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Flushing is one of the most debated practices in hydroponic cultivation. Whether you’re growing leafy greens like lettuce or high-value crops like legal cannabis, chances are you’ve heard arguments for and against this pre-harvest technique. But is it truly necessary? And what does the science say?
In this blog post, we dive deep into the practice of flushing hydroponic crops, examining what it is, how it’s done, and whether it delivers real benefits in taste, safety, and crop quality.
Flushing involves running clean, nutrient-free water through your hydroponic system for a few days before harvest. The goal? To remove excess nutrients, salts, and chemical residues from both the root zone and the plant tissues.
Proponents claim that flushing can:
Cannabis growers often flush their crops 5–14 days before harvest to reduce nitrogen and other mineral build-ups. This is believed to improve smoking quality, resulting in a smoother taste and cleaner ash. Some even use ice-cold water to stimulate plant stress responses, enhancing resin production.
While not all greens are flushed, it’s sometimes done for 1–3 days in systems with high EC (Electrical Conductivity). The key reason? To reduce nitrate levels, which are considered a contaminant in drinking water and can impact human health. This is especially the case in vulnerable populations like pregnant women where high blood nitrate levels can cause ‘blue baby syndrome‘.
Tomatoes, cucumbers, and strawberries are typically harvested continuously, making pre-harvest flushing impractical. These crops also require consistent nutrient levels until the very end of their growth cycle as they are harvested continually rather than on one set day.
Many commercial flush products are marketed with vague ingredient lists. However, most contain:
Pro tip: You can often replicate the effects of a commercial flush using diluted citric acid, saving money while still effectively reducing nutrient build-up.
Ultimately, the effectiveness of flushing depends on your crop type, growing system, and goals.
If you choose to flush, here’s how to do it safely and effectively:
| Crop Type | Flush Duration | Notes |
|---|---|---|
| Legal Cannabis | 5–14 days | Use clean or iced water; monitor EC runoff |
| Leafy Greens | 1–3 days | Best in high EC systems to reduce nitrates |
| Tomatoes, Cucumbers, Strawberries | Not recommended | Continuous harvest makes flushing impractical |
Tips for Successful Flushing:
Flushing remains a controversial yet common practice. For leafy greens, it’s about food safety and nitrate reduction. For cannabis, it’s often more about consumer perception and subjective experience than hard science.
Whether or not to flush depends on:
Flushing may not be a one-size-fits-all solution, but when done correctly, it can enhance product quality or meet food safety goals, depending on the context.
Before deciding, consider testing both flushed and unflushed batches and measuring taste, nutrient content, and EC levels. You might discover a practice that sets your crop apart—or one that you can skip entirely.
For a consistent high quality harvest use Liquid Gold, (Gold Leaf in the USA) right up until flushing.
Article by Dr Russell Sharp
If you would like to keep up to date with subjects just like this, you can listen to both our podcasts! Links can be found bellow:
Hydroponics Daily Podcast: https://open.spotify.com/show/207T7p7fw9sPjINfSjVXW2
Cereal Killers Podcast: https://t.co/eSEbBkTVHl