Idea sent in by Jane (Oakham). October 2011
If you have any small hardy seedlings and cuttings in pots, such as shrubs and perennials, that are either too small to plant-out in the borders or you don't have the space or time to plant them out before the frosts, then placing them in a plunge bed over-winter is the next best method to keep them safe. It would be a shame to lose all the hard work of rooting those cuttings only to lose them during a hard winter. Some gardeners like to protect them in a cold frame or poly tunnel but I prefer to keep shrubs and herbaceous plants outside and grow them hard, as this is nature's way, allowing the plants to follow their natural dormancy period in winter, making the plants stronger and better to grow-on in the spring.
From my own experience I find that hardy plants left to over-winter in cold frames can often make too much soft growth that is susceptible to mould and diseases, unless you are prepared to keep opening and closing the covers all winter long. However, if you just leave the pots out in the open, the soil inside can either dry-out (as watering is not a normal task in winter) or become overly waterlogged or can easily freeze solid and damage the plant's roots. Plunging the pots into the open ground, up to the depth of the soil in the pot, helps to protect the roots, keeps the pots moist and ensures the temperature remains constant. You can also cover the area in fleece if additional protection is required for very small seedlings etc. However, I would not recommend covering with leaves or straw as this blocks light and can cause the plants to rot.
Making a plunge bed in a sheltered corner of the garden is a simple task. Prepare the soil by digging over and adding plenty of organic matter and sharp grit for drainage. A raised bed is ideal as this ensures good drainage - very few plants enjoy sitting in waterlogged soil in the cold. Also if the soil is light and airy it will provide good insulation and is less likely to freeze. Simply dig the pots into the bed (up to the same level of soil in the pot), keeping them well spaced to ensure good airflow around the plants to prevent mould growing. Once spring arrives the pots can be left to grow-on until ready and lifted and replanted where needed in the garden. Also, as the plants have been kept in their pots the roots will receive the minimum of disturbance.