A Guide to Growing Kale in a Greenhouse

Kale, one of the hardiest and most nutritious leafy greens, is a favourite among keen gardeners. Growing kale in a greenhouse provides several benefits, including protection from extreme weather and pests, and extends your growing season, allowing you to enjoy fresh kale year-round. In our guide to growing Kale in a greenhouse, we will cover everything you need to know about growing kale in a greenhouse, from choosing the right varieties, caring for your plants, and protecting them from pests, to harvesting and cooking with this versatile vegetable.

Benefits of Growing Kale in a Greenhouse

Greenhouse gardening allows UK gardeners to cultivate kale with fewer challenges posed by the often unpredictable weather. Here’s why growing kale in a greenhouse is advantageous:

1. Extended Growing Season

Kale is a cold-hardy vegetable, but growing it in a greenhouse ensures a prolonged growing season. You can start growing kale earlier in spring and continue harvesting well into winter, without the risk of frost damage.

2. Protection from Pests

Common outdoor pests like pigeons, caterpillars, and slugs can wreak havoc on kale crops. A greenhouse provides a physical barrier to these pests, allowing your plants to grow unhindered.

3. Consistent Growth Environment

With the controlled environment of a greenhouse, you can maintain optimal conditions for kale growth, including the right levels of light, temperature, and moisture, ensuring stronger and healthier plants.

4. Hardening Off for Outdoor Planting

Young kale plants grown in a greenhouse can later be transplanted into raised beds or cold frames, allowing for an extended outdoor growing season as well. The greenhouse provides a nurturing environment for seedlings before they are ready to face the elements.

Choosing Kale Varieties for Greenhouse Growing

There are several varieties of kale to choose from, each offering different flavours, textures, and growth habits. Here are some of the best varieties suited for greenhouse growing:

1. ‘Curly Kale’ (Brassica oleracea var. sabellica)

This is the most common variety, known for its curly, dark green leaves. It’s a reliable and productive type that grows well in a greenhouse environment. It has a mild, slightly sweet flavour and is highly versatile in the kitchen.

2. ‘Cavolo Nero’ (Dinosaur Kale or Tuscan Kale)

Cavolo Nero has long, dark, wrinkled leaves and is favoured for its rich, earthy flavour. This Italian variety thrives in greenhouse conditions and is highly nutritious, making it an excellent addition to soups, stews, and salads.

3. ‘Red Russian’ Kale

This variety has flatter, frilly-edged leaves that turn red-purple in cooler temperatures. Red Russian kale is fast-growing and has a slightly sweeter flavour than other kales. It’s ideal for greenhouse growing due to its adaptability.

4. ‘Siberian Kale’

As its name suggests, Siberian kale is incredibly cold-hardy, making it perfect for year-round greenhouse growing in the UK. It has broad, grey-green leaves with a mild, cabbage-like taste.

How to Grow Kale in a Greenhouse

Growing kale in a greenhouse is relatively straightforward. By following these steps, you can enjoy a bountiful harvest of this nutrient-packed vegetable.

1. Sowing Kale Seeds

• Timing: You can start sowing kale seeds in early spring (March) for a summer harvest, or late summer (August) for winter harvesting. Kale thrives in cooler weather, making it a great crop for the UK climate.
• Seed Preparation: Sow the seeds about 1 cm deep in seed trays or small pots filled with well-draining, organic compost. Kale seeds typically germinate in 7–10 days.
• Spacing: Once the seedlings reach a height of around 10 cm, they can be transplanted into larger pots or directly into the greenhouse beds, spacing them 30–45 cm apart to give each plant ample room to grow.

2. Caring for Your Kale Plants

• Watering: Kale prefers consistently moist soil. Water your kale plants regularly, but avoid overwatering as this can lead to root rot. In the summer months, ensure the greenhouse is well ventilated to prevent heat stress, and provide more frequent watering during hot spells.
• Feeding: Kale is a leafy vegetable, so it benefits from nitrogen-rich fertiliser. Use an organic liquid fertiliser every 4–6 weeks to promote lush, green growth.
• Mulching: Mulch around the base of your kale plants to retain moisture and suppress weeds. This is particularly important in the warmer months to prevent the soil from drying out too quickly.
• Light: Kale requires at least 6 hours of sunlight daily. Ensure your greenhouse is positioned to receive ample light, especially during the winter months.

