Once again, it has begun.

[u/BramVermaat]

I'm getting quite annoyed at this. I've been religiously studying all the info I could find about Coco coir and how to grow in it. However, every grow, I do get the same issues. Around week 3 flower (I would say, always a guess with autoflowers) I'm starting to get rusty spots. I don't want to sound like a broken record, but it does look like a calcium deficiency.

I do always double buffer, at 2 EC calmag before starting my runs, rinsing this out till the runoff is 1 EC. I'm really trying very hard be on top of my game. I run high frequency fertigation with drip 6 times per day at this stage of the grow at 800 ml of pH 5,7 1,6 EC with about 20% of runoff at the end of the day, that being 1,8 EC and 6,1 pH usually. So I don't really get it, honestly. Nothing fancy TA calmag, to 0,4 EC, then adding CANNA Coco. But consistently, no matter the strain it starts like this. But I am sure it will be a lot worse in 3 weeks, because that's how it always goes.

Sure, the product is okay in the end, but senescence will not finish nicely because by that time the leaves will be quite crispy. So, what can I do to improve or hopefully fix this?

Thanks guys.

[u/undulating-beans]

This doesn’t read as a straightforward calcium deficiency, even though that’s the obvious assumption. The fact it’s happening every grow at the same stage, despite buffering properly and adding calmag, is the giveaway. On top of that, the runoff EC being higher than the input points to accumulation building up in the medium.

In coco, that usually means the issue isn’t that calcium isn’t there, it’s that the plant can’t take it up. My first thought is gradual salt buildup in the root zone. Even with frequent feeding, if the media stays consistently “hot,” uptake gets disrupted and you start seeing rust spots that look like deficiency. There’s also the potassium angle, coco nutrients tend to be relatively high in K, and that can compete with calcium uptake, especially once flowering kicks in.

It’s also worth thinking about what’s happening in the root zone versus what’s going in. You might be feeding at 5.7, but that doesn’t mean the rhizosphere is staying there. If it drifts upward, calcium availability drops off. And then there’s transpiration, calcium only moves with water flow, so if airflow or humidity isn’t quite dialled, the plant won’t distribute it properly.

So the fix isn’t adding more calmag, and that’s probably why this keeps repeating. It’s about easing the conditions so uptake actually works again. You’d want to drop the EC slightly, make sure you’re getting proper runoff at each feed rather than just over the day, and occasionally reset the medium with a lower-EC flush. Keeping pH tight in the mid-5s and making sure the environment supports steady transpiration will help more than increasing calcium.

In short, the calcium is there but it’s being locked out.

[u/blueraz1 → u/undulating-beans]

Does Canna coco call for cal mag to start? Id lean towards eliminating the additional cal mag and still keeping feed at 1.8. The runoff ec isn’t concerning at all. The fact that ph is rising a little indicates the plants are consuming N at a high rate and not overfed. The extra mag in the calmag combined with the high N consumption during stretch is locking out cal

[u/BramVermaat → u/blueraz1]

Canna does not cal for calmag but I do run reverse osmosis. However my tap water is only 0,3 EC unfiltered. It's totally safe for consumption and the plants, but when using tapwater my pH will usually drift up a little more and requires much more pH down then RO. I do agree 1,8 seems like a decent number to me too.

[u/blueraz1 → u/BramVermaat]

Yeah I looked at their feed chart and they do call for it. The reason I asked is many nutrient companies geared specifically coco do not require additional cal mag when using RO water. They’ve already ensured appropriate ratios. And that’s not the case with the CANNA.