Don’t be fooled by thinking because you are in Africa you are guaranteed year round weather. The Winelands along with the rest of the Western Cape definitely has summer and winter.
Summer can be hot and dry with temperatures in January and February regularly into high 20s and upwards. In fact as the Winelands do not benefit from the cooling breeze found on the coast you can often find it gets into the 30s. You can generally expect pretty consistent summer weather from December to through into April.
May/June and Oct/Nov are less predictable as they fall between the main seasons. It can be a great time to travel as it less busy, sometimes cheaper and there is chance you could still get perfect weather.
July-September is classified as winter. It is generally cooler, perhaps down to 8 or 9 degrees, so not cold by UK standards but this is when the Western Cape expects to receive the majority of it’s rainfall, vital to fill the dams that are virtually drained over summer.
There are still advantages to winter travel, some days can still be fine. If you get a cold rainy day, there are plenty of vineyards where you can cosy up in front of a log fire and enjoy a wonderful glass of local red wine.