Most cannabis growers stop fertilizing one to three weeks before harvest, switching to plain water for the final stretch of flowering. The exact timing depends on your growing medium, since soil holds nutrients longer than coco coir or hydroponic systems. This “flush” period is one of the most debated practices in cannabis cultivation, but the logic is straightforward: you’re letting the plant use up stored nutrients so they don’t end up concentrated in the final product.
Flush Timing by Growing Medium
The key variable is how much nutrient your medium retains. Dense, heavy substrates hold onto minerals longer and need a longer flush. Lighter or soilless systems release nutrients quickly.
- Sandy soil: 7 days of plain water before harvest.
- Porous loam: 10 to 15 days.
- Heavy loam or clay: 15 to 20 days.
- Coco coir or peat moss: 3 to 7 days, depending on how green the lower leaves still are. If the bottom fan leaves are already yellowing, 3 to 4 days is enough. If they’re still deep green, give it a full week.
- Hydroponic systems without a solid medium (deep water culture, aeroponics): 3 to 4 days.
- Hydroponic systems with a medium (drip irrigation, ebb and flow, wick systems): 4 to 7 days.
These timelines mean you’re counting backward from your expected harvest date. If you’re growing in a standard potting mix and estimate harvest in two weeks, you’d stop feeding now. In a hydroponic setup, you might keep nutrients flowing until just a few days before chop.
What the Science Actually Says About Flushing
Flushing is deeply rooted in cannabis growing culture, but the scientific evidence supporting it is surprisingly thin. A controlled study published in Industrial Crops and Products grew cannabis plants in identical conditions, then split them into two groups at the end of flowering: one kept receiving nutrients until harvest, while the other got only distilled water. The results showed flushing had limited effect on cannabinoid levels, terpene profiles, biomass production, or mineral accumulation in the plants. Cannabinoid concentrations changed in only 30% of samples tested, and terpene profiles shifted in just 3%.
Interestingly, the effects weren’t consistent across different cultivars. Some strains showed a slight increase in cannabinoid production after flushing, while one strain actually showed a decrease. The takeaway: flushing probably won’t make or break your harvest quality.
A separate review from HortScience confirmed that flushing is standard practice in controlled-environment cannabis production but noted it’s largely based on grower tradition rather than rigorous evidence. The one clear benefit researchers identified is practical: flushing reduces fertilizer use and limits the environmental pollution caused by over-fertilization, without damaging the chemical quality of the plant.
How to Flush Properly
If you choose to flush, the process is simple but has a few details worth getting right. You stop all nutrient solutions and switch to plain, pH-adjusted water. For soil, keep your water around pH 6.0. For hydroponic and coco setups, aim for 5.8 to 6.2. Maintaining proper pH prevents root shock and ensures the plant can still absorb the water it needs to stay alive through its final days.
For the initial flush in soil or coco, many growers run two to three times the pot’s volume in water through the medium to wash out residual salts. A practical approach for large containers: pour in half the water, let it soak for 20 to 30 minutes so it has time to dissolve built-up mineral salts, then continue pouring through the rest. After that initial heavy rinse, you simply water normally with plain water for the remaining days until harvest.
In hydroponic systems, the process is even simpler. Drain your reservoir, refill with pH-adjusted water, and let the system run as usual. There’s no salt buildup in a medium to wash out, so you’re just diluting whatever remains in the root zone.
Reading the Plant: The Fade
During the final week or two of flowering, cannabis plants naturally undergo senescence. The large fan leaves begin yellowing from the bottom of the plant upward as the plant redirects stored energy and mobile nutrients into the developing flowers. Growers call this “the fade,” and it’s a healthy sign that the plant is finishing up.
This natural yellowing looks different from a nutrient deficiency mid-grow. During the fade, the color change is gradual and starts with the oldest, lowest leaves. The buds themselves continue to develop normally, pistils darken, and trichomes keep maturing. If you see yellowing that starts at the top of the plant, affects new growth, or shows unusual spotting and browning during the final flush, that’s more concerning and could indicate a pH problem with your flush water or an issue unrelated to feeding.
The fade actually serves as a useful visual cue for harvest readiness. If you stopped nutrients a week ago and the lower fan leaves are turning gold while the buds look dense and frosty, you’re on track. If the plant is still dark green everywhere after a week of plain water in soil, you may want to extend the flush a few more days.
When Flushing Might Not Be Necessary
Given the limited scientific evidence, some growers, particularly in commercial operations, have started questioning whether a full flush is worth the time. If you’re growing in living soil or an organic, amended medium where nutrients are released slowly by microbial activity rather than delivered as dissolved salts, there’s very little to “flush” in the first place. The minerals in organic soil aren’t sitting in solution the way they are in a hydroponic reservoir or a coco pot fed with synthetic nutrients.
For synthetic-nutrient growers, the strongest argument for flushing is economic and environmental. You save a week or more of expensive nutrient solution, reduce runoff pollution, and the research suggests you won’t sacrifice quality. Even if flushing doesn’t dramatically change your flower’s chemical profile, it costs you nothing but a few days of plain water. The risk-reward math favors at least a short flush, especially in fast-draining mediums where it requires almost no extra effort.
If you’re running a short-flowering autoflower strain where every day of nutrient uptake counts toward final yield, you might keep the flush period on the shorter end, around 3 to 5 days regardless of medium, to avoid cutting into the plant’s productive feeding window.