Why VPD matters more than humidity alone during rooting

Relative humidity on its own doesn’t tell you how hard a cutting is working to hold water while it roots — vapour pressure deficit (VPD) does, because it accounts for temperature as well as humidity. A propagation tent reading 85% humidity at 22C behaves very differently to the same 85% at 30C; the warmer tent has a much higher VPD and cuttings lose water faster than they can replace it without roots.

The practical effect is wilting or stalled rooting that looks like a humidity problem but isn’t one that raising humidity alone will fix — cooling the zone, or shading it, often matters more. This is why PropagAfrica’s monitoring sensors track temperature and humidity together and calculate VPD directly, rather than leaving the grower to work it out from two separate numbers.

As a general target for softwood cuttings, aim to keep VPD low and stable during the first 7-10 days, then allow it to rise gradually as roots establish, to harden the plant off before it leaves the tent. Exact targets vary by species — our technical team can help set them for your specific crop during a site visit or virtual consultation.