The pre-harvest interval (PHI) is the minimum number of days, stated on a registered pesticide label, between the last application and harvest of the crop. It exists so that residues break down to levels the regulator considers acceptable. A PHI of zero days means the product may be applied up to harvest day if the label allows; it does not mean the product leaves no residue.
In practice
For cannabis the PHI is only meaningful when a product is actually registered for cannabis in that jurisdiction; a PHI written for lettuce or tomatoes does not transfer. Harvested cannabis is often tested for residues against strict limits, and flowers are smoked or concentrated, which magnifies any residue. Many growers therefore stop all sprays well before flower, regardless of label PHI. Record every application with date, product, active ingredient and rate, so the interval can be shown. Check what is registered for cannabis locally.
Don't confuse with
- Re-entry interval (REI): how long workers must wait before entering a treated area.
- Residual activity: how long a product keeps working.
- Flushing: unrelated; plain water does not remove pesticide residues from plant tissue.
