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19 posts in this topic


To cut a long story short, for now I can only get away with a microgrow under my desk. I am growing in a propagation tent 60cm tall and long and 40cm wide.


My plants started off fine, but have become very yellow and burned looking and bud growth has slowed as all the fan leaves are falling off. It's way too early to be just because of flowering imho.  I would have boosted nutrients, but with the burn at the tips, that'd seem like a bad choice.

I've dropped the light level down by 30% as I think the light was too intense at the distance from the flowers, I've also switched to plain correct pH water for a watering as the tip burn has always meant too much in the way of nutrients in previous grow.

I have insufficient experience of growing indoors and in strange circumstances like a micrgrow, to make a good judgement about what to do next.


Can you tell by the photos what might be wrong?  I'd really appreciate some advice before it is too late!  (Much more detail below if you want it, or skip to the photos at the end...)


My circumstances and location mean that it is impossible to grow unless it is very well hidden and camouflaged.

I figured I'd see if I could quietly grow indoors under my desk. I realised at the time it was a fairly big ask, with slim-pickings and problems along the way. Anyway...

My plants are all turning yellow and crispy. Please see photos at the end and offer any advice you can?

More Background

I had to make an ultra quiet and small grow room, which I did, using a propagation tent that fits basically under my desk. It's only 60cm tall and 40cm wide. I figured if people can micro-grow in a PC case, I could grow in that. I bought 2 sets of controllable 12cm PC fans for intake and a cooker hood carbon filtered-exhaust. I have a USB deskfan clipped to an internal pole to provide breeze. Temperatures in the tent range from 22c to 29c and RH from 30% - 60%. So far, so not bad - thanks Ona gel. I would have used my rhino filtered inline fan, but on trial it was far too noisy and the filter takes up most of the upper internal portion of the propagator, where the LED light needs to be.

I bought Royal Dwarf and Green-O-Matic varieties because they cover a spread of sativa / indica vibes and both stay under 60cm tall. With my fabric pots about 22cm tall and the 100W LED (SF1000) light tied to the ceiling of the tent, I had about 38cm between the pots and the light. I used a mix of about 60% compost - not enriched artificially, 20% Canna Coco Pro and 20% Perlite.

I germinated the seeds with the LEDs at about 50%. I LST'd them as soon as I thought it viable, though the GOM was tough and woody from a very early stage and I was loathe to risk breaking it. In hindsight I should have probably risked lower yields and fimmed the buggers at the 4th node perhaps?

Anyway, I stepped the light up, bit, by bit until it was at about 90% power. Everything went well for the first 3 weeks or so, despite a little leaf twist here and there, but on the GOM that happened with the very first set of leaves, so I thought, ho hum.

Very quickly a few leaves began to show some bronze spots and yellowing / crisping. I had been pH-ing the water to about 6.5 (down from 7.8) and the run-off was about the same, so I figured it wasn't pH. Saying that, a cheap garden-centre probe records the soil pH at about 6.8.

I started using some Biobizz Grow at 1/4 strength with each watering and introduced some CalMag at about 1/2 strength. our water is soft, so I figured the brown spots and crisping might be solved with that. Nope.

I realised I hadn't been leaving the soil to dry out quite enough. The top inch was dry, but the Royal Dwarf still had leaves dropping down, so I have spaced watering out about 2 days more, flood and drain. The plants are off the saucers, so aren't sitting in water. The leaves are no longer drooping. However, they are getting a bit jaundiced looking.

The problem continued into flowering which became noticeable by week 4. The odd leaf was easily removable and I added some Biobizz Top Max and Biobizz Bloom into the mix. The leaf tips were yellowing a touch, particularly on the Royal Dwarf, but both had more fan leaves going yellow and bronze, some at the bottom of the plant, some at the top.

Skip ahead to now, around week 6 since germination and I increased the nutrient dose a little. Still only 1/2 of the recommended dose, even for Autos in a standard soil mix. However, many to most of the fan-leaves have now come off. These plants finish in 8 - 10 weeks, so I will soon be at the stage where I'd expect to see some leaves going south during mid to late stage flowering. But this would seem to be a problem.

