How to Grow Garlic in a Greenhouse
Key Takeaways: Greenhouse Garlic Essentials
- ✅ Rust Protection: Growing under glass is the single best way to prevent the devastating garlic rust fungus common in the UK.
- ✅ The "Chill" Factor: Garlic needs a period of cold (0–10°C) to split into cloves. Don't heat your greenhouse too early!
- ✅ Container Depth: Garlic roots go deep. Use pots at least 20cm (8 inches) deep for successful bulbs.
- ✅ Seasonal Timing: Plant between October and December for the biggest, most flavourful bulbs.
- ✅ Expert Tool: A cold frame is excellent for starting cloves if your main greenhouse is full of winter greens.
While many UK gardeners view garlic as a strictly outdoor crop to be shoved in the ground and forgotten about, I’ve found that bringing it under glass changes everything.
If you’ve ever pulled up a crop of garlic only to find the leaves covered in orange spots and the bulbs small, you’ve been a victim of garlic rust or waterlogged soil. In our wet British winters, outdoor garlic often struggles.
Growing garlic in a greenhouse solves these problems instantly. You control the water, you eliminate the rain that spreads fungal spores, and you can harvest weeks earlier than your allotment neighbours. Whether you have a spacious aluminium structure or a compact wooden model, garlic is one of the most rewarding "set and forget" winter crops you can grow.
In this guide, I'll walk you through exactly how we do it here at Greenhouse Stores, ensuring you get fat, pungent bulbs every single time.
Why Choose a Greenhouse for Garlic?
You might be wondering if it's worth taking up valuable staging space for a crop that survives outdoors. The answer is a resounding yes, and here is why.
1. The Rust Solution
In the UK, garlic rust (a fungal disease) is rampant. It spreads via water splash on leaves. By growing inside one of our aluminium greenhouses, you keep the foliage dry. This simple change drastically reduces the chance of disease, keeping the green energy-generating leaves healthy for longer, which results in significantly larger bulbs.
2. Soil Control
Garlic hates "wet feet." In heavy clay soil or a waterlogged winter garden, cloves can rot before they even sprout. In a greenhouse, you control the moisture levels perfectly.
3. Early Harvest
Greenhouse garlic warms up faster in spring. You can expect to harvest your crop 3–4 weeks earlier than outdoor plantings, freeing up space perfectly in time for your summer tomatoes or cucumbers.
Choosing the Right Variety: Hardneck vs. Softneck
Before you start planting, you need to pick your team. Garlic comes in two main categories, and your choice depends on your taste and storage needs.
Softneck Varieties
These are the ones you usually find in supermarkets. They don't produce a flower stalk (scape), they store incredibly well (up to 9 months), and they are generally more tolerant of our variable British weather.
- Top Pick: Solent Wight (Perfect for UK climate).
Hardneck Varieties
These produce a curling flower stalk called a "scape" which is delicious in stir-fries. They have fewer but larger cloves and a more complex, intense flavour. However, they don't store as long (3–5 months).
- Top Pick: Red Duke (Fiery and fantastic).
Matt's Tip: For a steady supply, I plant 70% softneck for storage and 30% hardneck for that intense early-season flavour.
The Vernalisation Trap: A Critical Warning
Here is where many greenhouse growers go wrong. This is the content gap that most general gardening guides miss.
Garlic needs a period of cold temperatures (vernalisation), usually between 0°C and 10°C for about 30–60 days, to trigger the bulb to split into individual cloves.
If you plant garlic in a heated greenhouse that stays above 10°C all winter, you will end up with a "round." This is a single, solid bulb that looks like an onion. It tastes like garlic, but you lose that classic clove structure.
How to Manage Vernalisation in a Greenhouse:
- Don't Heat Yet: If you have heaters for sensitive plants, keep the garlic in the coldest part of the greenhouse (near the door or louvre vents).
- Use a Cold Frame: If your greenhouse is heated for exotic plants, start your garlic in a cold frame outside. Once the cold spell of Jan/Feb has passed, move them into the greenhouse for the spring growth spurt.
- Refrigerator Method: If you are planting late (February/March), put your bulbs in the fridge for 3 weeks before planting to simulate winter.
Preparing Your Planting Site
You have two options: planting directly into greenhouse borders or planting in containers.
Growing in Containers (Recommended)
This is my preferred method because garlic is efficient. You can place pots on shelves or tuck them into corners.
- The Depth Rule: Garlic roots can go down 60cm in open soil. In pots, you need a minimum depth of 20cm (8 inches). Standard shallow seed trays will not work.
- The Mix: Use a high-quality multi-purpose compost mixed with 20% grit or perlite for drainage. Garlic despises soggy compost.
Growing in Borders
If you have soil borders in your greenhouse, ensure you dig in plenty of well-rotted manure a month before planting. If you previously grew tomatoes in that spot, refresh the soil or add a general fertiliser like blood, fish, and bone, as tomatoes are heavy feeders that deplete the soil.
If you are prepping your cloves and mixing soil, a dedicated workspace makes life easier. Our range of potting sheds offers the perfect combination of storage and light to get your winter planting sorted comfortably out of the wind.
