenhouse to warm up several days before it is used to prevent seeds and young plants being chilled.

February

This month sees the days start to lengthen gradually and therefore you can slowly begin to increase the frequency with which plants are watered; nonetheless the soil should be kept on the dry side through the whole of the period from November to February. Wherever possible, plants should be watered by standing the pots in a bowl of water and leaving them until the surface of the compost glistens; the pots can then be removed and allowed to drain before returning them to the bench top. If watering plants from above, place the spout of the watering can under the leaves and try to water before midday.

Alpine houses can be well ventilated now, but for all other plants continue to maintain a dry atmosphere inside the greenhouse and ventilate on bright days only. The outside temperature may be improving compared to December and January but try to avoid exposing plants to fluctuations in temperature and aim to keep a relatively narrow temperature range of 5 - 15°C (42 - 60°F) inside the greenhouse. Keep frost-sensitive plants covered if you find it hard to maintain a minimum temperature of 5°C (42°F), especially on cold nights.

You can sow a bit more this month to get a head start before spring – good ideas ready for spring picking are early carrots, salad leaves and spinach. February is also the time to get potatoes chitting. Remember to give all your seed trays and pots a thorough scrub and make sure they are sterilised before use with a plant safe garden disinfectant. Sow seeds thinly in damp (not wet) compost and keep the container covered until germination has taken place; our propagator is the ideal location. Don’t forget to label all your seed trays clearly too – an all too easy thing to overlook!

If you have a soil bed inside your greenhouse, prepare this well by digging in good quality garden compost or very well rotted farmyard manure.

Over-wintered fuchsias and pelargoniums can also be kick-started this month if the greenhouse is protected against frost – pot them on, increase the watering and start to feed them to encourage growth.

Now is also the time to prune conservatory climbers such as bougainvillea and passiflora.  

March

This is traditionally the month that comes “In like a lion and out like a lamb” and greenhouse gardeners know this better than anyone! Although this is a great month for the greenhouse when things really get going with plenty to do, early spring can bring wide temperature fluctuations which can be a problem – a bright sunny day at the end of the month can result in the greenhouse overheating. Aim to keep the air temperature at around 7 - 18°C(45 - 65°F); this will involve heating the greenhouse at night and also ventilating and damping down the structure on cloudless days if required. Keeping the door and vents open more frequently as the month progresses helps to keep humidity levels down and prevent disease.

If your greenhouse is fitted with Bayliss roof vent openers, they may seem erratic as they do take time to adjust to the temperature fluctuations at this time of year. If you are concerned, please give us a call on 01730 826900.

You can also find plenty of useful advice from John Wood, Head Gardener at National Trust Hinton Ampner in Hampshire, about sowing seeds, as no doubt this will be your main focus in the greenhouse this month. Large-seeded, large-leaved vegetables such as cucumbers and courgettes can be planted into their own individual pots and placed in a propagator. In almost a month from now, the pots can be moved into the main body of the greenhouse for an early July harvest.

Seedlings need good light but young ones will still need protecting from the midday sun. If you do not have shades fitted to your greenhouse and find you need some – or additional ones – please contact us and we will be happy to advise you further.

Growing plants should be fed with a liquid fertiliser but take care not to overfeed your plants. A balanced formula should be used for leaf growth whilst a high potash feed should be used for flowering plants. Remember to keep actively growing plants damp but not soaking wet. The Hydroleca on your bench top helps to retain moisture.

Insects can start to be a problem this month – keep watch for greenfly, whitefly and red spider mite in particular. If evidence of bugs is found, spray with a greenhouse insecticide or investigate organic alternatives before the problem gets out of hand, as organic predators need to be ordered during early spring.

Pollinating insects are not yet present, so nectarine, peach and strawberry flowers must be pollinated by hand. Try to do this in the middle of the day and repeat for a few days to ensure success.

March is a good time to prepare hanging baskets for setting out in late May or June for a vibrant splash of colour. Fuchsias and pelargoniums can have their growing tips pinched out this month to encourage strong bushy plants.

April

Hopefully this month brings warmer weather with it, in which case you will need to keep your greenhouse ventilated. A good aim is to keep the temperature inside the greenhouse between 7 - 21°C (45 - 70°F). The roof vents should provide the majority of the ventilation, with the side vents only being used once the outside air is warmer. 

As the temperature increases, your plants will become thirstier and you will need to check regularly to see if watering is needed. Plants that are actively growing may well need watering several times a week but remember over-watering can be as much of a hazard as under-watering! 

Depending on the plants you are growing, the warmer temperatures may mean that the need to heat the greenhouse is reduced this month. Remember, however, that young plant growth is especially susceptible to low temperatures so check that heaters in the greenhouse are working whenever a frost is forecast. 

Find out what to sow and grow this month in our April Growing Guide.