With the flowers now only 10-15 cms from the light max, I have turned the light down to about 75% as that seems a comfortable level and I figured it will reduce the chance of burning, fox-tailing and so on.

I just figured that even if I was getting nute-burn (as the tips suggest I am) then the variety of nutrients I am feeding the plants should have sorted out the leave spots and yellowing, but it hasn't. The leaves that are affected were and are at the bottom and top of the plants. They are so short there isn't much difference between the two though, heh heh

I would love to know the most likely culprit(s). I'm thinking maybe the safest move would be to water without feed next time (though Biobizz seem to recommend a constant dose of nutrients) dropping the pH to 6.2 or so and turn the light down maybe to 65%?

Other Thoughts I've had

Could it be a nutrient lockout somehow? Or should I just be boosting the nitrogen nutes, despite the burn I am seeing? In which case, should I try taking the feeding pH down to more like 6.2 in case there is a lockout occurring despite only being a touch high at pH6.8 ?

Do I need to boost the cal-mag levels to the full dose recommended by the manufacturer? I felt I was only likely to be a little under, but I suppose it probably couldn't hurt to turn up the dose to full - even if there's already a fair baseline of calcium and magnesium in the water?

Is it light burn, if so, why at the bottom of the plant as well? I'm going to get some shorter, wider pots for next go to shave a few inches off the starting height and allow them a touch more headroom. I wanted to try smart pots this time though and couldn't get them in a shallow and wide format. I'm not sure how much I could dim the SF1000 and still give the buds enough light. I mean it feels like I could drop it further, though there isn't uncomfortable feeling heat at bud level.

Though the light is a lot closer than it would be in a typical grow-tent with plenty of headroom, it is serving a smaller area and is running at about 70% power. There is a breeze across the top of the plants, but with my hand under the LED for 60 seconds, it doesn't feel particularly warm or even hot. I'm left wondering If I drop the light much more, will it still provide enough light to flower well, if I haven't already completely scuppered my chances?

Should I cut nutrient levels back down?

I have included some photos of the grow, below. Ending with tonight's affected leaves.

Any advice at all would be gratefully received. I have read up a bit, but I don't think reading gives you as clear idea as experience. In this case, I am working with so many compromises and unknowns that I don't honestly know which symptom is strongest and needing tackling most.

I know a micro-grow will often have issues simply because of the downsizing, but I am definitely going wrong, I suspect in 2 or 3 places. So any advice will be gratefully received. Outdoor in soil was quite a bit less problematic.

This First Image is from a couple of weeks ago, before the problem advanced.

This is from the last couple of days.  Many fan leaves have fallen off and the other leaves are going yellow and burnt in the centre or at tips.

Thanks in advance for any help you can offer folks!

[Thanks too, to the admins for letting me in!]


Edited by MidgeSmith

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Welcome to uk420 @MidgeSmith :yep:


You'll get a few answers no doubt, 

here's my response. 



There's no need to pH when growing in well made compost,

the compost buffers what goes in up to a certain extent. 


I regularly see people coming onto the forum with problems caused by pHing in soil. 


Don't do it :)



Also it looked like the plants were a bit hungry and the Ca and Mg was possibly lacking or out of proportion to each other in the first photo,

but this could have been caused by pHing the feed/water. 


If you live in a hard water area you might need to get your water analysed(often Mg is not reported on water quality reports), 

or if you have soft/very soft water then you will need to add some CalMag to your feeds. 


Plants seem to require more nutrients under LEDs,

in particular Ca and Mg. 



The light does seem to be a bit closer than I'd like to be, 

better to turn it down a bit more (don't know what increments you can do) maybe 60% and see how they react. 



It's not the heat but the light intensity which can sometimes cause some problems similar to yours(can appear overfed) 


How much and how often were you feeding? 



30% RH is too low for a productive grow, 

your lowest RH will likely be when the lights are on and 60% with the lights off(i would imagine unless using humidifier and dehumidifier) 



So in summary...


