September
Seeds To Sow.
It may seem premature, but September is the time to start thinking about plants for next year. If you like cut flowers in your Allotment it can be expensive to create a cut flower bed of Herbaceous perennials, but many Herbaceous Perennial seeds can be sown from late Summer onwards including some Biennials such as Angelica. A packet of seed will produce numerous young plants for less than the cost of one mature plant and it does not take that long for them to reach maturity. In fact many Herbaceous Perennials sown this month will produce plants that flower the following year. Sown at this time they will germinate as easily as many other seeds, with one big proviso. A lot of Herbaceous Perennials will need to be “Stratified,” (given a Winter chill) by keeping them in your fridge for a couple of weeks, or so, before sowing. The other big point to remember is that you may well need to over Winter the young plants under cover for the first winter. After that though, they can be planted out in the Spring and they will be away.
One idea that has gained in popularity in recent years is to grow New Potatoes for Christmas. To do this you need to plant them the first week of this month at the latest. Almost any variety that has been kept from shooting will be suitable. Ideally they will have been kept be chilled as they do commercially, but if you are used to keeping your own seed Potatoes from one season to the next you should have no problem using your own. The problem is that the UK climate is not very conducive to the idea as any frost will take the tops in late Autumn, although, you can try them outside by using a combination of fleece and cloches. However, you will do much better in a greenhouse in Potato Grow Bags.
If you have large areas of your plot clear after taking things like Potatoes out you might like to sow something called Green Manure. The phrase “Green Manure,” covers any vegetation that is dug back into the soil to put goodness back into the soil and help condition it. However, a commercial seed mix for deliberate planting will contain things like Rye, Mustard and Clover seeds, but it can contain seed for other plants. Typically it will also contain some Legumes as these fix Nitrogen in the soil. Green Manure will help to suppress weed growth by covering the soil with its quick growing, luxuriant growth and smothering out the weeds. After the crop has matured it can be dug in whilst still green, before the Winter sets in, or left to die off and dug in, in the Spring as you might a mulch.

Plant Out.
It is too late to plant most crops for this year as they will simply not have time to mature before the cold nights come that will stop all plants from growing. However, there are a few things that you can grow such as quick to harvest Salad Crops. By now a lot of things will have been cleared from your plot leaving bare soil that will stay that way until the Spring. Salad plants might not be your first choice of vegetables to grow as the weather gets colder, but growing them will keep your plot active and help prevent weeds in otherwise empty patches. They can be grown in almost any small space, but are probably best grow under cloches at this time.
If you are growing a few flowers to make your plot look nice then it is time to plant out Wallflowers and Sweet Williams. These often come bare root and wrapped, in bundles, in wet newspaper. They may well have dried out a bit when you get them, so it is a good idea to soak the bundles in a watering can for a few minutes before planting. 
Spring Vegetable plants to crop early next year are something else that can go in now. It used to be just Spring Cabbage that was available, but these days there is a whole range of vegetables that you can buy.

Other Jobs To Do.
Most things will have been harvested by now, but shallots are one thing that is a bit later to end its growing season. As with Onions these will need ripening and drying off properly before attempting to store them, although most Shallots are grown for Pickling, in Vinegar as a means of preserving them almost indefinitely.
Keep an eye on your Hazel trees to see if the nuts are ready, but don’t be tempted to pick them until they are starting to dry and turn brown otherwise they may have no nuts in them. When they are dry enough to start dropping from the tree that is a sure sign that they need picking urgently. Even after picking, the nuts inside their cases will remain “Green,” moist and crisp, for some weeks before drying to the familiar texture that you get with shop bought nuts. Contrary to popular ideas you can safely eat them “Green,” and they are a whole different taste experience.
Other jobs to do include pruning out the old, brown, Early/Summer Raspberry canes. The canes to keep will be the young ones that are a different, fresher colour to the old ones that have turned brown, fruited and are of no future use. Tie in the Canes that you are keeping to support and protect them against any Winter storms.
Runner and French Beans will be coming to an end soon, so as the flowers stop forming it will be time to remove them. However, if you leave some of the bigger pods on the plants to mature and ripen they will swell and develop proper seeds inside that you will be able to save for use next year. When you do finally remove the old plants a lot of people like to cut off the top growth and leave the roots of the dead plants in the soil to rot away over Winter. The reason for this is that the roots are “Nitrogen Fixing,” and people believe this will add even more Nitrogen to the soil.