Spring marks the start of the annual wildflower takeover along the West Coast of South Africa
Johannesburg and the highveld is warm by day, cold at night, and dry. Cape Town and the Western Cape is mild by day, cool at night and receives regular rainfall.
The Kruger Park and surrounds is hot by day, mild at night, and dry.
September is an ideal time for beach holidays on the Indian Ocean coastline of KwaZulu-Natal, which tends to be temperate to hot over the nominal winter months, and very dry. Conditions on the coast of the Eastern and Western Cape are less predictable.
September is widely regarded to offer the best game viewing of any month in the Kruger National Park and other reserves, as animals congregate close to perennial water sources and the undergrowth clears to improve visibility.
September to November is peak whale-viewing season in Hermanus and the Western Cape.
Accommodation in tourist areas is usually very quiet in September, though it may fill up over the short spring school holiday that usually takes place in late September/early October.
Taking everything into account, September is one of the best months to visit South Africa, assuming you are free to travel then.