"Remember - always sow the seeds. You can catch up on everything else - but if you get behind on that, there's nothing you can do about it." ....
That means do it NOW! Every day, light is getting shorter and growth is slowing. - In the UK and Ireland we're facing some uncertainty of supplies due to Brexit - so anything you can sow now may give you some vitally important fresh salads etc, if there is a shortage of fresh winter vegetables!
Growing home-saved seed & supporting independent organic seed companies ensures diversity in seed supply & future food security
Sow outdoors in pots or modules - for planting in the tunnel or greenhouse later in September when summer crops are cleared and space is available - or direct sow in the polytunnel now if not too hot:
Cabbages 'Greensleeves', 'Greyhound' & other leafy non-hearting spring types, carrots ('Nantes' and other early finger types, possibly in long modules for transplanting), kales such as Cavalo Nero, dwarf green curled and Ragged Jack (or Red Russian) for baby leaves, lettuces (non-hearting leafy types such as green & red Lollo, Batavian, Jack Ice and Lattughino, Winter 'Gem' & winter butterheads like All Year Round), lamb's lettuce (corn salad), endives*, rocket, Swiss chards & 'perpetual leaf beets*, beetroot 'Bull's Blood' & 'McGregor's favourite' (for salad leaves*), peas (for pea shoots - Oregon Sugar Pod is a good variety), Claytonia* (also called miner's lettuce or winter purslane), American Landcress*, leaf chicories*, rocket, brocoletto 'Cima di Rapa'. All Oriental greens such as Vitamina, Mizuna, pak choi*, Choy Sum, mustards, Komatsuna, Tatsoi etc can be sown now with less risk of bolting now the weather is cooler, and are so productive and useful in polytunnels and greenhouses over winter, and can be grown in large pots if you wish. Also summer turnips*, summer spinach, salad onions*, leafy salad mixes, coriander*, chervil*, plain leaved and curly parsley* and broad leaved sorrel*.
Covering all young seedlings while in seed trays outdoors with a fine mesh-covered frame or cloche will give them protection from pests, early autumn strong winds or heavy rain. Cabbage root fly is still active in early Sept. and can devastate brassica crops. Be extra careful with watering and ventilation of seedlings now, in the damp autumn air, to avoid damping off. I find using a good, organic peat-free compost is best for autumn sowing in particular - I never experience any fungal diseases, as the plants have far more natural resilience that those grown in chemically fertilised peat composts.
Outdoors, sow in modules, in a seedbed for transplanting, or in situ where they are to crop - To possibly to cover with cloches or frames later on in autumn:
Early summer cauliflowers for next year, brocoletto 'Cima di Rapa', fast-growing early 'Nantes' type carrots for a late autumn crop, cabbages (red ball head, 'Greyhound' and leafy non-hearting spring types), leaf chicories*, endives*, salad onions*, Claytonia (winter purslane)*, lamb's lettuce*, American Landcress*, winter lettuces*, kales*, radishes, Oriental radish such as green skinned, red fleshed Mantanhong, or Pink Dragon (a great variety), rocket, summer spinach*, Swiss chard* and leaf beets*, oriental greens such as Choy Sum, Pak choi*, mibuna, mizuna, mustards 'Red & Green Frills', Chinese kale (Kailaan), Komatsuna*, and any of the fast-maturing salad leaf mixes.
On any empty patches of ground already cleared of crops that won't be used over winter:
Sow green manures now such as alfalfa, red clover, mustard (a brassica so be careful watch rotations) winter tares, field beans, fenugreek, phacelia and Hungarian grazing rye. These will help to protect and improve soil, mop up nutrients to stop them leaching in heavy rain - resulting in them being lost and polluting groundwater. Green manures or even weeds will 'lock-up' carbon, hold onto nutrients and feed worms later, when cut down and covered. Dig them in or cut down and leave on surface later after the first frosts, then cover with something waterproof to protect the soil, prevent nutrient loss and possible pollution. The worms will then work on incorporating the plant material into the soil over the winter - leaving you a perfect, weed free, warmer, more friable and more fertile soil to start your spring sowings next year. Don't leave manure or mulches uncovered now, otherwise you will cause pollution - whatever some of the devotees of that practice may say!!
Also sow a few hardy annuals, to flower over the winter in polytunnels and greenhouses, and early next year outside for bees and other pollinators which either don't hibernate or emerge in early spring. Bees and other insects are the basis of all life on earth - and they need all the help they can get now!
If you want new potatoes for Christmas -
You could also still plant a few sprouted potato tubers in pots before mid-Sept. - to bring into the greenhouse or tunnel later. 'Autumn planting ready' types are available now in garden centres - or if you have any small tubers of 1st or 2nd earlies you've kept from your spring crop, or 'Mayan Gold' or 'Apache' lifted in spring/summer - put them in the fridge for a couple of weeks - then bring into the warm and keep dark for a few days - this will initiate sprouting of shoots - Mayan Gold and Apache are great-tasting potatoes which are not day-length sensitive and will grow quite happily at any season of the year. Lady Christl is also good and always the fastest to bulk up but Sharpe's Express and Duke of York are also good. The sooner you plant them the better now. Give them really good air circulation once they are above the surface - to avoid late blight and don't wet the leaves when watering as doing this encourages it.
*Best sown in early September