What to Grow in Greenhouse August UK

Vegetables, Herbs & Winter Sowing Guide

Your practical guide to maximising August in your UK greenhouse - from winter crop preparation to managing summer abundance

August in your UK greenhouse isn't just about harvesting summer crops—it's your secret weapon for year-round growing success. While outdoor gardeners are starting to wind down, smart greenhouse growers know August is actually one of the most important months for establishing crops that'll keep producing through autumn and winter.

This is your last realistic chance to sow many summer crops for late harvests, while simultaneously being the perfect time to establish all those hardy winter vegetables that'll give you fresh salads when outdoor gardens are bare. But here's the thing—August greenhouse growing requires a completely different approach than outdoor gardening.

Your greenhouse can be both blessing and curse during August. Yes, it protects your plants and extends growing seasons, but it can also become a furnace that kills seedlings if you don't manage it properly. Get it right though, and you'll be harvesting fresh vegetables while your neighbours are buying expensive supermarket produce through winter.

I'll show you exactly what to sow now for continuous harvests, how to manage the heat that can kill your efforts, and the specific techniques that work in UK greenhouse conditions during our unpredictable late summer weather.

August Greenhouse Overview: Two Growing Seasons in One Month

August sits right at the crossroads of your greenhouse year. You're squeezing the last productivity from heat-loving summer crops while simultaneously establishing the foundation for autumn and winter harvests. This dual focus makes August uniquely challenging but incredibly rewarding for UK greenhouse gardeners.

Why August is Critical for UK Greenhouse Gardeners

The Six-Week Advantage
Your greenhouse gives you roughly six weeks advantage over outdoor growing—crops you sow in August will establish and start producing before outdoor conditions become too harsh. This timing difference is what separates successful year-round growers from those who shut down for winter.

Temperature Sweet Spot
August soil temperatures (16-22°C) are perfect for germinating hardy crops that need warmth to establish but will tolerate cold once mature. Wait until September and soil temperatures drop too low for reliable germination.

Light Levels Still Adequate
While days are shortening, August still provides 13-14 hours of daylight—enough for most crops to establish strong root systems before the really short days of winter kick in.

Key August Priorities

Maximise Remaining Summer Production
Don't neglect your established tomatoes, peppers, and cucumbers. August care determines whether they produce until October or peter out in September. Regular feeding, adequate water, and pest management keep them productive.

Establish Winter Crop Foundation
Hardy vegetables sown now will provide fresh harvests when shop prices soar and quality drops. Think of August sowings as your winter insurance policy against expensive, tasteless supermarket vegetables.

Prepare Infrastructure for Transition
Check heating systems now before you need them. Clean glazing for maximum winter light transmission. Service automatic systems that'll become critical as weather becomes more variable.

Manage Heat Stress Prevention
August heat can kill sowings faster than you can replace them. Proper ventilation, shading, and timing prevent costly failures when you're trying to establish important winter crops.

What to Sow in August for Winter Harvests

August sowing in your UK greenhouse focuses on hardy crops that'll establish quickly in warm conditions but continue growing as temperatures drop. These varieties are specifically chosen for their ability to handle the transition from summer heat to winter cold.

Leafy Greens for Continuous Harvests

Winter Lettuce Varieties That Actually Work

Winter lettuce in UK greenhouse conditions can provide fresh salads from October through March if you choose the right varieties and sow at the correct time.

'Arctic King' remains the gold standard for UK greenhouse winter lettuce. This butterhead variety handles temperatures down to -10°C and forms decent hearts even in low light. Sow in early August for October harvests that continue through winter.

'Winter Density' offers a more compact alternative that works brilliantly in smaller greenhouses. The tightly packed leaves resist cold damage and the compact habit means you can fit more plants in limited space.

'All Year Round' lives up to its name in greenhouse conditions. While outdoor sowings might struggle, greenhouse plants provide reliable harvests regardless of weather. Less cold-hardy than Arctic King but more productive in autumn. For additional variety recommendations, check the RHS growing guides for comprehensive plant information.

Sowing Technique for Winter Lettuce:
Sow in modules rather than direct seeding. August heat can prevent germination if seeds are sown directly into warm greenhouse soil. Use modules kept in the coolest part of your greenhouse, and transplant once temperatures drop in early September.

Spinach for Year-Round Production

Spinach sown in August provides harvests through the hungry gap when little else is available. Choose perpetual spinach varieties that keep producing leaves even after hard frosts.

'Perpetual Spinach' (actually a chard) produces tender leaves continuously once established. More heat-tolerant during establishment than true spinach, making it perfect for August sowings. Harvest outer leaves and the centre keeps producing.

