How long does legionella take to grow?

Under ideal laboratory conditions — warm water, plenty of nutrients, other microbes to prey on — legionella can double in a matter of hours. In an actual building water system the realistic timescale is days to weeks: long enough that a fortnight of warm, still water is a real hazard, and the reason the standard control interval for a little-used outlet is a week. The weekly flush is not an arbitrary figure; it is the system's growth clock being reset before colonisation can take hold.

The growth range and why lukewarm is the enemy

Legionella multiplies in water between roughly 20°C and 45°C, with the fastest growth in the warm middle of that band. Every control temperature in the guidance is set to keep water out of it: cold below 20°C, hot stored at 60°C in the calorifier and delivered at 50°C or more at outlets. The temperature control guide and the temperature chart set out the full picture. The danger zone is precisely where uncontrolled water naturally drifts: a cold pipe running through a warm ceiling void, a shower hose in a heated bathroom, a storage tank in a summer loft, water blended warm by a mixing valve and left standing.

Why biofilm shortens the clock

Free-floating legionella in clean, moving water multiplies slowly. Inside biofilm — the slime layer of microorganisms and debris that coats wet pipe surfaces — everything it needs is concentrated: nutrients, warmth and physical shelter from both heat and disinfectant. Growth within an established biofilm is faster, and the colony is far harder to dislodge once seeded, which is why dead legs and little-used outlets, where biofilm builds undisturbed, are where problems take root. Scaled shower heads and tap inserts are biofilm habitat for the same reason.

Dormant is not dead

Below 20°C legionella stops multiplying but survives, dormant, and resumes growth as soon as the water warms. This has two practical consequences. First, a system cannot be made safe by letting it go cold — seasonal warm-ups, hot weather on a loft tank, or a warm-up after a shutdown all restart the clock. Second, killing legionella requires heat held at 60°C and above, or disinfection; temperature control works by keeping water permanently outside the growth range, not by cycling through it. The conditions behind all of this are covered in what causes legionella to grow, and the companion answer on how long legionella takes to develop in standing water looks at the stagnation side of the same question.

What the growth clock means for your regime

Every routine in a monitoring schedule is calibrated against how fast colonisation can happen. Weekly flushing of little-used outlets beats the establishment window. Monthly sentinel temperature checks — logged with the temperature checker — catch a system drifting into the growth range before weeks of multiplication have passed. Shower heads are descaled quarterly or per risk assessment because scale feeds biofilm. And after any period of shutdown, the assumption is that growth has had its window, so the system is flushed through or disinfected before reoccupation, as set out in reopening after a shutdown. The regime, recorded in the log book, is the growth clock being interrupted on schedule.