I'd say turn the light down a bit. 


Stop pHing in any compost grow from now on.


And possibly reintroduce some bloom nutrients with a little bit of grow or fishmix if you want to stick with the Biobizz range. 


Check your water quality (online). 


Also it's probably worth getting a thermometer into one of the pots to check the actual root temperature (a heat mat with a thermostat may be necessary). 






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Welcome to 420! Root temperature would be most important, if your room temp is dropping to 22 (depending where you are measuring from) your compost temperature could be lower than this. If they drop below 18c you will start to get problems. 


I'm sure you will manage a smoke from these, and will have learned loads. Your next grows will improve for sure! 

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Thanks for the welcome and the advice fellas!


So would I be right in saying that the main recommendations would be:

1) Stop pHing the water.

2) Keep the light intensity 60% (as it is now) or below.


3) Go to a full strength (manufacturer recommended) of calmag with each watering / feed (had been a 1/4 - 1/2 strength).


4) Add back the 'Grow' at 1/4 strength and 'Bloom' at say 1/2 strength  to each watering?  It's not that I'm passionate about Biobizz, just that it was readily available and sounded like most people who had mentioned it seemed to think it was ok or good. I've also read a few saying they didn't rate it particularly.

Would that sound reasonable?

Cheers @Shumroom & @Openairbud!


A few extra explanations and updates below.  I had no notifications and had posted elsewhere a couple of days ago with no answers, so went ahead with my best guess in the meantime.

Yeah, I pH'd the water because it is pH 8.0 out of the tap and that seemed a bit high to be buffered.  I am happy to stop pH-ing if it really could be that bad.


My water is very soft, so I had added *some* cal-mag, but things continued to go downhill.  I hadn't posted the full dose, I'm loathe to do anything full-dose to autos so far. They don't seem to want much in compost, though with

Since I posted the article, the light has been on 60%, the damage appears to have slowed at least, maybe stopped.  The plants look a touch happier to me.

I had been feeding them a litre each, with about 1/2 recommended dose of Biobizz. Grow and Bloom, Calmag and TopMax in, ever 4 - 5 days. This gave the whole of the soil a wetting through and then a reasonable drying out.  The soil is staying reasonably loose at the top at least.

I had been pHing it down to 6.5 because as I keep saying - probably wrongly, soil buffering a whole 1.5 pH levels down seemed like a big ask.

I don't think that soil temperature could be a problem as it is in a used bedroom under a desk. This room is always between 18C - 24C.  The tent  temperature is pretty much in the range 20C - 29C and humidity is now more like 45 - 75.  I was previously 40ish - 60.   (30 - 60 was a typo).


Edited by MidgeSmith
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If I'm honest, the 1st pic looks like one of my classic over watered soil grows that I tried to fix with more feeds. Switching to coco sured this for me. :yep:



Also, the fan is best pointing away from plants as direct blowing can fry some leaves. As for air exchange, everything improved when I replaced PC fan with decent EC fans. EC can go slow and quiet, means the intake could be passive and decent filter put to use. Maybe re-use the pc fans to gently push the air around in the tent.


It's a good size tent to turn into a nursery. This spring, you could churn out some outdoor genes and plant them in a field somewhere with the off chance of bringing a kilo home in October. Then put the tent back into flower mode straight after. :guitar:


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Yes, the first pic was when I realised they'd enjoy another day or two to dry out between waterings.  The top inch was dry, but the bottom had held the moisture well, so my usual rule of thumb wasn't quite right.

Also Yes, regarding the breeze being too direct.  I was keen to keep the flower-top temps down, but wondered if I was chaffing them as well, cheers, I have been moving it a bit, but not making it entirely indirect, as I will for a while.

The main reason that I got the propagation tent, is to getting the drop on the outdoor season indeed :) This was just a bit of a proof of concept really (and to cover myself for meds a bit longer). With improvements based on each try, it might be worth a seasonal pop with a small plant in the propagator, as well as bringing the spring flowers on.


Edited by MidgeSmith
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Apologies for the delay in getting back to you. 


