Month by month
Udzungwa in September
September closes out Udzungwa's prime hiking stretch with 17 mm of rain over 2 days, tied for the fewest rain days anywhere on the calendar, while the daytime heat starts visibly climbing toward October's peak.
If you only want the answer
September is still firmly inside the dry season, with trails at their best, but the days are warming fast and it pays to start walking earlier than you did in July or August.
Dry, and noticeably warming
Rain stays minimal in September at 17 mm over just 2 days, matched only by October for the fewest rain days of the year, with Sanje holding a steady but reduced flow after two months of near-zero rain. What moves is the temperature: day highs jump to 27.2°C from August's 25.1°C, a bigger single-month rise than anywhere else on the record outside the wet-to-dry transition, and the first real sign that the heat building toward October's peak has started.
Nights stay mild at 16.5°C, warmer than July and August's cold trail-camp lows, so the discomfort in September shifts from cold nights to warm afternoons rather than disappearing.
Why the extra heat changes your plan more than the rain does
The trails themselves are unaffected, still firm and largely leech-free, and every route from the short Prince Bernhard walk to the five-day Lumemo Trail remains open without restriction. What the rising heat changes is timing: the climb up Sanje's 450 metres or the early stages of Mwanihana are noticeably harder in a September afternoon than a July one, and guides who already push for a 07:00 start have more reason to hold you to it now.
The Udzungwa red colobus keeps the same canopy routine it has all year, unaffected by the warming days, and September remains a solid month for it on both Sanje and Mwanihana.
The practical takeaway
If you have a choice between an early September date and a late one, take the earlier date for slightly cooler walking, though the difference is a matter of degree rather than a real trade-off. Guide assignment is still same-day at Mang'ula, and the only bookings that matter are for special campsites, porters and multi-day trek permits, exactly as in every other month.
September is also a fair fallback for the Mwanihana and Lumemo routes if June through August did not fit your schedule, since the trail conditions have barely moved from their driest point in the year.