'Giant Winter' spinach forms larger leaves than summer varieties and handles cold exceptionally well. Takes longer to establish but provides bigger harvests through winter months.

'Space' spinach offers a compact option for smaller greenhouse spaces. The upright growth habit maximises production in limited space while maintaining cold tolerance.

Oriental Greens Collection

August is actually the ideal time for oriental greens in UK greenhouse conditions because it avoids the bolting problems these crops face with earlier sowings.

Mizuna provides mild mustard-flavoured leaves that add interest to winter salads. The finely cut leaves look attractive and the plants keep producing even after multiple harvests.

Pak Choi 'Hanakan' was specifically bred for autumn sowing and winter production. The compact heads handle cold well while providing substantial harvests of tender leaves and crunchy stems.

Rocket 'Apollo' gives you that peppery bite that's often missing from winter salads. Hardy varieties continue producing even when outdoor rocket has long since finished.

Brassicas for Spring Harvests

Spring Cabbage Setup

Spring cabbage planted now won't harvest until next March-May, but they're worth the wait. These provide fresh vegetables during the expensive spring months when stored crops are finished but new season growth hasn't started.

'Durham Early' remains the most reliable variety for UK greenhouse growing. Compact plants fit greenhouse spacing while producing decent-sized heads that store well once harvested.

'Spring Hero' offers improved disease resistance and more uniform heading. Slightly more expensive seed but worthwhile for the reliability, especially in greenhouse conditions where air circulation might be limited.

Sowing Method: Use modules to start spring cabbage, then transplant when plants have 4-5 true leaves. This gives you better control over spacing and reduces root disturbance compared to direct sowing.

Purple Sprouting Broccoli Planning

While you won't harvest purple sprouting broccoli until next March, August sowing is critical for plants that'll produce well. These need time to establish strong root systems before winter.

'Early Purple Sprouting' starts producing spears in February-March, extending your harvest season when little else is available.

'Late Purple Sprouting' continues the harvest into April-May, bridging the gap until summer vegetables begin production.

Root Vegetables for Storage

Quick-Growing Roots for Autumn Harvest

Not all August sowings are about winter crops. Some quick-growing roots sown now will be ready for October-November harvest before storage crops from earlier sowings are ready.

Turnip 'Sweetbell' grows fast enough for autumn harvest while remaining tender. These small, sweet roots store well and provide variety when your main root crops are in storage.

Beetroot 'Chioggia' with its distinctive striped flesh adds visual interest to autumn salads. The compact roots don't need much space and the leaves can be harvested separately as salad greens.

Early Carrots 'Adelaide' can still be sown in early August for November harvest. Choose early varieties rather than maincrop types that won't have time to develop properly before cold weather.

Radish 'Cherry Belle' provides quick satisfaction—ready in just 25-30 days from sowing. Perfect for filling gaps between slower-growing crops or providing quick harvests while waiting for main crops to mature.

Herbs for Winter Windowsills and Greenhouse Growing

August herb sowing focuses on varieties that'll provide fresh flavour through winter months when dried herbs just don't compare to fresh-picked alternatives.

Mediterranean Herbs That Thrive

Parsley for Winter Harvests

Both flat-leaf and curled parsley sown in August will provide fresh leaves through winter. Flat-leaf parsley offers better flavour for cooking, while curled parsley handles cold slightly better and looks more attractive in mixed plantings.

Sow parsley in modules because the seeds are slow to germinate and benefit from consistent moisture control. Once established, parsley plants continue producing even after light frosts, making them perfect for unheated greenhouse growing.

Coriander Success Tips

Coriander requires careful timing in August. Early August sowings often bolt in residual heat, while late August sowings establish better as temperatures moderate. Use bolt-resistant varieties like 'Slowbolt' or 'Leisure' for better success rates.

Succession plant coriander every 2-3 weeks through August and September. Each sowing provides 3-4 weeks of harvest, so multiple sowings ensure continuous supply through autumn and early winter.

Chervil for Delicate Flavour

Often overlooked, chervil provides delicate anise-flavoured leaves that enhance winter soups and salads. This herb actually prefers cooler conditions, making it perfect for August establishment and winter production.

Chervil doesn't transplant well, so direct sow where plants will grow. The fine seeds need consistent moisture but good drainage—perfect conditions in well-managed greenhouse soil.

Dill for Cooking and Flowers

Dill sown in August grows slowly through autumn, providing both leaves for cooking and eventual flowers for seed collection. The feathery foliage adds texture to greenhouse plantings while providing useful harvests.

