A polytunnel does not have to be used only for annual crops. A polytunnel can also allow you more protection and therefore more variety when it comes to perennial crops. Asparagus is one perennial crop that could be well suited to polytunnel growing. Asparagus needs a sheltered and sunny spot where it is protected from strong wind – a polytunnel is perfect for providing that protection and can also help avoid problems with over-watering that can come from unpredictable UK weather.
There is nothing quite like cutting your own asparagus, however you will need to think about where you will put a permanent bed for the plants. Soil type is not important, but good drainage is essential. Pick a sunny spot sheltered from strong winds. Dig bed for planting in autumn and incorporate plenty of organic matter.
While it is possible to grow asparagus from seed, most gardeners will choose to grow asparagus from crowds that are 1-3 years old. These crowns are best planted in around March or April – a little later if the ground is waterlogged or particularly cold.
Before planting, it is necessary to prepare a growing area for asparagus. First, dig a trench that is approximately 30cm wide and about 20cm deep. Enrich the bottom of the trench with compost or well-rotted manure and then make a 10cm high ridge along the bottom of the trench. Place the crowns on top of this ridge, carefully spreading out the roots to either side and gently re-fill the trench with the excavated soil, making sure that the bud tips are still just visible above the soil surface. Water and then mulch well with a weed free organic matter.
It is vitally important that you weed well and carefully around your asparagus plants as they cannot handle any competition and the roots are close to the surface and easily damaged. For this reason, it is best to weed by hand rather than using a rake. There should be a clear 45cm between each row of asparagus and between the plants in each row.
Patience is important when growing asparagus. It should not be harvested for the first two years after planting. In the third year, spears can be harvested from mid-April for around six weeks and then for eight weeks in subsequent years. You should stop harvesting after this period to allow the plants to retain energy for continued growth and so you can enjoy another harvest the following year.
When harvesting asparagus spears, make sure that you use a sharp knife to cut them off just below the surface of the soil. It is best to harvest the spears when they are no more than 12-18cm tall. In warm weather it is best to harvest every two or three days for the best quality spears.
In autumn, cut down the stems of the asparagus once they have turned yellow and give the plants protection from frost over the winter months so you can continue to enjoy your asparagus for a number of years in your polytunnel or elsewhere in your garden.