Claire Mummery: Clean and replenish your garden soil this winter with green manure crops

Broad beans are a good crop to plant to help replenish nutrients in the soil. Photo/Getty

After a terrible summer, rife with pests and disease, it’s never been more important to rest and replenish our garden beds by growing some green manure crops over winter.

If you’ve never heard of, or grown, green manure crops before, they are usually “cover” crops that are used to cover the soil over winter while not in use. They actively replenish the soil, returning the nutrients that have been spent all summer long, growing your food.

The process is that you sow your crop of choice from seed then, once it grows, you dig it into your bed before it flowers. By digging the plant material back into the soil, you increase the organic matter it contains as the plants decompose, releasing nutrients such as phosphorous, trace minerals, potassium and nitrogen that will be available for your summer plants to feed on.

If you wait too long and the crop produces flowers, all the good nutrients go up to the flower and your soil will miss out.

Green manure crops can also increase your plants’ production and improve the soil structure, opening it up, so it’s an incredible gift you can give to your garden in thanks for all the bounty it has produced for you over the season.

The types of green manure crops are broad beans, lupins (lupines angustifoli), peas, oats, mustard and phacelia to name a few.

Mustard is a quick-growing green manure crop. If you have suffered serious fungus like powdery mildew this summer, I highly recommend planting a quick mustard crop as this cleans the soil of harmful fungi. It also reduces the harm of future diseases.

Mustard seed is a small seed that needs moisture to germinate so always make sure you sow the seed on to a moist area. Prepare your area by removing the previous crop, water the soil and scatter your seed over the area, then rake in if your bed permits.

Sow the seed densely to suppress weed germination by blocking out the light. I recommend 30g per 1sq m. Either lightly cover with soil and water or just put a light sprinkling of water over the seeds each day until germination (about five to 10 days). You can expect your crop to be ready for digging around four to six weeks later.

Lupins are also fairly quick to grow, at six to eight weeks. They are a vigorous grower and stand well together against the wind. They have deep tap roots and are a member of the legume family, which are big nitrogen-fixers. Their deep roots burrow their way into your soil and pull up minerals, aerating your soil at the same time.

Lupin seeds like to be sown no more than 2cm deep to enable good germination and vigour.

Scatter or sow in drills (lines), using a guideline of 40-50 seeds per square metre and water. Then, cover with a 2cm layer of soil/ compost/ potting mix or push each individual seed down. Water the soil again.

Sowing a small area, pot or raised bed? Soak seeds for 24 hours prior to sowing. Lupins can take up to 14 days to germinate. For faster germination, cover the soil with moist cardboard for seven days, then remove it.

Broad beans are slightly longer to grow as a cover crop. Broad beans like to be sown in a spot that gets at least two to three hours of sun per day. They are self-supporting and do not require staking.

Simply sow the broad beans in rows 5cm deep and approximately 15cm apart. Push the bean into the soil, with the dimple in the seed coat facing upwards. This will save time for your plant and ensure better germination as the shoot will have further to travel to reach the light if sown upside down and could run out of its own food before it gets to the surface where it can photosynthesise to start producing its own.

Cover with wet cardboard to speed up germination. Germination can take two to four weeks. My top tip - by soaking your seed prior to sowing, you can halve this time.

Broad beans are like any other bean when it comes to watering - you will only water it once when planted and then no more water until you see the first two leaves appear. Watering before this stage can cause the bean to rot and germination to be poor.

Make the effort while planning your winter garden to decide which beds you will rest over winter. If you like to have home-grown nourishment all year round, my advice for you is to have one garden bed in your rotation with a green manure crop over winter to replenish it, which is what I do at home.

It’s a total no-brainer if you’re a summer gardener only because it takes care of your beds for you while you hibernate inside and prevents weeds from taking over your beds instead.

Happy gardening this autumn/winter.

For more gardening advice, visit growinspired.co.nz and discover Claire Mummery’s online gardening course at www.growinspiredacademy.com