Herb Growing Schedule

Extending Summer Crops Through August

Don't abandon your summer crops just because you're planning for winter. Proper August management can extend tomato, pepper, and cucumber harvests well into October, maximising your investment in these heat-loving plants.

Maximising Existing Plants

Tomato Management for Extended Harvest

August tomato care determines whether plants produce until October or collapse in September. Focus on maintaining plant health while managing the transition from peak summer to cooler autumn conditions.

Continue feeding with high-potash fertiliser, but adjust frequency based on growth rates. Slower growth in late August means less frequent feeding prevents salt buildup that can damage roots.

Remove lower leaves progressively as they yellow. This improves air circulation and reduces disease pressure as humidity increases with cooler nighttime temperatures. Never remove more than 2-3 leaves per week to avoid shocking plants.

Support heavy trusses that developed during peak summer growth. The weight of ripening fruit can snap branches, ending production prematurely. Use soft ties or clips that won't damage stems as they continue growing.

Managing Heat Stress in Established Plants

Even well-established summer crops can suffer in August heat waves. Watch for signs of heat stress: wilting despite adequate moisture, poor fruit set, or yellowing leaves despite proper feeding.

Increase ventilation during peak heat periods. Automatic vent openers set to 18°C ensure vents open before stress occurs. Manual vents should be opened early morning and closed only after temperatures drop in evening.

Provide temporary shading during extreme heat. Shade cloth or even old bedsheets can prevent temperature spikes that stop fruit development or cause flower drop. Monitor local conditions using UK weather forecasts to plan your August greenhouse activities around heat waves.

Late Summer Sowings for Extended Harvests

Quick-Growing Options for September Harvest

Some crops can still be sown in August for late summer harvest, extending your productive season before winter crops take over growing space.

French Beans 'Delinel' is a dwarf variety that matures quickly enough for August sowing. Plants will produce pods through September if protected from cold snaps. Use modules for better transplant success.

Courgette 'Eight Ball' produces compact round fruits on bushy plants that fit greenhouse spacing better than traditional trailing varieties. August sowings can produce harvests into October with basic protection.

Lettuce for September Use

Quick lettuce varieties sown in late August provide fresh salads during the September gap when summer lettuce finishes but winter varieties aren't ready yet.

Choose fast-maturing varieties like 'Tom Thumb' or 'Little Gem' that handle temperature fluctuations better than large heading types. These bridge the harvest gap effectively.

Heat Management for Late Sowings

Positioning Strategies

Place August sowings in the coolest areas of your greenhouse. North-facing positions or areas shaded by larger plants provide better germination conditions than full sun positions that worked for earlier sowings.

Timing for Success

Sow seeds in early morning or evening when temperatures are lowest. Hot soil can prevent germination or kill emerging seedlings before they establish.

Cover sowings with damp newspaper or cloth until germination occurs. This maintains moisture while preventing overheating that kills seeds in warm greenhouse soil.

August Greenhouse Management Tasks

August greenhouse management requires balancing the needs of established summer crops with creating optimal conditions for new sowings. This month demands more active management than any other time of year.

Heat Management Essentials

Ventilation Strategies That Work

Proper greenhouse ventilation is absolutely critical during August. Inadequate ventilation kills more August sowings than any other factor in UK greenhouses.

Set automatic vent openers to trigger at 18°C rather than the 20-22°C you might use earlier in summer. This earlier opening prevents temperature spikes that stress plants and prevent seed germination.

Ensure cross-ventilation by opening both roof and side vents. Hot air rises and exits through roof vents while cooler air enters through side vents, creating circulation that prevents stagnant hot spots.

Consider electric fans for air movement during still, hot days. Even small clip-on fans can prevent the stagnant conditions that encourage fungal problems and heat stress.

Shading Solutions for August

External shading works better than internal shading for reducing greenhouse temperatures. Shade paint applied to outside glass reflects heat before it enters, while internal shading just moves hot air around.

Coolglass Shading Paint becomes transparent when wet, allowing maximum light during overcast periods while providing cooling during sunny weather. Apply in early August for best results.

Shade Cloth Systems offer more control than paint. 40-50% shade cloth provides adequate cooling while maintaining sufficient light for plant growth and seed germination.

Natural Shading using tall plants like cucumbers or beans can provide localised cooling for heat-sensitive sowings without reducing light throughout the entire greenhouse.

Watering and Feeding Adjustments

August Watering Best Practices

Watering becomes more complex in August as you're managing both water-hungry established crops and newly germinated seedlings with different needs.

