What to Grow in a Greenhouse in June

June marks a pivotal moment in the greenhouse gardening calendar. As summer temperatures rise and daylight hours reach their peak, your greenhouse becomes a productive powerhouse capable of nurturing everything from heat-loving tomatoes to crisp salad leaves. Whether you're tending a small greenhouse or managing a larger structure, June presents unique opportunities and challenges that require careful planning and attention.

The month brings with it the perfect conditions for establishing tender crops that will provide abundant harvests throughout summer and into autumn. A greenhouse is a great asset to any vegetable plot, enabling gardeners to make the most of the sun. Even the smallest, unheated structure will allow gardeners to extend the seasons and produce good crops of a wide range of vegetables.

Essential Heat-Loving Crops to Plant in June

June represents the final opportunity to establish many warm-season crops from seed. The rising temperatures create ideal germination conditions, whilst still allowing sufficient growing time for a productive harvest.

Tomatoes: The Greenhouse Staple

Early June is the very latest you can plant tomato seeds for cropping this year, however seeds will quickly germinate as the warm temperatures provide an ideal growing environment. If you've missed the seed-sowing window, young plants from garden centres can still be successfully established during June.

Choose varieties suited to greenhouse cultivation such as 'Sungold' for sweet cherry tomatoes, 'Gardener's Delight' for reliable cropping, or 'Shirley' for larger fruits. These varieties have been selected for their ability to thrive in the controlled environment of a greenhouse, producing consistently good yields even in the variable UK climate.

When planting tomatoes in June, ensure they have adequate support systems in place. Cordon varieties will need strong canes or strings, whilst bush varieties benefit from being grown in large containers or growbags. Ensure climbing plants such as cucumbers and melons have sufficient support and tie cordon tomatoes into strings or canes.

Cucumbers: Cool and Refreshing

Cucumber plants thrive in the warm, humid conditions that June brings to your greenhouse. Sow cucumber seeds in early June for a continuous supply of crisp, refreshing cucumbers throughout the summer months. Compact varieties like 'Telegraph Improved' and 'Mini Munch' work particularly well in smaller greenhouse spaces, whilst climbing varieties can be trained up strings or canes to maximise growing space.

Cucumbers are particularly hungry plants that require consistent feeding and watering. The warm June temperatures mean they'll establish quickly, but they'll also need daily attention to watering needs. A drip irrigation system can help maintain the consistent moisture levels these plants crave.

Peppers and Chillies: Adding Spice to Summer

Plant pepper and chilli seedlings in June to ensure a plentiful harvest of spicy peppers later in the season. Varieties such as 'California Wonder' peppers and 'Scotch Bonnet' chillies thrive in the warmth and protection of a greenhouse. These heat-loving members of the nightshade family benefit enormously from greenhouse cultivation, as outdoor growing in the UK can be unreliable.

Sweet peppers like 'California Wonder' provide reliable crops of thick-walled, sweet fruits, whilst chilli varieties can range from mild 'Hungarian Hot Wax' to fiery 'Scotch Bonnet' varieties. Both require warm soil and consistent temperatures to establish well, making June planting ideal as soil temperatures remain consistently above 15°C.

Quick-Growing Crops for Succession Planting

June is perfect for establishing succession plantings of fast-growing crops that will provide continuous harvests throughout summer. This approach prevents the glut-and-shortage cycle that often plagues gardeners.

Salad Leaves and Leafy Greens

Lettuce, spinach, and rocket can all be sown in June for harvest within 4-6 weeks. Rather than sowing entire packets at once, plant small amounts every two weeks to ensure a continuous supply of fresh leaves. This technique, known as succession planting, maximises productivity whilst preventing waste.

Choose heat-tolerant varieties for June sowings, as temperatures can stress cool-season crops. 'Little Gem' lettuce and 'Red Salad Bowl' varieties cope well with warmer conditions, whilst 'Mizuna' and 'Mustard Red Frills' add peppery flavours to summer salads.

