When should I stop feeding before harvest?

Q: When should I stop feeding before harvest?

Don't stop feeding before harvest. Stopping nutrients or flushing with plain water in the last 1-3 weeks is a harmful Myth: the plant is still packing on weight and resin. Feed through to the chop at stage strength. The GenetixBay approach is to taper only nitrogen over the last 3-4 weeks, down to 0-20 mg/L in the final days, while keeping P, K, Ca, Mg, S and micros fed.

Short answer

Don't. Feed cannabis through to harvest at stage strength. Stopping nutrients or flushing with plain water in the final 1–3 weeks is a Myth, and it does harm: it starves the plant while it is still adding most of its final weight, trichomes and resin. Taper only nitrogen over the last 3–4 weeks, to 0–20 mg/L at the finish.

Why

Cannabis stores what it takes up. Nutrients are built into proteins, chlorophyll and cell walls, so plain water at the end does not rinse them out of the flower. In Utah State work, leaf nitrogen fell during a flush but flower nitrogen did not change ^[3].

Controlled flushing trials found no gain in quality, minerals or cannabinoids. Short flushes of 2 weeks or less showed no consistent yield change; flushes of 3 weeks or more cut dried bud weight ^[2]^[4].

Nitrogen is the one element worth trimming. Lower nitrogen raised cannabinoid and terpene concentration while yield rose with N up to about 160 mg/L ^[1]. That is a deliberate trade-off, made by cutting N alone, not everything.

The numbers

Weeks before harvest	Nitrogen	Everything else
4	100–150 mg/L (normal late flower)	Normal late-flower strength
3	~75% of that	Unchanged
2	~50%	Unchanged
1	~25%	Unchanged
Final days	0–20 mg/L	Unchanged

What to do

Keep feeding at your late-flower EC; don't switch to plain water.
If you use single salts or per-element dosing, step nitrogen down as above.
With a bottled two-part, keep feeding normally rather than diluting everything.
Watch runoff EC; flush the root zone only if salts have built up.
Judge harvest by trichomes, and put your effort into a slow dry and cure.

Common mistakes

Flushing for two weeks "for taste" and losing weight and resin.
Blaming dark ash on nutrients instead of drying and curing.
"Tapering" by diluting a two-part, which cuts calcium and potassium too.
Panicking over lower fan leaves fading in the last 2–3 weeks; that's normal.

Full article: Per-element feeding tricks
Main article: Pre-harvest flushing: a myth · Nitrogen
Answers: fixing lockout · deficiency in late flower
Terms: nitrogen taper · root-zone flush

Follow-up questions

Does flushing before harvest improve taste?

No. Flushing before harvest does not improve taste or ash colour; that is a Myth. Controlled studies found no gain in flower quality, minerals or cannabinoids, and in one taste trial tasters preferred unflushed flower. Harsh smoke and dark ash come from fast drying, rushed curing and how the material burns, not from nutrients left in the plant.

How do I taper nitrogen before harvest?

Taper nitrogen in steps from about 4 weeks before harvest: around 75% of your late-flower nitrogen at 3 weeks out, 50% at 2 weeks, 25% in the last week and 0-20 mg/L in the final days. Keep phosphorus, potassium, calcium, magnesium, sulfur and micros at normal strength. A diluted two-part bottle cannot do this; single salts or per-element dosing can.

Is there ever a good reason to flush?

Yes, one. A root-zone flush washes accumulated salts out of the medium when runoff EC sits roughly 1.0 mS/cm or more above input, or you see salt crust, spreading burnt tips or lockout. It treats the medium, not the plant, can be done at any stage, and is followed by normal feeding. It is not a pre-harvest ritual.

Sources

Saloner A, Bernstein N, 2021. Nitrogen supply affects cannabinoid and terpenoid profile in medical cannabis (Cannabis sativa L.). Industrial Crops and Products 167:113516. doi.org/10.1016/j.indcrop.2021.113516

Alden MJ, Faust JE, 2025. Exploring the legacy practice of flushing in controlled-environment production of high-CBD cannabis. HortScience 60(10):1818-1825. doi.org/10.21273/HORTSCI18752-25

Hershkowitz JA, 2024. Nutrient management of cannabis in controlled environments. MS thesis, Utah State University. digitalcommons.usu.edu/etd2023/178

Stemeroff J, 2017. Irrigation management strategies for medical cannabis in controlled environments. MSc thesis, University of Guelph. hdl.handle.net/10214/12125