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.

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 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. On sunny days with no clouds in the sky, you may well find that it is necessary to use your shades. If your greenhouse is not already fitted with shades, do give us a call and we will be happy to advise you as to the cost and the best location for shades for your particular structure.

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! Actively growing plants will also need feeding but again you will need to take care - always follow the instructions on the packaging and remember ‘little and often’ is the golden rule, as overfeeding can cause distortion, leaf damage or root scorch.

Continue to keep an eye out for pests, particularly greenfly, whitefly, vine weevil and red spider mite, as these are all active at this time of year. Slugs are also out in force, and they can rapidly destroy a trayful of carefully nurtured seedlings.

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. 

As well as continuing to sow half-hardy annuals, now is a good month to start taking cuttings to increase your plant collection. If you are growing fruit, hand pollinate at midday by transferring pollen using either a soft brush or a ball of cotton wool and repeat over a period of several days to ensure success.

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.

As the weather continues to warm up, the main focus is to keep the temperature inside the greenhouse below 24 - 29°C (75 – 85°F) during sunny spells. Often this will mean active use of shades and monitoring their use every day according to the weather. 

Ventilation is also paramount this month. Make full use of both the side and roof vents if the temperature is over 24°C (75°F) and supplement further by hooking the door open if necessary. In sunny periods you can also damp down the floor and benching. This not only lowers the air temperature as the water evaporates but also raises the relative humidity.

Whilst it should not be necessary to heat the greenhouse during the day in most parts of the UK, keep a check on night time temperatures as night time frosts are still a possibility.

Plants are actively growing this month and staking and tying should be done as plants grow. Unlike in the garden, remember that roots cannot extend out into open soil to seek out extra food. Regular feeding and watering is therefore important this month to ensure healthy, vigorous growth. ‘Little and often’ remains the golden rule when feeding and for general purposes a balanced formula with nitrogen, phosphates and potash in roughly equal proportions should be used. If you wish to stimulate leaf growth, a high-nitrogen feed should be used and for plants in flower or fruit use a high-potash fertiliser. You will also need to water plants regularly and thoroughly and must check the dryness of the compost every day – some plants will need daily watering.

With a greenhouse full of healthy specimens, space may become limited inside the greenhouse. Now is the ideal time to make full use of your cold frames if you have them and start the hardening-off of the early sowings. 

Last but not least, this month 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.

Ventilation is now also essential. Open roof and side vents to their maximum and the door as required to keep the temperature down inside the greenhouse. On warm days, ventilation should be carried out in the mornings and evening and it may even be necessary to keep the side vents open overnight. If you are due to be away on holiday and there is nobody to look after the greenhouse whilst you are away, we recommend the side vents are left open and the blinds left down. Automatic roof vents (including Bayliss) will continue to open and close as they adjust to the day and night time temperatures and are a great aid in keeping the greenhouse as cool as possible in such circumstances.

In hot spells, damping down inside the greenhouse regularly is a must and should be carried out in the morning or early afternoon by spraying the floor and benching. As well as monitoring the air and cooling the greenhouse using shades and vents in combination, damping down has the added benefit of discouraging red spider mite - at this time of year it can be a particular menace.

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!

Plants destined for the garden are moved out this month. Make sure that the danger of frosts has passed before putting out half-hardy plants and also make sure that specimens grown inside the greenhouse have had a period of hardening- off before they are moved into the garden. Some greenhouse plants, especially citrus, can be moved into the garden during the period between the end of June and early October. This not only improves their health but also gives you more space for growing under glass.

As you move more and more of your plants outdoors, give the greenhouse a good tidy and clean up. Do not leave dirty pots, discarded plants or dead leaves lying around – these can all harbour dirt and disease leading to problems in the future. If you feel the need for some motivation, Sarah Wain, Garden Supervisor at West Dean Gardens in West Sussex, is full of good advice on how to set your greenhouse sparkling! Her handy tips and tricks can be found here.