Water established crops deeply but less frequently. This encourages deep root development and prevents waterlogged soil that encourages root diseases during humid August weather.

New sowings need consistent moisture but not waterlogging. Use spray bottles or fine-rose watering cans for newly emerged seedlings that can be damaged by heavy watering.

Time watering for early morning when possible. This gives plants time to dry before cooler evening temperatures, reducing fungal disease pressure that increases as nights become longer and cooler.

Feeding Programmes for Transition Period

Adjust feeding schedules to match slowing growth rates as daylight hours decrease. Overfeeding in August can cause soft growth that's vulnerable to disease and cold damage.

Use high-potash feeds for fruiting crops like tomatoes and peppers to encourage ripening of existing fruits rather than excessive vegetative growth.

Reduce nitrogen feeding for crops you want to slow down or prepare for winter dormancy. Excess nitrogen in August creates soft growth that doesn't handle autumn temperature fluctuations well.

Pest Prevention in Hot Weather

August heat creates perfect conditions for several greenhouse pests that can destroy crops quickly if not managed properly.

Red Spider Mites thrive in hot, dry conditions. Maintain adequate humidity through careful watering and avoid letting greenhouse air become too dry. Spider mites indicate insufficient humidity more than any other problem.

Whitefly Populations peak during August heat. Use sticky yellow traps to monitor populations and introduce beneficial insects like Encarsia wasps before populations explode.

Aphid Management becomes critical as they seek out new growth on both established crops and fresh sowings. Regular monitoring and early intervention prevent infestations that can destroy months of work. Follow government pest management guidelines for sustainable approaches to greenhouse pest control.

For detailed pest identification and treatment strategies, check our Complete UK Greenhouse Pest Control Guide which covers all the common August problems and natural solutions that work.

Preparing for Autumn Transition

August preparation work determines how smoothly your greenhouse transitions from summer abundance to winter productivity. Small tasks completed now prevent bigger problems later.

Infrastructure Preparation

Heating System Checks

Test heating systems now while you don't need them desperately. Check electric heaters for proper operation, clean paraffin heater wicks, and ensure gas connections are secure and leak-free.

Service automatic controls that manage heating cycles. Battery-powered thermostats need fresh batteries, while electronic controls should be tested for accuracy using a separate thermometer.

Glazing and Insulation Assessment

Clean greenhouse glazing to maximise light transmission as days shorten. August cleaning is easier in warm weather and ensures maximum light penetration when autumn reduces natural light levels.

Check existing insulation materials for damage or deterioration. Bubble wrap insulation that's been up all winter may need replacement before autumn installation.

Inspect greenhouse structure for damage that could affect heating efficiency or create cold spots. Small repairs completed in August prevent larger problems during autumn storms.

Planning September Activities

Succession Planting Schedule

Plan September sowings while August sowings are establishing. Succession planting ensures continuous harvests rather than gluts followed by shortages.

Order seeds for autumn sowings now while varieties are still available. Popular winter varieties often sell out by September, leaving you with inferior substitutes.

Growing Area Preparation

Prepare dedicated areas for winter crops by removing finished summer plantings and refreshing soil. August soil preparation allows settling time before September plantings.

Label growing areas clearly to avoid confusion when managing multiple crops at different growth stages. Good organisation becomes critical as greenhouse space becomes more crowded.

Practical Growing Techniques for August Success

August growing success depends on adapting standard techniques to cope with heat stress while ensuring good establishment of crops that'll carry you through winter.

Sowing Methods That Work in August Heat

Module Sowing Advantages

Module sowing gives you control over growing conditions that direct sowing can't match during August heat. Seeds sown in modules can be placed in the coolest areas of your greenhouse and moved as conditions change.

Use quality seed compost that retains moisture without becoming waterlogged. August sowings are particularly vulnerable to both drought and overwatering as you try to balance heat stress with adequate moisture. These techniques align with organic growing principles that promote sustainable greenhouse gardening.

Consider larger modules for August sowings. 3cm modules provide more root space and moisture retention than smaller cells, giving plants better drought resistance during establishment.

Direct Sowing Guidelines

When direct sowing is necessary, prepare soil carefully to optimise conditions. Add compost or well-rotted manure to improve moisture retention without creating waterlogged conditions.

Cover direct sowings with damp newspaper or horticultural fleece until germination occurs. This maintains consistent moisture while preventing overheating that kills seeds in warm soil.

Water soil before sowing rather than after. This ensures adequate moisture for germination without washing seeds too deep or creating surface crusting that prevents emergence.

Companion Planting for August

Strategic Plant Combinations

August companion planting focuses on using established plants to create favourable microclimates for new sowings rather than just pest deterrence.

