May in the greenhouse

Settling In and Gearing Up

There’s something about May that feels like opening a gate into the proper growing season. The days stretch out, the sun has a bit more bite to it, and the greenhouse begins to buzz with energy. It’s a month of transition – the slow crawl of spring finally tipping into something closer to summer. And in the greenhouse, that means plenty to do, but plenty to enjoy too.

If April was about sowing and potting on, May is when things really begin to settle into their summer rhythm. By now, tomatoes, aubergines and peppers should be well under way in the greenhouse, either in their final positions or nearly there. I like to wait until early May before planting out into borders or grow bags, just to be sure we’re past any late frosts. Even in a greenhouse, a cold snap can nip the tops of young plants and set them back.

Tomatoes, Peppers & Aubergines: Getting Established

Tomatoes should be strong and stocky now, with their first trusses forming. I like to get them into the border if I can – soil-grown tomatoes seem to have a deeper flavour, though grow bags and pots will do the job just fine. Make sure whatever system you use, you've got your supports in place early. Spiral canes, vertical strings, or even sturdy pea sticks will work, as long as they're solid. There’s nothing worse than a collapsing tomato halfway through July.

Tip: When planting, I always bury the tomato stem a little deeper than it was in the pot. Tomatoes can grow extra roots along their stems, so a deeper planting helps them anchor and feed better. A handful of worm castings or well-rotted compost at the base of the hole never goes amiss either.

Tomaten richtig pflanzen und stützen

Deine Tomaten sollten jetzt kräftig und kompakt sein, oft schon mit den ersten Blütenständen. Wenn möglich, pflanze sie direkt in den Boden – das sorgt meist für ein intensiveres Aroma. Alternativ funktionieren auch Töpfe oder Growbags problemlos

Wichtig ist eine stabile Rankhilfe von Anfang an:

Spiralstäb
Schnüre nach oben
stabile Pflanzenstäbe

Ein späteres Nachrüsten wird schnell zum Problem – besonders, wenn die Pflanzen im Sommer schwer werden.

Tipp: Setze Tomaten tiefer ein als im Topf. Sie bilden zusätzliche Wurzeln entlang des Stängels und wachsen dadurch stabiler und kräftiger. Eine Handvoll reifer Kompost oder Wurmhumus im Pflanzloch unterstützt zusätzlich.

Companion Planting with Marigolds (Natural Pest Control)

While your tomatoes are settling in, consider popping in a few marigolds around the base. I do this every year, and not just because they brighten things up. Marigolds are brilliant companion plants – their scent deters aphids and whitefly, which can be a real nuisance in the close quarters of a greenhouse. They also attract beneficial insects like hoverflies, which help keep pests in check. French marigolds are particularly effective, and they seem to thrive in the same warm, slightly humid environment that tomatoes love. Plus, there’s just something cheerful about that splash of colour under the canopy of green.

Hardening Off: Preparing Plants for the Season

May is also the month of the plant shuffle. Those tender young things you’ve been cosseting on the windowsill or in a heated propagator will need to adjust to life in the real world – or at least the sheltered world of the unheated greenhouse. It’s all about hardening off. Start by moving plants into the greenhouse during the day and bringing them back in at night for a week or so. Then leave them overnight in the greenhouse, covering them with fleece if it looks like a cold one.

Sweetcorn, courgettes, cucumbers, and even bedding plants all benefit from this step. It might feel like a faff, but it makes them tougher and more resilient once they go outside.

Jungpflanzen abhärten

Im Mai beginnt die sogenannte „Abhärtung“. Pflanzen, die bisher drinnen oder im beheizten Anzuchtkasten standen, müssen sich langsam an die Bedingungen im Gewächshaus gewöhnen

So gehst du vor:

Tagsüber ins Gewächshaus stelle
Nachts wieder reinholen (ca. 1 Woche
Danach auch nachts draußen lassen (bei Bedarf mit Vlies schützen)

Das gilt besonders für:

Zuckermais
Zucchini
Gurken
Sommerblumen

Dieser Schritt sorgt für widerstandsfähigere Pflanzen im Freiland.

What to Sow in May

There’s still time to sow plenty in May. French beans do well in deep pots or root trainers, and I like to start a few indoor cucumbers around now for a reliable crop. Salads can be sown in trays or modules for transplanting out or growing in gaps. I usually keep a tray of mixed lettuce going all month long, slotting them in between slower crops or using them to fill space in the greenhouse itself.

May is also a good time to sow basil, coriander and dill. Herbs do well in the heat of a greenhouse and are quick to bolt if grown outside in a dry spell. Keep basil warm, watered and nipped out regularly to prevent it flowering too soon. It’s one of my favourite crops to grow alongside tomatoes – they like similar conditions, and there’s nothing better than the scent of both mingling in the warm air.