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. If the weather is very hot, even opening all the vents and keeping the door hooked open may not be enough to keep the temperature comfortable. More than likely, you will have to make full use of shades and continue to damp down inside the greenhouse. Both of these will help but you may even need to bring in an air circulating fan to provide forced ventilation and keep the temperature down. Our Edwards heater may be just the answer if you do not already have one – details can be found here.

As in June, scorch from the sun can be damaging and some form of shading is essential for a wide variety of plants covering a diverse range such as flowering pot plants, rooted cuttings or cucumbers in flower. Blinds on the inside of a greenhouse can be difficult to use if you are growing tall plants such as tomatoes and cucumbers and can also have a tendency to collect debris, such as pests and leaves. Externally mounted shades - such as those we offer – avoid these drawbacks and are very easy to use. We would be delighted to discuss the options with you if you would like to add shading to your structure or even supplement existing shades that you have.

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. Spray promptly if you notice signs of their presence or investigate biological alternatives and remember that hanging good old sticky yellow fly catchers also has its place in your armoury! Other garden visitors such as butterflies looking for places to lay their eggs and even birds can find their way into the greenhouse through open vents and doors, bringing a different array of problems with them. You may wish to consider adding bug-screens to your greenhouse – if you give us a call on 01730 826900 we will be happy to advise you what is possible.

This month is a good time for taking cuttings as roots will form quickly in the warm conditions. With most plants now growing vigorously, don’t forget to establish routine feeding. A liquid feed at fortnightly intervals is suitable as a basic treatment, but fruit-bearing plants such as cucumbers and tomatoes need feeding every 7 - 10 days.

August

The battle against pests and disease continues this month. Keep an eye out in particular for grey mould. If leaves, fruit and flowers should succumb, destroy the affected parts, spray with a systemic fungicide and take steps to improve ventilation inside the greenhouse. To help prevent disease, take care when watering to ensure leaves are not splashed in the evening and never leave old leaves and fruit lying about. Dead-heading flowering plants regularly helps to combat disease and extends the flowering season.

Keeping the greenhouse ventilated remains vital. Keep the vents and door open on warm days, though as the temperature begins to drop over the course of the month, close the side vents before nightfall so that only the roof vents remain open during the night.

Plants will continue to require regular watering and vigorous plants in small containers will still need to be soaked daily. Continue to routinely feed actively growing plants too, as this remains a necessity.

A potash-rich feed will improve fruiting and flowering; 10 - 14 days should be left between feeds.

As in July, August is a good month for taking cuttings for plants in the garden. Cuttings typically fall into 3 groups – alpines, such as Armeria, shrubs such as Weigela, and half-hardy types such as Pelargonium. Don’t take more cuttings than you will be able to handle but it is a good idea to strike a few extra – failures can occur, despite a healthy dose of positive thinking!

Now is also the time to plant bulbs. Make sure you have an adequate supply of pots, bowls and bulb fibre.

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.

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. Aim to keep the temperature in the 10 - 21°C (50 - 70°F) range and close the vents in the early evening to retain some of the sun’s warmth overnight.

Cooler temperatures will also reduce the need to damp down. Only spray the floor and benching on warm days and, even then, make sure the job is completed by midday.

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.

If you are growing shade-loving plants such as ferns, these will need to continue to be kept shaded. For most plants however the prime concern is achieving maximum light levels rather than controlling heat and you should be using your shades less and less as the month progresses.

Salad days and the joys of eating the fruits of your labours al fresco are now drawing to a close. The end of September or early October is the time when tomatoes and cucumbers are cleared out of the greenhouse and frost-sensitive plants from outside are brought in. There may be a gap between the two and, if so, this is an excellent opportunity to fumigate the greenhouse with a fungicidal smoke.

Don’t wait too long before bringing in tender specimens as one unexpected sharp frost is all that is needed to kill off a beloved plant! Do make sure that pots of chrysanthemums, fuchsia etc are free from pests before bringing them into the safety of the greenhouse.

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.

