What is a Garden Propagator?
A propagator is an enclosed growing environment designed to provide seeds and cuttings with the warm, humid conditions they need to germinate and root successfully. In the UK, where spring temperatures can be unpredictable and frosts linger well into April, a propagator allows gardeners to get a head start on the season — sowing tender crops weeks before it would be safe to do so outdoors.
From a simple unheated tray with a clear lid to a sophisticated thermostatically controlled heated unit, propagators come in a wide range of types and price points. This guide helps you choose the right one and get the best results from it.
Types of Propagator
Unheated Propagators
The simplest and most affordable option — essentially a seed tray with a clear plastic or polycarbonate lid. The lid traps warmth and humidity, creating a more stable microclimate than an open tray. Unheated propagators work well on a warm windowsill or in a heated greenhouse, where ambient temperatures are already reasonably warm.
Best for: hardy annuals, vegetables that germinate at lower temperatures (lettuce, brassicas, peas), and gardeners with a warm indoor space or heated greenhouse.
Not ideal for: heat-loving crops like tomatoes, peppers, aubergines, and basil, which need consistent warmth of 18–25°C to germinate reliably.
Heated Propagators
A heated propagator has a built-in electric heating element in the base that warms the compost from below — the most effective way to deliver heat to germinating seeds. Bottom heat dramatically speeds up germination and improves success rates for heat-loving crops.
Basic heated propagators maintain a fixed temperature (usually around 18–21°C) and are suitable for most common vegetables and flowers.
Thermostatically controlled propagators allow you to set a precise temperature, giving you full control over the germination environment. These are the preferred choice for serious propagators growing a wide range of crops with different temperature requirements.
Best for: tomatoes, peppers, chillies, aubergines, cucumbers, basil, and other heat-loving crops; early sowings from January onwards; anyone propagating in a cold greenhouse or unheated room.
Windowsill Propagators
Compact propagators designed to fit on a standard windowsill. They're ideal for small gardens, flats, and anyone without a greenhouse. Look for models with adjustable vents in the lid to control humidity and prevent damping off.
Large Bench Propagators
Designed to sit on greenhouse staging, these larger units can accommodate multiple seed trays simultaneously. Some models have tall lids to allow for taller seedlings before pricking out. Ideal for gardeners who sow large quantities of seed each season.
Propagation Mats (Heat Mats)
A heat mat is a flexible electric mat that sits under seed trays, warming the compost from below. They're used with a separate unheated propagator lid or cloche. Heat mats are a cost-effective way to add bottom heat to an existing propagation setup and can be used with any size or shape of tray.
Key Features to Look For
Thermostat
A thermostat is the single most useful feature on a heated propagator. It allows you to set a precise temperature and maintains it automatically, regardless of ambient temperature. Without a thermostat, a propagator in a warm room can overheat, cooking seeds rather than germinating them.
Vented Lid
Ventilation is essential to prevent damping off — a fungal disease that kills seedlings at soil level. Look for lids with adjustable vents that can be opened progressively as seedlings emerge and grow.
Depth and Capacity
Consider how many trays you want to propagate at once and how tall your seedlings will grow before pricking out. Some propagators have interchangeable lids of different heights.
Energy Efficiency
Heated propagators run continuously, so wattage matters. A 60–80W propagator running for 12 weeks costs relatively little to run, but check the wattage before buying if energy costs are a concern. Thermostatically controlled models are more efficient as they cycle on and off to maintain temperature.
What to Sow in a Propagator: A UK Seasonal Guide
January–February
- Chillies and peppers — need a long season; sow as early as possible at 21–25°C
- Aubergines — similar requirements to peppers
- Pelargoniums — for summer bedding; sow at 21°C
February–March
- Tomatoes — sow at 18–21°C; prick out when first true leaves appear
- Cucumbers — sow on their side at 21°C to prevent rotting
- Celery and celeriac — need light to germinate; don't cover with compost
- Bedding plants — lobelia, petunias, antirrhinums
March–April
- Courgettes and squash — sow individually in small pots at 18°C
- Basil — needs warmth; sow at 18–21°C
- French and runner beans — can be started in a propagator for an early crop
- Sweetcorn — sow in deep modules at 18°C
Propagator Tips for Better Results
- Use fresh, good-quality seed compost — not general-purpose compost, which is too rich and can inhibit germination
- Sow thinly — overcrowded seedlings are harder to prick out and more prone to damping off
- Water from below — stand trays in water until the surface darkens, then drain; this avoids disturbing seeds and reduces damping off risk
- Label everything — seedlings look remarkably similar; always label trays with variety and sowing date
- Open vents gradually — once seeds germinate, open vents a little each day to acclimatise seedlings to lower humidity
- Don't leave the lid on too long — once seedlings are up, remove the lid entirely to prevent leggy, weak growth
- Harden off before planting out — move seedlings to a cold frame or sheltered spot outdoors for 1–2 weeks before planting in their final position
Troubleshooting Common Propagator Problems
- Seeds not germinating — check temperature (too cold is the most common cause); check seed viability; ensure compost is moist but not waterlogged
- Damping off — improve ventilation; water from below; use fresh compost; avoid overcrowding
- Leggy seedlings — insufficient light; move to a brighter position or use a grow light; remove propagator lid sooner
- Condensation blocking light — open vents to reduce humidity; wipe lid regularly
Shop Propagators at Selections
Browse our full range of heated and unheated propagators to get your seeds off to the best possible start this season.