The first time I grew veggies using nutrient film technique (NFT) hydroponics, I did not know anything about flushing. Turns out, it is a top-secret trick to improve the flavor, aroma, size, and overall quality of my harvest! However, when should you start flushing?
Generally, nutrient film technique (NFT) hydroponics can be flushed 7–14 days before harvesting. Flushing is the process of replacing the nutrient solution with plain water to prevent salt build-up that can reduce harvest quality. When done, the flavor, aroma, texture, and overall quality of the hydroponic vegetables and herbs improve.
Why do you need to flush? Is it really necessary? Does it vary among different hydroponic systems? These are just some of the questions that I will answer for you today! Let us move forward!
On average, flushing is done 7-14 days before harvest in nutrient film technique (NFT) hydroponic systems. However, the general rule for ideal flushing time is when the hydroponic vegetables and fruits are already ripe, and when leafy vegetables and herbs are big enough for harvest..
Before we start, let us first discuss what flushing is.
Flushing is the process of providing your hydroponic system with plain water to wash out excess nutrients that may build up in your hydroponic veggies and herbs. This is done at least a week before you harvest your plants.
Thus, looking at your plant’s readiness—i.e. ripeness or size maturity—is an essential step in flushing.
Fool-Proof Way to Check for Ripeness
For fruiting veggies such as tomatoes and eggplants, a tested way to ensure their ripeness is by checking their trichomes. Now, what are trichomes? These are the hair-like structures that you can feel when touching a plant’s stem or leaf. We can also see them in vegetables such as ladyfingers!
In general, when fruiting veggies are ready for harvest, their trichomes change in color. Usually, during the growing phase, they begin as clear hairs, turn milky white, and then finally change to amber.
Once trichomes become milky white, you can start flushing because that signifies the peak of the fruit’s ripeness. To properly check their color, you can use a magnifying glass.
Flushing is not necessary but is highly recommended for nutrient film technique (NFT) hydroponics. This is because flushing can improve the quality of hydroponic veggies and herbs by removing possible sources of excess salts and compounds.
If you are looking for an easy and free way to produce a high-quality harvest, flushing might be your best buddy!
But what will happen if you do not flush?
The nutrient water where your hydroponic plants are growing can also be a ground for salt and nutrient buildup. These excess compounds may cause bitterness in your veggies and herbs. We do not want that!
Flushing was also found to enhance the smoothness, smell, and taste of your hydroponic veggies and herbs.
Flushing too early can decrease the harvest and quality of hydroponic plants. On the other hand, flushing too late can produce a chemical taste and aroma that are not ideal for consumption.
When you replace the nutrient water with plain water too early before harvest, you are starving your plants of food. Remember that the flowering and fruiting stages are critical phases for nutrient availability. Considering this, you need to ensure that your plants are ready for harvest first before flushing.
On the other extreme, flushing too late results in bitter vegetable or herb harvests. This bitter taste can be attributed to the uptake of excess nutrients and harsh compounds. Aside from the bitter taste, there is also a tendency that your harvest will smell like chemicals. Thus, flushing too late will also affect the overall quality of your hydroponic veggies or herbs.
Flushing time in hydroponics varies depending on the specific growing medium used in the nutrient film technique (NFT) hydroponic system. Earlier flushing is recommended for systems with growing mediums.
In general, hydroponic growers that do not use any growing medium in their nutrient film technique (NFT) system can flush around 7 days before harvesting.
On the contrary, hydroponic systems that use growing mediums like coco coir may require flushing as early as 2 to 3 weeks before the anticipated date of harvesting.
Why do we need to flush earlier in systems with growing mediums? This is because nutrients can also be stored and built up in the growing medium.
1. Remove all the nutrient water from your hydroponic reservoir.
2. Clean your hydroponic reservoir using food-grade hydrogen peroxide or bleach. Make sure that there are no signs of algae because it may harm your system.
3. Rinse your hydroponic reservoir.
4. Fill it with plain water. However, it is optional to use flush boosters available on Amazon like the one below.
5. Check the pH of the water in the hydroponic reservoir. A pH of around 6.0 is acceptable.
Learn more about pH levels in our article best pH for hydroponics.
6. Finally, turn the air pump on to recirculate the water in your nutrient film technique (NFT) hydroponic system.
Similar to nutrient film technique (NFT) hydroponics, flushing is advised, but not necessary for other systems like deep water culture (DWC) and ebb and flow. However, in wick system hydroponics, regular flushing is necessary to prevent algae buildup.
In deep water culture (DWC) hydroponic systems, you can flush 2-3 days before harvesting.
Maybe you are wondering, why is deep water culture able to handle flushing in just 2-3 days before harvest? This is because in this system, the process of topping off is also done almost every week. In topping off, gardeners are advised to add plain water to account for lost water due to evaporation and plant use.
Meanwhile, since ebb and flow systems are somehow similar to nutrient film technique, flushing 7 to 14 days before harvesting is also advised.
In wicking systems, flushing regularly is required because it is prone to nutrient buildup in the wicks. This nutrient build-up also facilitates the growth of algae—which is not ideal for hydroponic systems.
Thus, through regular flushing in wick systems, you can lessen the possible incidence of algae bloom.
Learn more about this in our article about how often should I change hydroponic water.
Can I harvest without flushing?
Yes, you can harvest in hydroponics without flushing. However, the harvested hydroponic veggies or herbs may not be of their best quality. This means some of them may have astringent aroma, bitter taste, smaller fruits, and harsh textures.
Can I reuse hydroponic water?
You can use your hydroponic solution to water plants grown in soil in your garden. However, it is not advisable to reuse the hydroponic solution in another growing season in the hydroponic system. This is because nutrient build-up may happen which may result in excess salts and compounds affecting the quality of your plants.
Should I check the water pH after flushing?
Yes, it is best to check the water pH after flushing. It must be maintained at around 6.0 to continue its normal growth. When you change the water, the plant may experience physiological stress. Thus, if you expose the plant to water with a completely different pH than the previous pH, it may respond negatively—its leaves may fall and further wilt.
Flushing improves overall harvest quality. In most hydroponic systems, including the nutrient film technique (NFT), this process is done 7–14 days before harvesting. Without growing mediums, flush 7 days before harvesting. When using a growing medium, flush as early as 2–3 weeks before harvest.
When flushing is done too early, plants may experience nutrient starvation. Conversely, flushing too late may result in nutrient build-up that can result in a bitter taste.
Furthermore, other hydroponic systems like deep water culture (DWC), ebb and flow, and wick systems are ideally also flushed 2–3 days before harvest, 7–14 days before harvest, and every week, respectively.
- Fertilizer drawn forward osmosis process for sustainable water reuse to grow hydroponic lettuce using commercial nutrient solution by Chekli, L. et al. in Separation and Purification Technology.
- “Intermittent Root Flushing with Ozonated Water Promotes Growth of Japanese Mustard Spinach (Brassica rapa var. perviridis) grown in a Nutrient Film Technique Hydroponic Culture—Preliminary Results” by Ishii, M. et al. in Ozone: Science & Engineering.