3. Hardening Off and Planting Out

If you plan to transplant kale into outdoor raised beds or a cold frame, start by hardening off the plants. This process involves gradually exposing the young plants to outdoor conditions by placing them outside during the day and bringing them back into the greenhouse at night. After about 1–2 weeks of hardening off, your plants will be ready to handle outdoor temperatures.

• Raised Beds: Raised beds provide excellent drainage and warmer soil temperatures, making them ideal for kale. Plant the kale 45 cm apart to allow for full growth.
• Cold Frames: Cold frames are a great intermediate step for kale plants moving from the greenhouse to the outdoors. They provide protection from harsh weather while still exposing the plants to more natural light and temperature fluctuations.

Common Pests and Diseases

Even in a greenhouse, kale can still face some common pests and diseases. Here’s how to protect your crop:

1. Aphids

Aphids can suck the sap from your kale plants, leading to stunted growth. To manage aphid infestations, introduce beneficial insects like ladybirds or use organic insecticidal soap.

2. Cabbage White Butterflies

The larvae of cabbage white butterflies (caterpillars) can quickly devour kale leaves, use fine netting to prevent butterflies from laying eggs on your plants.

3. Slugs and Snails

While less of a problem in a greenhouse, slugs and snails can still find their way inside. Use organic slug pellets, copper tape, or handpick them off the plants if you notice any.

4. Powdery Mildew

This fungal disease can affect kale grown in humid environments. Ensure your greenhouse is well ventilated to prevent mildew, and remove any affected leaves to stop the spread.

Harvesting Kale

1. When to Harvest

Kale can be harvested as early as 60–90 days after planting. You can begin by harvesting the outer leaves while leaving the inner leaves to grow, ensuring a continuous supply. For baby kale, harvest when the leaves are small and tender, typically 30–40 days after sowing.

2. How to Harvest

Use scissors or a sharp knife to cut the leaves at the base of the plant. Avoid tearing the leaves by hand, as this can damage the plant. Kale is a “cut-and-come-again” crop, meaning you can continue harvesting leaves from the same plant over several months.

3. Storing Kale

Freshly picked kale can be stored in the refrigerator for up to a week. Alternatively, kale can be frozen for long-term storage. Blanch the leaves in boiling water for 2–3 minutes, then cool them quickly in ice water before freezing.

Health Benefits of Freshly Picked Kale

Kale is often referred to as a superfood due to its impressive nutritional profile. Here are some key health benefits of incorporating freshly grown kale into your diet:

1. Rich in Vitamins and Minerals

Kale is an excellent source of vitamins A, C, and K, which support eye health, immune function, and bone strength, respectively. It’s also high in calcium, iron, and potassium.

2. High in Antioxidants

Kale contains powerful antioxidants like quercetin and kaempferol, which help neutralise harmful free radicals in the body, reducing inflammation and the risk of chronic diseases.

3. Supports Heart Health

The high fibre content in kale can help lower cholesterol levels, supporting heart health. Its antioxidants also protect against oxidative stress, which can lead to heart disease.

4. Boosts Digestion

Kale is rich in fibre, which aids digestion and promotes a healthy gut. The leafy green also contains compounds that support liver function and detoxification.

Recipes Using Home-Grown Kale

Freshly picked kale adds a nutritious, flavourful element to a wide range of dishes. Here are some easy ways to use your home-grown kale:

1. Kale and Potato Soup

A hearty and warming dish, kale and potato soup is perfect for colder months. Simply sauté onions, garlic, and potatoes, then add vegetable stock and fresh kale for a comforting bowl of goodness.

2. Kale Chips

For a healthy snack, try making kale chips. Toss kale leaves in olive oil, sprinkle with salt, and bake in the oven until crispy. Kale chips are a crunchy, nutritious alternative to crisps.

3. Kale Pesto

For a twist on traditional pesto, substitute basil with kale. Blend fresh kale leaves with garlic, olive oil, pine nuts, and Parmesan cheese for a vibrant, flavourful sauce that pairs well with pasta or grilled vegetables.

In conclusion, our guide to growing kale in a greenhouse offers British gardeners a reliable way to enjoy this nutrient-rich vegetable year-round.