They give your soil the nitrogen it's looking for, introduce organic matter, provide mulch and compost material, plus flower beautifully. Justin Newcombe loves lupins.
After a hard year on the crop rotation merry-go-round, your soil needs a cup of tea and a nice lie down. And what better bed to lie in than a bed of lupins?
Lupins are part of the legume family which also include beans, peas and peanuts. As well as producing snappy flowers, lupins are a wonderful tonic for your soil because they generate nitrogen. As lupins grow, absorbing nitrogen from the atmosphere, they store it in rhizomes attached to their roots. Once the plants are cut down to the soil level, bacteria break down the rhizomes and the nitrogen becomes accessible in the soil for nitrogen-hungry plants such as lettuce and spinach.
When the plants are juvenile, the foliage as well as the roots can also be dug into the ground to fix nitrogen. To do this, grow your lupins to around 400mm tall, then trim them down to about half, leaving the clippings on the soil surface. Once they grow back again, the whole lot can be dug into the ground or cut back and composted over. This process is called manure or green cropping. For your garden to get the most out of it, lupins should be dug into the ground a month or two before planting.
You need to remember that the bigger you grow your lupins, the harder and more carbonic they become which makes decomposition in the soil difficult.
Carbon uses nitrogen to break itself down so if you let them grow too big, then dig them into the ground, the benefits will be negligible. Once they have flowered, lupins become quite woody and may take up to six months to break down properly. They will compete with other nitrogen-hungry plants and will be more of a liability than a help. If you do decide to let them grow big (as I have done here), cut them right down at surface level but leave the roots in the ground. The tops make excellent compost material or can be chopped up finely and used as a mulch under fruit trees while the roots will still provide generous amounts of nitrogen as well as organic matter to improve soil tilth.
Other benefits of green cropping include weed control and soil protection, so if you're planning to leave you beds over winter or you have a prolonged period during summer, lupins are a great space filler.
Sowing lupin is very easy, even on rough ground, as the root system is vigorous. It's a good idea to till the soil before you start and add some wood ash as a top dressing.
If you intend to cut smaller plants down and dig them in, then sow the seed more intensively. But if you intend to grow the plants on to a more mature stage, you can sow more sparsely, as I have here.
For first time gardeners, lupins give you that initial little bit of nitrogen your soil is looking for. They inject a whole lot of organic matter straight into the soil, provide some great mulch and compost material with some cracking flowers to boot.
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