Radishes and Spring Onions

These quick-maturing crops can be ready for harvest in as little as three to four weeks from sowing. Radishes mature quickly and can be ready for harvest in as little as three to four weeks. Plant them between slower-growing crops to make efficient use of space, or use them as indicator crops to mark where slower-germinating seeds have been sown.

French Breakfast radishes provide mild, crisp roots, whilst Cherry Belle varieties offer round, bright red roots perfect for summer salads. Spring onions can be harvested at any stage, from pencil-thin scallions to more mature bulbing onions.

Advanced Greenhouse Crops for Experienced Growers

For gardeners looking to challenge themselves, June offers opportunities to establish more exotic crops that truly showcase the advantages of greenhouse growing.

Aubergines: The Heat-Loving Challenge

Aubergines require consistent warmth and careful attention to detail, making them perfect for experienced greenhouse gardeners. Plant young aubergine plants, either grown from seed or newly bought, into their final growing position... in early May for growing in an unheated greenhouse, and in late May/early June for growing outdoors, after hardening them off.

These plants need temperatures consistently above 20°C to produce full-sized fruits, making greenhouse cultivation essential in most parts of the UK. Choose varieties like 'Black Beauty' for traditional large fruits, or 'Little Fingers' for smaller, quicker-maturing aubergines perfect for shorter growing seasons.

Water aubergine plants generously and consistently throughout the growing season. Plants in containers are particularly susceptible to drying out and may need watering at least once a day in hot weather. The high water requirements and need for consistent feeding make these plants more demanding than basic salad crops.

Melons: The Ultimate Greenhouse Treat

Growing melons successfully in a UK greenhouse represents the pinnacle of greenhouse gardening achievement. Melons need 12–20 weeks from sowing to harvest depending on variety and growing conditions. June plantings will provide late summer harvests of sweet, homegrown melons that far exceed anything available in shops.

Cantaloupe varieties like 'Sweetheart' and 'Edisto' are well-suited to greenhouse cultivation, whilst honeydew types require longer growing seasons. Each plant should produce two to four melons. The key to success lies in providing consistent warmth, adequate nutrition, and proper support for developing fruits.

Pollination becomes critical with melons, as in a greenhouse, you may need to hand-pollinate them due to limited access for natural pollinators. This involves transferring pollen from male flowers to female flowers using a soft brush on warm, sunny days when flowers are fully open.

Essential Greenhouse Management for June

As temperatures rise throughout June, greenhouse management becomes increasingly critical. The difference between a thriving greenhouse and a struggling one often comes down to proper ventilation, watering, and temperature control.

Temperature Control and Ventilation

If temperatures are allowed to build up (typically more than 27°C/81°F) plant damage can occur, making ventilation absolutely crucial during June. The ideal greenhouse temperature range sits between 20-25°C during the day, with night temperatures not dropping below 15°C for warm-season crops.

Automatic vent openers prove invaluable during this period, as they respond to temperature changes even when you're not present. Fit automatic vent openers to ensure roof vents open even when you are not around. These simple devices use expanding wax to open vents when temperatures rise and close them again as conditions cool.

For greenhouses without adequate ventilation, additional measures may be necessary. One other very simple way to bring heat down is to wet the floor of the greenhouse. Get the hose or watering can, and give the ground a good soaking - this is an especially good idea if you have a paved or concrete base which absorbs more heat than soil.

Watering and Humidity Management

June's increasing temperatures mean plants require more frequent watering, but the approach matters as much as the frequency. Deep, less frequent watering encourages strong root development, whilst light, frequent watering can lead to weak root systems and increased disease susceptibility.

Water plants thoroughly, ensuring the soil is evenly moist but not waterlogged. Container-grown plants, particularly those in smaller pots, may require daily watering during hot spells. Installing a drip irrigation system can help maintain consistent moisture levels whilst reducing the daily workload.