The extra humidity that was so vital in the warmer months is now no longer required and you should stop damping down inside the greenhouse as moist air can cause a problem by encouraging the growth of grey mould. Continue to ventilate the structure a little each day between mid-morning and early afternoon by using the roof vents in isolation – the goal is to bring fresh and moving air into the greenhouse rather than to reduce the inside temperature. Do not ventilate on damp or foggy days and make sure the vents are closed at night.

Watering may also be eased off this month and you should check that each plant requires watering before applying it. Be careful when watering too – do not splash the water about and try to keep it off the floor, benching, leaves and crown of the plants. You may find it helpful to water plants before midday so that any inadvertent splashes have time to dry out before sunset.

Inspect plants for evidence of pests and disease – diseased or yellowing leaves should be removed and spray if grey mould or whitefly has become a problem; spraying should be carried out in the morning. Slugs and snails may also be present and measures to combat these should be taken if you spot their tell-tale slime trails or evidence of nibbled leaves! Some snails hibernate and these may be attracted to the warmer climes of a greenhouse, as may slugs if the outside temperature dips below -5°C (42°F).

Late autumn is an ideal opportunity to think about your plant collection for next year and start collecting seeds – you will have plenty in your own garden and all for free! If you have like-minded gardening friends, why not get together and swap? Store any seed you collect in clearly marked paper bags over the winter – memories are likely to be fallible come spring! Click here for John Wood, Head Gardener at National Trust Hinton Ampner's top tips advice.

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. It may not be necessary to carry out this task until November or December in milder parts of the country.

November

With plants safely brought inside, the emphasis this month is on maintaining a healthy environment inside the greenhouse. 

Just as last month, it is important to maintain a minimum temperature of 5 – 7°C (42 – 45°F) to protect frost-sensitive plants. If your greenhouse is unheated, it will be necessary to cover tender plants with fleece or newspaper at night if a frost is forecast.

Try to keep the air inside dry. On dry and sunny days side vents can be opened if they are away from wind but they should be closed mid-afternoon so the sun’s heat will be conserved before nightfall. Vents should be kept closed on damp or foggy days.

All plants (except flowering plants) should be watered sparingly and you should avoid splashing the floor, benching or foliage when watering, as this will also help keep the atmosphere dry inside the greenhouse.

Pots should be spaced out on the bench top to ensure plants receive maximum light. Continue to check for signs of grey mould and whitefly and treat if either is present. Flowers should be dead-headed and diseased or yellowing leaves should be removed as a further measure to reduce the spread of disease.

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. If you have soil beds inside your greenhouse, now is the time to dig these over. Clear rubbish from inside the greenhouse and then scrub the benching and shelves with a garden disinfectant. Clean the frame and glass next, both inside and out, and take this time to check that gutters and roof valleys are free of fallen leaves and debris, as a build-up of these can cause blockages and encourage algae growth which can lead to leaks over time. Remember to bring your plants back into the greenhouse once the big tidy up is complete – sounds obvious but it is easy to overlook as you are admiring your new shiny greenhouse after all your hard work!

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.

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. 

If plants are not in flower, the soil must be kept rather dry at this time of year. This is particularly true for cacti, succulents and pelargonium. Only damp down the floor if it is really necessary and be sure to do this job mid-morning so the greenhouse has ample time to dry out.

Pot cyclamen will now be approaching their best but be sure to only water when the top of the compost is dry to the touch.

Onion seed can be sown in a little heat towards the end of the month – your reward will be large onions next summer.

Cut down the last of the winter flowering chrysanthemums and store them in their pots underneath the benching. This month is also the time to start taking the first batch of chrysanthemum cuttings.

Continue to keep an eye out for pests and diseases and treat if necessary. Dead-heading and removal of yellowing or diseased leaves should also be a key focus this month. 

You may be dreaming of a white Christmas and if snow is forecast, be sure to keep the shades on your greenhouse tightly rolled up on the ridge if you have these fitted to avoid damage to the shade from fallen snow.

All should be relatively calm and peaceful inside the greenhouse this month, fitting the mood of the season. With this year’s hard work done, you can look forward to all being merry and bright next year!

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