Plant heat-sensitive sowings in the shade of tall summer crops. Tomato and cucumber plants provide natural shading that creates cooler conditions for lettuce and herb establishment.

Use fast-growing crops as living mulch around slower-establishing plants. Radishes planted around spring cabbage provide ground cover that reduces soil temperature while giving quick harvests.

Beneficial Plant Partnerships

Marigolds planted near tomatoes continue providing pest deterrence while adding colour to greenhouse displays as summer flowers begin fading.

Basil near peppers enhances flavour development in fruits while the aromatic leaves help deter flying insects that become more problematic in August heat.

Nasturtiums work as trap crops for aphids that often move onto new sowings. Position them where they'll attract pests away from vulnerable seedlings.

August Greenhouse Growing Calendar

Plan your August activities week by week to ensure nothing gets forgotten during this busy transition month.

Daily August Priorities

Troubleshooting August Growing Problems

August greenhouse growing presents unique challenges that require specific solutions. Understanding common problems helps you prevent failures and recover quickly when things go wrong.

Heat Stress Problems and Solutions

Identifying Heat Stress

Heat stress shows up differently in various crops. Tomatoes develop poor fruit set and blossom end rot. Lettuce bolts prematurely or fails to germinate. Seedlings wilt despite adequate moisture.

Poor Germination Troubleshooting

Common Causes in August:

Soil Temperature Too High: Use a soil thermometer to check. Seeds won't germinate if soil exceeds 25°C. Solution: sow in cooler areas or wait for evening temperatures to drop.

Inconsistent Moisture: August heat dries soil quickly. Solution: cover sowings with damp newspaper and check daily. Water gently to maintain moisture without waterlogging.

Old or Poor-Quality Seeds: Heat accelerates seed deterioration. Solution: use fresh seeds and store unopened packets in refrigerator between sowings.

Wrong Timing: Some crops are sensitive to day length changes beginning in August. Solution: check specific variety requirements and use varieties bred for late summer sowing.

Managing Multiple Crops Simultaneously

August greenhouse management becomes complex when you're caring for established summer crops while nurturing new sowings for winter harvest.

Essential Equipment for August Growing Success

August greenhouse growing requires specific equipment to manage heat, maintain optimal conditions, and ensure successful establishment of winter crops.

Equipment Setup for August Success

Ventilation Systems
Install automatic vent openers on both roof and side vents. Set trigger temperatures to 18°C rather than higher settings used earlier in summer. This early opening prevents dangerous temperature spikes that can kill sowings within hours.

Monitoring Equipment
Position digital thermometers at plant level rather than eye level. Ground-level temperatures can be significantly different from air temperatures, especially important when managing sowings in modules on staging.

Shading Solutions
External shading works better than internal alternatives. Coolglass shading paint provides automatic adjustment—becoming transparent during overcast periods while providing cooling during sunny weather. For independent equipment reviews and comparisons, Which? provides unbiased testing of greenhouse equipment.

For all your August greenhouse equipment needs, from louvre window vents to raised beds and planters, visit Greenhouse Stores for everything you need to make August growing successful.

Make August Your Most Productive Greenhouse Month

August in your UK greenhouse is about maximising opportunities. You're harvesting the rewards of summer growing while investing in winter abundance. The key is understanding that successful August growing requires active management—this isn't a month to set things up and hope for the best.

Focus on heat management first. Get your ventilation and shading sorted before you start sowing because failed sowings cost time you can't recover this late in the season. Once conditions are right, be bold with your sowings—your greenhouse gives you growing opportunities that outdoor gardeners simply don't have.

Remember that crops you establish now will carry you through the expensive winter months when fresh vegetables cost a fortune and taste like cardboard. A few pounds spent on seeds and proper equipment in August can save you hundreds on grocery bills while providing fresh, nutritious food your family will actually want to eat.

The transition from summer abundance to winter productivity doesn't happen by accident. It requires planning, proper techniques, and quality equipment that works reliably when you need it most.

For everything you need to make August greenhouse growing successful—from automatic ventilation systems to seed starting equipment—visit Greenhouse Stores for expert advice and reliable equipment that helps your greenhouse produce year-round.

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More Seasonal Growing Guides:

• Complete UK Greenhouse Pest Control Guide
• 6 Essential Greenhouse Growing Tips for Beginners
• Best Position for a Greenhouse UK
• How to Prepare the Ground for a Greenhouse Base

Essential Equipment:

• Greenhouse Thermometers
• Automatic Vent Openers
• Complete Greenhouse Accessories Range