Humidity management becomes equally important, as A further important factor in guarding against heat damage within a greenhouse or conservatory is the maintaining of a good level of atmospheric humidity during bright sunny weather. This can be achieved through damping down - watering hard surfaces and paths - which creates cooling evaporation whilst raising humidity levels.

Harvesting Early Summer Crops

June brings the excitement of harvesting the first crops from spring plantings whilst simultaneously establishing plants for late summer and autumn harvests.

Leafy Greens and Salads

Enjoy a bounty of fresh salad greens harvested from the greenhouse in June. Lettuce, spinach, and arugula are ready for picking throughout the month, offering crisp, tender leaves for summer salads. The key to maintaining quality lies in harvesting at the right time - early morning picks often provide the crispest leaves with the best flavour.

Pick outer leaves regularly from cut-and-come-again varieties, allowing the plants to continue producing throughout the season. This approach maximises yields whilst maintaining plant health, as overgrown lettuce becomes bitter and prone to bolting in warm weather.

Root Vegetables and Quick Crops

Radishes planted in March and April reach perfect harvesting size in June, providing peppery additions to salads and sandwiches. Radishes planted earlier in the season... mature quickly and can be ready for harvest in as little as three to four weeks.

Early carrots, particularly baby varieties, also become ready for pulling during June. These tender, sweet roots bear little resemblance to their mature counterparts, offering delicate flavours perfect for summer cooking.

Pest and Disease Management in June

The warm, humid conditions that benefit plant growth also favour many common greenhouse pests and diseases. Early identification and prompt action prevent minor issues from becoming major problems.

Common June Pests

Keep an eye out for aphids, whitefly, and spider mites, which thrive in warm, sheltered environments. These pests multiply rapidly in greenhouse conditions, making early intervention crucial.

Red spider mites become particularly problematic in hot, dry conditions. Mist the leaves regularly (ideally twice daily) with tepid water to discourage red spider mites and improve fruiting. These tiny pests are barely visible to the naked eye but cause distinctive yellowing and stippling of leaves.

Whitefly can be controlled through sticky yellow traps placed among plants, whilst beneficial insects like ladybirds help control aphid populations naturally. Regularly inspect plants for signs of infestation, and employ organic pest control methods such as neem oil or insecticidal soap to keep populations in check.

Disease Prevention

Monitor for fungal diseases like powdery mildew and botrytis, which can spread rapidly in humid conditions. Good air circulation proves essential for disease prevention, making adequate ventilation systems crucial for greenhouse health.

Powdery mildew appears as white, dusty patches on leaves and stems, particularly affecting cucumbers, courgettes, and peas. Removing affected leaves immediately and improving air circulation often prevents spread to healthy plants.

Botrytis, or grey mould, thrives in cool, humid conditions and affects wounded or weak plant tissue. Maintaining good hygiene by removing dead plant material and avoiding overhead watering during cool periods helps prevent outbreaks.

Planning for Autumn and Winter Production

Whilst June focuses on establishing summer crops, successful greenhouse gardeners also plan ahead for autumn and winter production. This forward-thinking approach ensures year-round productivity from your greenhouse investment.

Late Summer Sowings

Late June represents the perfect time to sow winter salads and brassicas for autumn harvests. Hardy lettuce varieties like 'Winter Density' and 'Arctic King' can be sown now for harvesting from September onwards, providing fresh greens when outdoor crops finish.

Oriental vegetables like pak choi, mizuna, and tatsoi thrive in the cooler conditions of early autumn, making June sowings ideal for October and November harvests. These crops often grow better in the shorter days of autumn than in the long, hot days of midsummer.

Space Planning and Crop Rotation

Measure out your greenhouse beds and floor space for growbags. Check you have room to space out all the summer greenhouse crops you plan to grow. Effective space planning ensures maximum productivity without overcrowding, which can lead to poor air circulation and increased disease pressure.

Consider the mature size of plants when planning layouts. Tomato plants can reach 2 metres or more in height, whilst bushy crops like peppers and aubergines require substantial horizontal space. Mini greenhouses benefit from vertical growing techniques to maximise limited space.