Most growers when they talk about flushing are actually describing
something called leaching. Also, a lot of folks think it's just something to do a
week or so before harvest, rather than periodically throughout the lifecycle. So
what is leaching? Well, it's really all about reducing the concentration of
dissolved minerals around your plants' roots and in the growing media. Maybe
you've increased your nutrient solution strength too quickly and your plants
aren't responding well. Perhaps you've even noticed some tip burn? Not good.
Example: you're growing hydroponically, let's say in a soilless mix: coco coir
and perlite perhaps? And you want to feed your plants a nutrient solution at
1,000 ppm, that's 2.0 millisiemens per centimeter, anyway you test the runoff
and-- hey, look at that-- it's all the way up to 3.0 millisiemens, or 1500 PPMs. No
wonder your plants look a little frazzled, huh? So, what can you do? Well, you
can panic and top feed your plants pure water ;that'll certainly lower the
concentration quickly around the root zone, but it also risks creating a form
of osmotic stress: plants struggling to adapt to sudden changes in solute
concentrations around the roots. It's preferable to apply a dilute nutrient
solution, say 500 or 750 PPM, that's 1.0 or 1.5 millisiemens, and water through,
testing the runoff periodically until it's around your target 1,000 ppm. Note
the symmetry between my corrected, more dilute nutrient solution and the
original leachate reading. Why do mineral-based nutrients tend to
accumulate in the root zone? I'm so glad you asked that!
Remember, plants transpire to keep themselves cool; they uptake lots of
water, but actually hold on to very little. Most of it simply passes through
the plant and eventually back into the air through tiny pores in their leaves
as water vapor. There's also a considerable amount of water lost to
evaporation. Now I don't you two worry unduly about transpiration; it's natural
and necessary, and well, it's just what plants do, but in the process of sucking
up all this cooling water, some nutrients inevitably get left behind.
In other words, plants uptake more water than nutrients relative to their ratio
in the nutrient solution. This is why hydroponic growers who use recirculating
systems top up their reservoirs with water or half-strength nutrients every
day or two. If they've neglected to do this their plants would end up feeding
on an ever concentrating nutrient solution as time went on, eventually
leading to toxicity problems, and plant tissue damage. Leaching then starts with
testing the leachate or runoff. If it's over 300 PPMs higher than your input
nutrient solution, you top feed with a milder nutrient until the runoff is
where you want it. Like I said before, if you're 300 PPMs over your input, leach
with a solution that's 300 PPMs under. Some growing systems are more prone to
salt build-up than others. If you top feed your
plants with drippers or manually with a watering can, then you're probably going
to be okay, so long as you water regularly, and achieve 20 to 30%
runoff each time. Passive wicking systems and flood and drain, where plants
are fed from the bottom and the capillary action of the roots draws up
the nutrient solution are more prone to salt build-up. As well as periodic
leaching, there's also the more widely known, pre-harvest flash. This is what most
folks mean when they're talking about flushing; it's typically performed by
growers cultivating plants which produce the majority of all of their harvest in
one it at the end. Yeah, okay, like most determinate tomato varieties, for example.
Usually, two weeks before the harvest growers start to lower their nutrient
concentrations. Just don't go straight into it with pure water; first go half
strength, and eventually say in the last week, to quart strength. The idea is to
encourage the plant to use up any residual nutrients within itself so that
only pure plant flavors remain. Reverse osmosis water mixed with the flushing
agent is the most aggressive flush and should be reserved for the final few
days. The final flush is a balancing act: start it out too early, and you deny your
plants important nutrients where they arguably need it the most and it could
hit your yield; too late and you risk leaving excess residual mineral stored
in plant tissue. Enzyme-based products and compost tea can be a good prelude
to a pre-harvest flush, as they help to break down any dead root material and
allow nutrients from these to be fed back to the plant. Reducing the nutrient
concentration signals to the plant that the end is near, just like when you stop
offering seconds to your in-laws at holiday dinners, and can speed up the
ripening process. Your plants' leaves, especially older lower leaves, will begin
to turn pale and may even turn reddish brown as mobile elements are drawn from
them into the newer growth, buds, flowers, and fruits. This is a sign that your
final pre-harvest flush is working. Also, when we start to starve the plant like
this, we effectively force it to draw sugars from its larger older leaves into
the flowers and fruits, themselves. Tobacco growers, for instance, feed with
plenty of water prior to harvest to reduce nitrate content in the leaves,
helping to improve flavor, and create a better burn. Growers in NFT and DWC
systems have the easiest time flushing because there's really not any growing
media to leach of nutrients. You simply change out the reservoir and you're done.
Coco coir growers may find it hard to flush out calcium and magnesium, as this
media tends to hold on to these elements for dear life. Using a coco and perlite
mix makes it easier to leach effectively. Hydroton expanded clay balls are
relatively easy to leach, as they don't really hold on to nutrient solution.
However, if they do undergo a drying out period, the nutrient salts can dry on the
outer surface and become insoluble. You see these often on the surface of the
container where they can dry out under light and air movement. Grodan stonewool is
another easy pre-harvest flusher. It has zero CEC so it easily lets go of
residual nutrients. Mother Earth grow stones are another great choice for the
same reason; peat based potting mixes are harder to flush. Organic soil growers
shouldn't worry about flushing at all. Simply build up the initial soil recipe,
you can check out mine here, use the liquid organic feet or top dress
additional nutrients as and when required, and then switch to water a few
weeks before harvest. To sum up: periodic leaching and pre-harvest
flushing should be done gradually and carefully. It's really something quite
specific to high-value, consumable crops. You really won't find commercial
cucumber hydroponic growers talking about this. However, the moral of this
story is always be aware of the nutrient concentrations in your root zone, as well
as in your reservoir. I hope you found this helpful! Please don't forget to
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