There should be no problem buffering from pH 8.0. 


Be careful not to overdo it with the CalMag, 

maybe just stick with half strength. 


Some CalMag is also high in Nitrogen for some reason, 

Check out the NPK ratio on the bottle :yep:


So if the CalMag you have is one that has Nitrogen you probably won't need to use any/much grow at all. 


I'm not much other use in the feeding of autos other than to say that often the less is more approach is better than overfeeding. 



Get a thermometer into the soil to check it's at least 20°C(you might be surprised) how cold a wet medium gets,

but ideally a heat mat(with a thermostat control) under the saucers to bring them up higher(like 24-26°C).


Check out yootoob videos by Dr Bruce Bugbee. 


He really knows how his stuff when it comes to growing canna and using LEDs :oldtoker:



Try to stick to metric or imperial or it confuses me(medicated user here :)). 



Using LEDs generally you don't get the same kind of radiant heat that you get from HPS etc,

sometimes the driver is warmer than the boards. 


Getting your environment sorted will pay off, 

I've seen some great microgrows, 

@Prophet comes to mind maybe you can offer some advice here. Tia






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I try not to comment on auto grows, not my bag, but they look like they're about to be cut down after a three week flush.  Hungry as fuck. 

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Something is wrong either very wrong with your PH measurements or your tap water is hard, soft water will have a PH below 7. I think your issue is either stemming from PH adjusting your water (if your PH measurements are off your adjustments will be way off) which isn't needed when growing in compost, or there is something off with your wet/dry cycle or a combination of the two. How much water are you giving them and what size pots are they?



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Hi all, thanks for all the ideas and comments.

My water is classed as soft, 39.81 ppm but in summer it rose to roughly ph 8.  Actually, I measured it again recently and actually it had fallen to around 7.5 or maybe just a shade less.

I've used a couple of meters with the same result. My own meter currently reads within 0.2 of each extreme of the buffer solutions expected values.  I'm not an expert, but that seemed ok. I recalibrated it to be sure.

I am giving them a litre each (in 2 gallon pots) every 4 - 5 days or so.  I am waiting until I am worried the soil will become impervious to water and the plants seem less boyant than before.  Week 1 / 2 they only needed one watering.  Week 3 I watered them twice and they were overwatered.  Well, the Royal Dwarf showed signs of that.  The GOM doesn't seem to mind too much what I do to it apart from the heatstress / calmag deficiency looking signs.

I think cutting the lights down to 60% has helped A LOT and the water only flush I have been doing the last few days (waiting for dry out) may have too.

Biobizz CalMag is nitrogen free. I am considering going to full recommended strength CalMag and 1/4 strength nutrients again from next feed.  Does that sound like a plan?

I keep wondering though... Could the entire problem have been leaf-burn from the light intensity and then I chemically burnt the poor things trying to get nutrients into them?

P.s. Ha ha @Sumroom "Try to stick to metric or imperial or it confuses me(medicated user here :)). " I've always bounced between metric and imperial, due to precision on one hand and purchasing medicines in archaic quantities on the other.  So it's just down to which is closer usually.  I'd usually stick to one and I have to say, I was wondering if anyone would find it annoying :lookaround: Soz.

Edited by MidgeSmith
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4 hours ago, fatboy77 said:

I try not to comment on auto grows, not my bag, but they look like they're about to be cut down after a three week flush.  Hungry as fuck. 

Fair play!  Good  as they can be, autos wouldn't strictly be mine, under more relaxed circumstances.  At the moment though, it is a matter of auto or not at all really.

This should have been a walk in the park really, but I suppose some things you don't know until you know.  I didn't quite fly from the blocks on adapting to a microgrow and LED with soil. From this perspective, hydroponics were easier!!

I WILL get to a full height tent and non-auto plants again, when I have the space and liberty to do so.

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lollollolGallons and litres


Is that a UK gallon or US? lollol


I'd still say to go ¾ strength with the CalMag to begin with. 


Remember that flushing in soil is kind of a last resort unlike with hydro. 