May

Your greenhouse is more than likely to be packed with vibrant healthy plants this month, so you can look forward to spending more time in your greenhouse to stay on top of watering and ventilation to keep them in tip-top condition. To find out how to keep your greenhouse at the optimum temperature, what to sow and harvest this month and how to support your tomatoes, check out our growing guide here.

This month also heralds the start of the social season with the RHS Chelsea Flower Show traditionally taking place in the third week of May. If you have already ordered your tickets from the RHS why not pop along and visit our stand whilst gathering inspiration and replacing your seed and bulb stocks at the world’s greatest garden show? We’d be delighted to see you!

June

The weather may well be hotting up but now is the time to keep your cool!

With the sun high in the sky, sun scorch can be a problem this month. Tell-tale signs are growth slowing down, flowers losing their colour and leaves showing signs of scorch. To combat this, you will need to make full use of your shades.

If your greenhouse is unheated, summer crops should be doing well. Melons and cucumbers should now be being trained and side shoots on tomato plants should be pinched out. Other crops such as chillies, aubergines and salad should also be growing well but do keep a watchful eye out for pests.

As an alternative to vegetables, your greenhouse can be used as a flowering display house. Ideal plants for this purpose include: coleus, pelargoniums, foliage begonias, small sunflowers, celosis – the list is endless!

Download our growing guide here to find out more on what to grow and how to care for your greenhouse this month.

 

July

As summer reaches its peak, the main challenge facing greenhouse gardeners this month is preventing the greenhouse from getting too hot. Ventilation is vitally important and you will have to make sure there is good air circulation inside the structure during the night as well as during the day. Whilst the high air temperatures will cause soil to dry out quickly, watering must be carried out without fail and it is essential that compost is checked every day to ensure it does not dry out.

Remember to watch like a hawk for pests and diseases. Red spider mite, greenfly, whitefly, thrips, grey mould and mildew are all prevalent at this time of year and combatting them is a major focus for the greenhouse gardener. 

This month is also a good time for taking cuttings as roots will form quickly in the warm conditions.

Read July's Growing Guide in more detail here.

August

With plants in flower and salad crops at their peak, August provides the ideal opportunity to take stock and reflect on your successes (and not so successes!) and learn from these when making next year’s plans. You can also use this part of the year to collect and store some seed from your own plants over the next few weeks.

Find out what other jobs to do this month in our downloadable Growing Guide.

Thinking ahead, now is a good time to contact Alitex to ask us to quote for an annual Clean and Care service of your greenhouse – we can book you in for either this autumn or next spring to best fit in with the greenhouse gardening season.

September

This month can be of mixed fortune, bringing a mix of warm and colder weather and you will need to control the temperature inside your greenhouse carefully as a result. Cooler temperatures will also reduce the need to damp down. 

Plant growth also becomes less active as temperatures fall and compost stays moister for longer; you can therefore reduce the frequency with which you water. Keep watch for fungal diseases however, as they tend to appear this month.

Find out what other jobs to do this month in our downloadable Growing Guide.

October

You may find this month that you will need to introduce heat into your greenhouse as the outside temperature starts to drop. As a good rule of thumb, you should aim to maintain a minimum night-time temperature of 5 - 7°C (42 - 45°F) if you intend to keep your greenhouse frost-free. If frost is forecast, you may need to protect tender plants with a horticultural fleece or newspaper.

As winter approaches, make sure that all half-hardy plants have been brought into the greenhouse. In colder parts of the UK you may wish to insulate your greenhouse using bubble wrap. Find out what else you can be sowing and planting in October with our downloadable Growing Guide for October

November

November is a good time to give the greenhouse its annual clean and tidy up – the other opportune time is spring. Choose a bright, dry day and allow a day for the task as there will be lots to do! Place pots in a safe place in the garden and move tender plants indoors. 

Sarah Wain’s blog on our website is full of handy tips and tricks on cleaning your greenhouse – her enthusiasm for maintain a spotless greenhouse is boundless!

If the task is just too daunting or time is too limited, give us a call and see if we can help by carrying out a Clean and Care service for you. This is also an ideal opportunity to check over your greenhouse to see if you would like to make any additions to your fixtures, fittings or accessories in time for next year’s season.

If you haven't already then you need to be lifting your temperature sensitive tubers. Take a look at our November Growing Guide on how to lift and store your Dahlias this winter. 

December

Continue to monitor the temperature carefully this month. Frost-free greenhouses should be kept at a minimum night time temperature of 5 - 7°C (42 - 45°F) and a maximum temperature of 13 - 16°C (55 - 60°F) during the day – any warmer may result in weak and lanky plant growth.

Some ventilation will be necessary to remove condensation and to keep the air moving inside the greenhouse. Do tackle this carefully though to avoid creating draughts that are prone to causing dangerous drops in temperature and killing plants; the advice given for the months of October and November will help you. 

All should be relatively calm and peaceful inside the greenhouse this month, fitting the mood of the season. Take a look at our Decemeber Growing Guide for our gardening gift idea. 

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