• Professor Sprout

Cannabis Grow Guide PART 3: What Causes Weed to Have Black Ash?

Updated: Apr 24


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Before we get to the results of my great black ash experiment, I think it is important to do a little housekeeping and scratch a few items off the list of potential causes of black ash.  These are some of the big questions we ask ourselves when confronted with horrible black ash in our joints, blunts, and bongs, and I have been able to answer many, if not all of them.


The Plant that Shocked Me


I’ll be the first to admit, I am not the most skilled grower.  I fuck up on a regular basis.  But I learn from my mistakes and every time I grow I get a little better and add a few new tools to my arsenal and a few more tricks up my sleeve.  I think one of the reasons my seeds turn out so healthy and vigorous is because the weaker and fragile genes just can’t survive my (lack of) growing skills.  I work on a limited budget in a challenging growing environment and so I’m forced to deal with less than ideal equipment and shabby climate controls that result in much greater difficulty maintaining a stable environment for my plants.  As a result, only my strongest and most robust genetics ever make it into the seed packs. 


A while back I had a plant that I had to remove from my grow room due to poor health and terrible performance.  This sickly little thing was never right from the start.  It always had some sort of nutrient deficiency and grew at less than half the rate of it’s sisters.  In cases like this I usually pull the plant out and compost it, giving the other plants extra room to fill in the space left behind by the failed plant.


Following my usual protocol I pulled the plant and decided to harvest a few salvageable buds before composting the rest.  Once I had clipped a few decent looking flowers and set them out to dry, I moved on to testing the soil to see what was wrong.  I immediately determined that the soil was far too nutrient rich for this particular sativa-dominant strain.  I had also made the grave error of overwatering (a rookie mistake that has been the downfall of many) which severely stunted root growth creating a situation whereby the plant could not utilize all the food it had been given.  So as the grow progressed the over fertilization problems just continued to worsen and the plant was never able to recover.  


Getting back to the buds I had clipped from the culled plant, I let them dry just enough such that they could be ground up and rolled into a few joints.  I was almost completely sure that these would burn to a very dark black ash; if not due to the over-fertilization then most certainly due to the fact that they were far from being properly dried and aged.  Regardless, I attempted to light a completely green, non-cured, over-fertilized, overly-damp joint right away.  It took a little cooking with the flame before the joint would stay lit, but to my utter surprise it was burning as white as snow!  The flavour was awful and tasted of burnt chlorophyl but that could be improved by curing, the important thing was the ash was gleaming white.

So how could a plant that was grown poorly, harvested too early, over-fertilized, over-watered, non-flushed, and rolled up into a soggy joint possibly burn white? 


Perplexing isn’t it.


However, this big fuck up (and a couple of previous fuck ups) answered a number of questions I had.  


Question 1:  Does Wet Cannabis Cause Black Ash?  Answer: No 


This joint was way too wet to stay lit without constantly hitting it with flame, but the ash was still completely white.  Once these samples were cured and dried to a much more acceptable 55-60% moisture, they burned to a fine-white ash and did not require constant lighting.  Clearly black ash isn’t about moisture.


Question 2:  Does Non-Cured Cannabis Cause Black Ash?  Answer:  No


The buds that I chopped off the plant were sampled before they were properly dried and cured, yet they burned white.  They tasted awful, yes, but the burn was near perfect.  Therefore proper curing is still highly recommended in order to improve flavor and smoothness, but curing is not what makes cannabis burn to a clean and white ash. 


Question 3:  Is Lack of Flushing the Cause of Black Ash?  Answer:  It Depends.


In cannabis growing circles, flushing is a technique used to rinse away (or flush) all built up nutrient salts that remain in the medium or soil.  Once the plant no longer has any food in the medium, it cannibalizes itself by using up internal stores of nutrients giving the plant a very depleted and drained look come harvest time.  


After testing my failed plant’s soil with a commercially available soil testing kit it was revealed to have an excess amount of all major nutrients and even had a irrigation runoff level of 6500 parts per million - well in excess of what it should be.  Even considering this grossly over-fertilized soil, the buds still managed to burn to a white ash.  Therefore excess nutrient in the soil does not cause black ash, with one important caveat: this particular plant was grown in 100% organic soil.  I was to find out later that lack of flushing with non-organic nutrients will indeed create black ash.  More on that in a minute.