Doesn't have to be an auto to do a micro grow.






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It doesn't have to be, but I figured it should help.  I've only grown autos twice and photoperiods 3 times.  I just thought it would be wise to get the naturally smallest plants I could, which seemed to be these two and maximise the amount of light they could use by having a longer light-cycle throughout.  The idea was to remove variables while I got the environmentals right and to top up my stash in the process. Either way, it all adds to the learning.

Btw you totally got me there, sorry it is a US Gallon, I was thinking it was a UK gallon. I looked back at the details online and its about 7.5L of soil mix in each one.

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I think you've largely answered your own questions. LED's do need a bit of headroom, at least in the earlier stages of growth as they will burn leaves.


I've had a very similar issue to you, many times with my micro grows which i believe is down to damp, cold roots. You have to back off the watering until the pots become light and leaves start to droop. I've attached a pic of mine below at week 4 (photo, not auto). At this stage I'm watering half a litre, a week. Pumping nutrients in (in my experience) will do more harm than good. I stay on tap water until around week 6-8.


It also looks like you've removed most of the fan leaves. Best to not go too mad as these are sugar-stores which the plant will need later. I'm not sure you'll be able to save this one if I'm honest


In summary I'd:


- Turn the LED down (which you've done)

- Give the soil a bit of a flush to remove any built-up nutrient

- Turn off the oscillating fan, no need in a grow of this size

- Let it dry out & observe a good wet/dry cycle


Just my thoughts, I'm no expert!



Week 4: (plain water, no nutes, dry looking soil)





Edited by Prophet
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Hi Prophet,


Yes I believe you are totally on the money there.  I've not crapped up this much before and I suspect I have just tried too hard to cover bases which didn't need covering! At least it is a learning experience.

After the excessive watering of week 3, I believe my watering schedule has been reasonable, though I could stretch them out one further day.  To be honest, I was keen to put some less nutrient dense and non-pH-corrected water through them to try to soothe the roots this week. I will go back to more sparing water now though.  There's only a few weeks left for them to finish up and then this one is done, lesson learned.

I didn't add any nutes at all to my autos outside in the greenhouse until the last 3 weeks and then gave them a fortnight of gentle nutrient reinforcement.  They stayed healthy to the end and yielded well. I panicked with these as I was experimenting with lighter soil and rather than deficiency in nutrients, I believe it was just an excess of heat and possibly the nutrient / pH / water problem.

I've clearly pretty much ruined these with too much light and nutrients.  I don't think these will recover per sé, though there is still *some* green left.  They're putting what energy they can get into flowering now.  I didn't remove any fan leaves that didn't start to fall by themselves, but neither are they making much in the way of new fans now as you might expect.


I added the (non-oscillating) fan to try to help the tops of the plants stay cool as they were getting to 30C and I was concerned that was too high.  I thought I could compensate for the strong, close light by adding a breeze.  I suppose that was a bit like turning on a fan to compensate for a nuclear blast. Perhaps the breeze is adding to the stress and drying the damaged leaves out though.

Next time, I will probably invest in some ready made growing soil or even the compost I used before, with light up to 55% max, no internal fan, or gentler fan, maybe a propagation heatmat too.  I will definitely reserve nutes until later flowering - if they seem to need them - too.

I will probably only use the propagation tent for experiments, propagation and winter growing too.

Thanks for chipping in, I appreciate the input!  My plants looked gloriously green and lush like yours until about week 3, sadly now, they look like they've been aggressively defoliated and jaundiced which I guess they have! At least there is still some green on the buds and I won't let them light or nute burn again!

One last question.  Is there any point putting a heatmat under the saucers for indirect convection heat?  I'm quite enjoying these fabric 'smart' pots, but have them on stands to allow air to get around the whole of the fabric pot, advice I got from another forum.  If I stick them on a heat mat, the bottoms will stay more damp.  Would you say that's a compromise I need to bear, put the saucers on top of the heatmat and the pots straight on top of the saucers and be satisfied with that?



Edited by MidgeSmith
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