Hardy annuals and some veg crops can be sown in autumn and then overwintered to plant out in spring. This gives them a head start on spring-sown plants and can lead to earlier flowering and cropping times. However, despite being hardy, they need protection from heavy frost, so it’s a good idea to overwinter them with some protection.
That said, it can be difficult to keep seedlings, germinating seeds and young plants alive over winter, especially if you don’t have much indoor space to house lots of pots. Overwintering them in a greenhouse or cold frame can work effectively, but it’s a good idea to follow a few key rules, to ensure the best results.
Clean greenhouse glass
Make sure your plants have access to the best light levels by cleaning greenhouse glass before the winter season gets underway. This prevents seedlings from becoming leggy and ensures they’re still able to photosynthesise. Clean glass is particularly important in winter when light levels are naturally lower and there are fewer hours of daylight.
Make sure you take care when cleaning greenhouse glass. Wear gloves when handling glass (or metal) and avoid leaning on the glass panes. Use long-handled tools for higher sections and always work in pairs if you’re using ladders.
Sweep out your greenhouse
As well as ensuring good light levels, it’s important to remove debris that might harbour mould spores and get rid of slugs and snails. With fewer pests and a reduced chance of fungal issues, your plants are more likely to survive.
Avoid overwatering
Plants need less water in winter as many will have become dormant or will be growing slowly. Reduce the frequency of watering and only water when the soil feels dry to the touch. Watering in late morning is preferable, so that compost doesn’t remain too wet overnight when temperatures are likely to drop. Keeping your plants on the dry side protects roots from rotting in cold, wet winter conditions.
Keep heat in your greenhouse
Insulating your greenhouse with bubble polythene is a cost-effective way to protect plants over winter while still allowing light in. Using a thermometer will enable you to keep an eye on the temperature and add further insulation or heating if necessary. If you’ve only got a small number of seedlings to protect, and you have access to electricity, try using a heated propagator.
Ensure good ventilation
We tend to think of ventilating greenhouses in summer, but it’s also important to ensure good air circulation in the winter months by opening windows and vents for an hour or two in the middle of the day, providing the weather isn’t windy or icy. This helps prevent the fungal problems that can build up in a damp atmosphere. Good ventilation also provides plants with the right balance of oxygen and carbon dioxide so they remain healthy.
Make sure plants aren’t kept too close together. Aim to have gaps between each container or plant so they don’t touch and stand pots on slatted benches or place them on pieces of wood or pot feet if they need to stand on the greenhouse floor.