Question #4:  Do foliar sprays cause black ash?  Answer:  Yes.


As an organic grower I rely on natural methods of disease and pest control.  I don’t use chemical pesticides and fungicides, and instead utilize organic solutions like neem oil, and essential oils like peppermint and rosemary oil along with ladybugs to control spider mite infestations.  I’ve also found that spraying plants with a dilute solution of baking soda or alkaline mineral water to help prevent powdery mildew.  However, even though these substances are organic, they should not be utilized once the plant begins flowering since the residue will remain on the flowers and ultimately contaminate your end product.


When I first started growing medicinal cannabis for myself, we experienced a very wet and cool autumn that brought with it the perfect conditions for powdery mildew and bud-rot.  At this time I only had a couple of plants in my garden and one particularly susceptible strain of Bubba Kush got hit hard with the dreaded white-fuzz of powdery mildew.  Not knowing any better, I sprayed it heavily with a treatment of organic neem oil.  I was pleased to see that within a few days all signs of the mildew had vanished but when the plant was harvested and smoked, the buds left a nasty garlic taste in the mouth and burned completely black and wouldn’t stay lit.  It also left a very congested feeling in my chest which scared me enough that I tossed the entire harvest into the compost.  But I learned my lesson.  


I no longer use any foliar sprays on my plants (organic or otherwise) once flowers have formed.  I also wash my plants down with filtered water a few days before harvest in order to rinse off any dust or other contaminants that may remain on the flowers.  Warning:  If you chose to spray down your plants with water prior to harvest it is essential to provide them with excellent air flow and dehumidification until you chop them down.  Water trapped inside the dense buds is a recipe for disaster and will most certainly cause bud-rot.


Question #5:  Do Synthetic Nutrients Cause Black Ash?  Answer:  It Depends.


Like I just stated, I am primarily an organic grower.  However, the very first time I test grow a strain I will often use synthetic mineral salts.  This gives me total control over the plant’s nutrition, and can be useful when growing a strain for the first time when it is unknown what the plant’s specific nutrient requirements will be.  


In one such instance I was growing a trio of beautiful specimens of Grape Chronic for the first time in coco-coir medium using a popular brand of hydroponic nutrients.  The plants were all looking beautiful but one plant in particular looked absolutely fantastic.  It showed no nutritional deficiencies and was packing on impressive flower weight day by day.  Due to this plant’s particular vigour and fast growth rate, I fed it a 25% higher concentration of nutrients than the other two plants.  


As is recommended by most hydroponic growers, I cut off the nutrients to all plants on week 7. I had expecting this strain to take at least an additional two weeks to ripen.  During these two weeks these plants would be given plain water only, thereby forcing them to use up their internal nutrient stores.  However, the plant that was getting fed the higher nutrient strength ended up ripening a full week before the other two plants, and so I cut it down early.  The other two plants finished in the expected 9 weeks and completed their flush as planned.


As you may have guessed, the plant that got fed the heavier nutrient dose with only a week of flushing burned black, while the other two burned white.  So synthetic nutrients don’t always cause black ash, but they can if fed in excess or if not flushed properly.


Some Unanswered Questions


These observations provided some excellent insight into what causes black ash in cannabis, but there were still some questions I had that remained unanswered.  First, in the case of the Grape Chronic plant just mentioned, was it the over fertilization or the lack of flushing that caused the black ash, or a combination of both?


Second, what causes black ash in organically grow cannabis?  I had already proven to myself that over fertilized and non-flushed organically grown cannabis can still burn white (as in the case of my prematurely harvested sativa mentioned earlier), but I clearly recall an instance where I had grown an absolutely gorgeous Showgirl plant outdoors in organic soil that resulted in terrible ash quality.  This plant was grown without any synthetic nutrients or foliar sprays at all, was harvested on time with no signs of pests, disease, or deficiencies, yet burned to a dark black ash that wouldn’t stay lit.  So what could be the culprit here?

To find the answer to these final questions I had to conduct my great black ash experiment; the results of which will be posted in the next and final instalment of this series.


Back with more soon.


- Professor Sprout


Read Part 4

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