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Do You Need to Flush?

Do You Need to Flush?

Alex Grady

Most growers flush at some point - but when and for how long should you do it? Here's what you need to know.

What's flushing?

It’s where you apply nothing but water to your root zone (absolutely no nutrients at all).

When do you flush?

It’s common to flush plants in the final stage of growth, typically the last 1-2 weeks.

Some growers also flush for 1 – 2 days mid-cycle.

Why flush?

Flushing at the end

It forces plants to use the nutrient stored up inside them.

If unused or in excess, these nutrients can alter the taste and quality of the final yield.

Some growers also find that flushing at the end helps bring out preferred characteristics – like a slight colour change.

Ultimately, by flushing at the end, you’re making sure it’s the final produce you’re tasting, not stored nutrients.

Flushing mid-cycle

Flushing for 1 – 2 days reduces nutrient intake quickly. Some growers flush mid-cycle to correct damage done by overfeeding. You can do this at any point of growth.

Some growers also flush for 1 – 2 days when switching to a flowering cycle. Doing this quickly rebalances the preferred NPK ratio in the growing media ready for the change in growth.

Do I always need to flush at the end of a crop?

There’s some debate.

Some people argue that it depends on the type of feed and growing method you use.

Most people agree that if you grow organically, flushing isn’t as important as it is when you use mineral based nutrients.

There are also some flush products. Some help plants use excess nutrients, others make flushing easier by binding up nutrients in the media.

One that does this is Canna Flush. It draws out and holds onto nutrients in plant tissue & growing media, so that it's easier to flush out.

How do I flush?

It’s as simple as just feeding plants water.

If you grow in a hydroponic system that doesn’t use media, flush for around 7 days at the end of your harvest.

If growing in a media (like soil or coco) you need to start earlier to ensure the nutrients that have built up in your media will be flushed out too.

Most people using media start 2 weeks before harvest. Or you can halve the nutrient strength in the second to last week, then move onto water in the final week.

There are some awesome pre-flush nutrients that help add weight to your final harvest. You'll still need to flush with pure water for one week though.

What are pre-flush nutrients?

You switch to these for a few days right before your final flush. They completely replace your base nutrient. You may have heard them called 'final flowering' boosters.

SHOGUN Dragon Force

This forces plants to ripen in the last two weeks of flowering. Use Dragon Force to get increases in essential oils, terpenes and get an overall bigger yield.

Start 2 weeks before your harvest. Use 2 - 4ml for 10 days, then flush with pure water for 4 days.

New Millennium Winter Frost

New Millennium Winter Frost

Restart ripening after fruit and flowers have already reached their peak with a dose of this. It crashes your plants' systems, forcing them to bulk up, ready for reproduction. It's a natural, winter effect.

You know Winter Frost is working because you'll see purple, red & burgandy colour shifts as a side-effect.

Use 2 - 3ml the week before your final flush. Use for roughly 5 - 7 days (soil or coco) or 2 days (hydro).

Canna Flush

Canna Flush

Use Canna Flush to remove excess nutrients from your plants and media. It helps create a really clean flavour. Lots of growers use it to ready their media for re-use.

Start 2 wees before harvest. Use 2ml/L (media) or 4ml/L (hydro) for 3 days, then switch to pure water until harvest.

How do I know I’m flushing for long enough pre-harvest?

Flushing is a bit of a balancing act. Flush for too long and it can hurt your yield. Too short and your flavour is poor.

When flushing for the right amount of time, lower leaves should turn yellow, or even a red-brown colour. This’ll happen roughly halfway through your flush.

This fading out of the older leaves is a sign that the nutrient reserves are being used up and transported inside the plant to the developing fruits and flowers.

What’s a flavoured flush?

Some companies sell flavoured flush nutrients that they claim adds a flavour (like pineapple or blueberry) during the last two weeks of flowering. These are mostly available in North America.

We don’t believe they work.

Ultimately, flavour is determined by your plant’s genetics. You can make the flavour stronger, but you can’t change it altogether.

Is there an alternative to flushing?

Yes. You can slowly work down your nutrient strength over the last 3 weeks of your crop instead. Start at week 6, where the nutrient solution will be around 1.6 EC (16 CF), and work it down to:

  • End of week 6 - 1.4 EC (14 CF)
  • End of week 7 - 1.2 EC (12 CF)
  • End of week 8 - 0.6 EC (6 CF)

By keeping it low at this time (while plants are maturing) you lower the risk of excess nutrients being stored in the plant at all.

flushing by hand

Are there any down sides to flushing?

Only if you over-do it mid cycle.

Some growers panic and flush too heavily, for too long. Make this mistake and you can go the other way and underfeed plants!

If you’ve overfed plants and the leaves are just starting to curl over or burn at the tips, go easy! Try using a half strength nutrient solution instead of applying lashings of water.

Doing it slowly like this causes less osmotic stress.

The best method, of course, is not to overfeed plants in the first place!

If the EC rises in a recirculating hydro system, plants are using less nutrients and more water. Reduce your nutrient strength before plants get overfed.

In a hand-watered or drip system you can check the EC of run-off. If it’s higher than normal, either get more run-off when you feed or reduce the nutrient solution EC.

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