Watering, Mulching & Maintenance

This month, watering becomes more important. Days are warmer and plants are drinking more. But resist the urge to overdo it. Inconsistent watering can lead to problems later – split tomatoes, blossom end rot, or sulky peppers. I water early in the day, giving plants time to dry off by nightfall. A good soak every few days is better than a daily dribble.

Mulching helps the soil hold on to moisture too. If you’ve got grass clippings, leaf mould or old compost, spread a thin layer around your plants. It keeps roots cool and reduces evaporation.

Richtig gießen – weniger ist mehr

Mit steigenden Temperaturen wächst auch der Wasserbedarf. Dennoch gilt: gleichmäßig und gezielt gießen.

morgens gießen
lieber seltener, dafür durchdringend
Staunässe vermeiden

Unregelmäßiges Gießen kann Probleme verursachen wie:

platzende Tomaten
Blütenendfäule
Wachstumsstörungen

Mulchen hilft: Eine dünne Schicht aus Grasschnitt, Laub oder Kompost hält die Feuchtigkeit im Boden und schützt die Wurzeln.

Pest Monitoring & Natural Control

As plants grow, so do the pests. Look under the leaves and along the stems for any clusters of bugs or discolouration.

If you spot anything early, it’s easier to deal with. I introduce beneficial insects like ladybirds or lacewings if needed. You can also use a gentle soap spray, but only in the early morning or evening when the sun isn’t strong.

Bean Growing: Strong Starts in the Greenhouse

May is the perfect time to get runner beans and French beans started in modules in the greenhouse. The warmth gives them a strong head start, and sowing in trays or root trainers protects the seeds from mice, which have a cheeky habit of digging them up if you direct sow outside. I like to sow one bean per module with a bit of depth, using multi-purpose compost with added vermiculite. Keep them well watered but not soggy. Within a week or so, you’ll see sturdy shoots pushing through, and they’re easy to harden off and plant out once the risk of frost is gone. It also means you can be more selective when planting, choosing the strongest plants and spacing them exactly how you like. Starting beans this way gives you a reliable, uniform crop and saves a lot of frustration.

Final Thoughts: May in Full Flow

So, there you have it – May in the greenhouse. A month of shifting gears, of planting out, potting on and gearing up for the abundance to come. Give everything a bit of time, a bit of attention, and your greenhouse will reward you in spades.

Happy growing!

Bohnen im Gewächshaus vorziehen

Stangen- und Buschbohnen lassen sich im Mai ideal im Gewächshaus vorziehen. Das bringt klare Vorteile:

schnelleres Wachstum durch Wärm
Schutz vor Mäusengleichmäßige Keimung

Säe je eine Bohne pro Modul in lockere Erde (z. B. mit etwas Vermiculit). Halte die Erde feucht, aber nicht nass. Nach etwa einer Woche zeigen sich kräftige Keimlinge, die du später gezielt auspflanzen kannst.

Fazit: Ein Monat voller Bewegung

Der Mai bringt Dynamik ins Gewächshaus. Pflanzen wachsen sichtbar, Strukturen entstehen und die Grundlage für eine reiche Ernte wird gelegt. Mit etwas Aufmerksamkeit, regelmäßigem Gießen und den richtigen Handgriffen schaffst du ideale Bedingungen für die kommenden Monate

Viel Freude im Gewächshaus und eine erfolgreiche Gartensaison!

About the author of this article

Rob is an experienced gardener specializing in the cultivation of fruits, vegetables, and other edible plants. He writes for several British gardening magazines, appears regularly on television and radio, and shares his passion for growing his own food on social media at @robsallotment

In addition, Rob collaborates with well-known gardening companies in the UK, including Suttons, Thompson & Morgan, Dobies, and The Organic Gardening Catalogue. Together, he develops new and exciting varieties of fruit, vegetables, and flowers that are available at garden centers across the UK and Europe.

When he’s not writing or in front of the camera, Rob can usually be found in his kitchen garden—come rain or shine, and almost always accompanied by his faithful dogs, Nipper and Reggie.

Photos: Robert Smith and Darren Laki / Text: Robert Smith / May 2025

JULIANA GROUP

Juliana Group is a leading global supplier of quality residential greenhouses. Founded in 1963 by Mogens A. Stærmose in Odense, Denmark, the family-owned company is today owned by the third generation, Nikolaj Stærmose. The headquarters remains Odense-based with subsidiaries in the UK and Germany and exports to more than 20 countries.

’We help people grow’. That is why our brands Halls, and Juliana set the frame for experiences that are as natural as they are magical. Despite our long history, we’re future-focused and curious about how we can improve the sustainability of our greenhouses. Learn more about Juliana and find inspiration for life in the greenhouse in our inspirational universe.