Any kind of legumes will make for an ideal green manure

Planting a green crop and digging it back into the soil can be done before autumn, says Fiann Ó Nualláin

AS YOU have been steadily harvesting your potatoes and other crops over the past few weeks, space they once occupied is often left fallow. You may want to rest it a little before the next successional sowing, or even have it sit out a season until it fits back into your crop rotation plans. Well, it doesn’t have to be idle and you can choose to sow in a green manure — then you can be one step ahead of the soil fertility game.

My advice as always is — don’t wait until late autumn to dig in manure, grow a green manure now. When you think that potatoes are heavy feeders and will deplete your soil rapidly — why not sow in a green manure straight after?

Any fast-growing crop of legumes (you can use edible beans and peas as well as commercial green manures from the legume family), will through their physiology (root nodules with a symbiotic relationship with bacteria), fix nitrogen from the atmosphere back into the depleted soil. Then when you dig the temporary crop back into the soil in a few weeks to a few months, all that plant material will break down into good humus and further boost soil fertility.

If you grow edible French beans or sugar snap peas you can have a meal for yourself before you dig in the stems, leafs, and roots to give a feed to your soil. Green manures are more than natural fertilisers you can use them too with the bonus of all the advantages of companion-planting science.

Just as one would plant Tagetes to deter nematodes, so you can sow Mustard (a great green manure in its own right), to confound the potato eel worm by tricking it into breeding at the wrong time. Mustard is a brassica so avoid in succession to other brassicas, or if club root is active in your soil.

The extra benefits of green manure is, apart from the fact that where there is a plant there is no room for a weed, these crops will also prevent soil from leeching nutrients or succumbing to wind erosion. They cover and protect areas that you will make even more productive later on.

Green manures are traditionally sown in late summer or autumn but any time into an empty bed or harvest-stripped location is fine. You don’t have to sow the whole bed or wait for the whole plot to empty, you can sow as you go. Small patches as you need — as the harvest leaves a patch.

The most popular green manure is Grazing rye (Secale cereal) — it’s an annual crop with extensive root systems, sown August to November, traditionally to improve soil structure as it grows and then to be dug in as green manure the following spring.

I have some sown from a clearance back in July and I will dig that in around November before I plant out some overwintering crops. While I like the wisdom of some traditions and the link to all gardeners gone before —you don’t have to stick to tradition. Its okay to experiment and make things work your way.

Green manures are valuable as living mulches to suppress weeds, but also as good company, in that they provide cover for beetles, frogs and other pest predators and when they flower, they also attract pollinators and aphid-eating insects too. So a strip of green manure can be used as a natural divider between crops or beside a row to reap theses rewards.

Over-wintering green manures have the added benefit of holding in existing fertility by not letting rain leech nutrients away and Secale cereal sometimes sold as Hungarian grazing rye is the best variety of green manure for winter use too. It germinates in 7-14 days, so if you want to wait until November you can figure out best sown times for your area’s first frosts.

Surface sow the seed at a rate of 16g-18g per square metre, you may rake, but do lightly tamp down. I also recommend covering the sown area with a fleece; the extra heat helps speed germination, but it also offers vital protection against foraging birds at this time of year and later on, too.

If you want to let it run its traditional span then come March/April you can strim or mow it back and then dig it in. Digging-in can be hard work but worth it. I recommend that you leave the dug-in site alone for further four weeks to let it fully break down in the soil.

Then you can plant in potatoes and tubers, crowns or legumes. But from experience, if it is to be followed by carrots, parsnips or any small seed, then I would dig in a month earlier and let it sit for two full months – the thing it never says on the packet is that as it breaks down it produces a temporary seed toxin that generally dissipates within six weeks, but which will inhibit germination of smaller seeds while still active. Other green manures that you try include:

- Mustard (Sinapis alba): an annual crop than can be sown March to September – let grow for two or three months before digging in.

- Phacelia (Phacelia tanacetifolia): an annual crop sown April to August and generally dug in after two or three months. It can overwinter in mild areas.

- Trefoil (Medicago lupulina): utilised as an annual or biennial, sown in March to September — it can be dug in after two or three months or allowed to overwinter. If you have a larger allotment and need to block off a section this is ideal, as it can be left for 1-2 years after sowing;

If you plan to green manure over winter then try Winter field bean (Vicia faba): sown September to November and left for two or four months after sowing. Really good for heavy soils.

Or Winter tares (Vicia sativa): sown July to October for overwintering and a spring dig in.

Remember too that you don’t have to follow your potatoes with green manure, you can precede them with it.

Fiann’s tips for the weekend

- It is timely to harvest up the bulk of your potatoes — do let them sit in sunshine to dry for two to three hours before storing. They will keep longer if cured this way.

- The tradition is to lift onions and shallots once the foliage has started to die back — which commences any day now, but please don’t bend over the tops to speed the process as it only reduces their storage potential.

- If possible, allow pulled onions and shallots to dry on the soil surface. If weather is uncooperative, cure them (let skins harden) in a dry, well-ventilated shed .

- You can plant overwintering onion sets from now.

- Now is also the time to think of other overwintering vegetables — so continue to sow the likes of turnip, spinach and Oriental vegetables and plant out any plugs under a little fleece to stop the pigeons until established.

- Take a bit of time to reflect on how much you have achieved this year — the bounty of your harvests so far. Then, why not feed the garden some thanks — it’s almost as good as green manure for it and it’s really good for your own mind-body-spirit.