Step-by-Step Planting Guide
The best time to plant garlic in a greenhouse is from October to December. Planting before Christmas allows the roots to establish while the soil is still relatively warm, ensuring the plant is ready to fly when spring arrives.
- Break the Bulb: Carefully separate the bulb into individual cloves. Do this just before planting to keep them fresh.
- Select the Best: Only plant the large, fat outer cloves. Use the small inner ones for cooking—they produce small bulbs if planted.
- Do Not Peel: Leave the papery skin on the clove; it protects against rot.
- Position: Plant the clove flat end down, pointed end up.
- Depth: In pots, cover the tip of the clove with about 2–3cm of compost. In the ground, plant slightly deeper (3–4cm).
- Spacing:
- In Pots: You can squeeze them in a bit tighter than outside. Allow 10–12cm between cloves.
- In Borders: Allow 15cm between cloves and 30cm between rows.
Care and Maintenance
Once your garlic is in, it's relatively low maintenance, but "low" doesn't mean "no."
Watering
Overwatering is the enemy. In winter, your greenhouse garlic will hardly need water. Check the soil moisture with your finger. If it's dry an inch down, give it a drink. As the days lengthen and temperatures rise in March and April, increase watering significantly. Bulb formation requires water.
Weeding
Garlic does not like competition. It has narrow leaves that don't shade out weeds. Keep your pots and borders weed-free to ensure nutrients go to the bulb, not the intruders.
Feeding
Garlic is a hungry crop during its rapid growth phase.
- Winter: No feed required.
- Spring (March): As soon as you see new green growth, apply a nitrogen-rich fertiliser (chicken manure pellets are great) to boost leaf growth.
- Late Spring (April/May): Switch to a high potash feed. Interestingly, the feed you use for growing tomatoes in a greenhouse is perfect for this stage, as it promotes bulb swelling rather than leaf growth.
Pest and Problem Solving
While greenhouses protect against rust, they can introduce other issues if you aren't careful.
Managing Humidity
High humidity can lead to fungal issues like white rot or downy mildew.
- Ventilation is Key: Keep your greenhouse vents open on mild winter days. Good airflow is essential.
- Spacing: Don't pack pots too tightly together. Air needs to circulate between the leaves.
Pests
Aphids can sometimes find their way into a greenhouse early in the year. Keep an eye on the soft new growth. If you see them, squash them by hand or use a mild soapy water spray.
Harvesting and Curing
The moment of truth!
When to Harvest
Greenhouse garlic usually finishes 2–4 weeks ahead of outdoor crops.
- Hardneck: Harvest when the bottom leaves start to yellow, but the top 5–6 leaves are still green.
- Softneck: Wait until the foliage goes floppy and lies on the ground.
Don't wait until the leaves have completely died back, or the bulb wrappers may split, reducing storage life.
The Curing Process
You cannot store fresh garlic immediately; it needs to cure (dry).
- Lift the bulbs gently with a fork (don't pull the stems).
- Brush off excess soil (don't wash them!).
- Lay them out on wooden greenhouses staging or a wire rack in a dry, airy place out of direct sunlight.
- Leave them for 2–3 weeks until the skins are papery and the roots are dry and brittle.
- Trim the roots and stems (or braid the stems if they are softneck) and store in a cool, dry place.
Summary: Your Winter Growing Strategy
Growing garlic in a greenhouse is a fantastic way to utilise space that often sits empty during the winter months. By following these steps, you secure a crop that is cleaner, healthier, and earlier than anything grown outside.
Your Next Step:
Check your greenhouse space. Do you have room on the floor or staging? If you're short on space or want to keep your greenhouse purely for tender plants, consider adding a cold frame to your garden arsenal. It’s the perfect halfway house for garlic and frees up your main greenhouse for early spring seedlings.
Browse our range of greenhouses for sale to find the perfect winter growing companion.
Frequently Asked Questions
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Can you grow supermarket garlic in a greenhouse?
We generally advise against this. Supermarket garlic is often sourced from warmer climates (like Spain or China) and may carry diseases that can infect your soil. It's also often treated with growth inhibitors to stop it sprouting on the shelf. Always buy certified disease-free cloves from a garden centre or seed merchant.
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My garlic has produced a flower stalk, what should I do?
This is a "scape," and it happens with Hardneck varieties. Snip it off as soon as you see it curling. This redirects the plant's energy back into swelling the bulb. Don't throw the scape away—chop it up and use it in stir-fries or pesto!
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Why are my garlic leaves turning yellow in winter?
A little yellowing of the tips in deep winter is normal due to cold stress. However, if the yellowing is extensive, it could be waterlogging. Check your drainage immediately. In late spring, yellowing is a natural sign that harvest time is approaching.
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Can I grow garlic in grow bags?
Yes, standard tomato grow bags can work, but they are often quite shallow. If you use them, cut them in half crosswise and stand them on end (like a pot) to get greater depth for the roots. Ensure you puncture drainage holes in the bottom.
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Do I need to heat my greenhouse for garlic?
Absolutely not. Garlic is hardy and survives freezing temperatures. In fact, as mentioned in the Vernalisation section, heating the greenhouse too much can prevent the bulb from forming cloves. Keep it frost-free if you wish, but don't keep it tropical!
