When later flowering plants are not producing nectar due to weather or other reasons, beekeepers should ensure their colonies are not starving.
An indication of starvation could be that bees are lethargic, or may even be particularly aggressive.
On checking the frames, there should be plenty of stores on them. If not, you will need to think about feeding.
A sugar syrup solution made up of granulated sugar and water 1:1 (e.g. 6kg to 6 litres of water). The sugar and water should be placed in a pan and heated slowly, continuously stirring. The solution should not boil, but if continuously stirred, the solution will become clear and have a slight yellow colour. It is then ready to leave to cool, then put in a container.
Once the last Supers have been removed (and you may start commencing your Varroa treatment, e.g. Apivar) a mixture of sugar syrup 2:1 water is required to help the bees prepare for Winter.
(As a Learner Beekeeper, I have been using Ambrosia on occasions, especially Autumn.
The sugar syrup in Ambrosia is already inverted, so it saves the bees having to
go to the trouble of inverting ordinary sugar syrup -
The advice given from experienced beekeepers is (in September especially) to not feed too much at one time, as you still want the Queen to have space to lay in the brood box. Too much feed may cause workers to fill the frames with feed/stores and the Queen then has no room to lay. It is important she lays to build up the colony strength to see through the Winter.
You can heft the hive to check how much is stored, if it is heavy, the bees may have enough stores, if it is light, it is recommended that you feed.
If the weather is warm enough in September (16 degrees or more), you can usually have a quick check of the frames in the brood box to see if the Queen has space to lay.
For more info on Autumn feeding, click here
The National Bee Unit has a very useful leaflet on feeding (Click here to download)