Editor’s Note: This month the “Nico’s Nuggets” inbox received over two dozen letters inquiring about discolored, dried out, burnt or wilting leaves. Rather than post all those questions for my weekly Q & A column, I thought I would simply thank all those readers for writing in (you know who you are – THANKS!) and give some insights into funky leaf issues in this feature, as they can be quite distressing. Here we go….
First and foremost, do not panic! You have already done a great job of being aware and identifying potential plant problems. You must have good garden presence and be in tune with your plants to notice things like curling leaf tips, certain discolorations, and other warning signs. The next step is to assess the root cause and make any necessary changes.
Let’s start with some basics. As plants mature, the older leaves will naturally begin to change color. They will darken, and then pale. Some may show purple stems. They may begin to sag a bit (especially if they are large). This is all normal, and these leaves can be safely removed. Removing these leaves enables the plant to move on and concentrate on new growth (or flowers) rather than try to repair aging leaves. Removing a few leaves daily during the flowering stage is considered healthy pruning. Remember, the older (and bigger) the leaves get, the less efficient they are at producing energy for the plant and at some point there are diminishing returns, meaning those leaves are using more energy than they are making.
Pests, Molds & Environment
That being said, sometimes newer, smaller leaves begin to discolor early on in the plants development and this can be disconcerting. Before you panic, take close inspection of your garden. Are all the plants having the same problem? Or is it just one or two plants out of many? If all your plants are having the same issue, then you have a systemic problem within your grow system or environment. A systemic problem is one that is related to issues inherent in the overall system, rather than due to a specific, or isolated, factor. A systemic problem could be with your nutrient program or with insect pests or molds within your growroom or garden area. If your problem is systemic and most of your plants are affected, the first step is to closely examine your plants. Check on the undersides of leaves with a magnifier and look for mites. Check your medium for root aphids. Check leaves and flowers for botrytis or powdery mildew. All these pests can affect your plants’ health and the look and growth of your leaves.
If you do not have pest issues, but do have leaf discoloration (or even if you have just one or two plants with leave discolorations), the problem may extend beyond pests to environmental conditions, watering schedule, or nutrient deficiencies. Environmental conditions usually mean temperatures problems (over 82 F) or humidity issues (over 60 percent). If it is only one or two plants, check to see that they are not the ones directly under the lamp or in a particular heat-pocket zone within your garden. If your leaves are drying out and getting crunchy and brittle near the edges – or if they begin to severely sag and wilt – the issues is likely due to either too much or too little water. However, if you have various discolorations on many leaves, across several plants, the issue is most likely a nutrient deficiency.
Nutrient deficiencies are tricky and come with some common misconceptions. The biggest misconception is that a nutrient deficiency means you are not feeding your plants enough. Instead, the truth is more likely that you are feeding your plants too much and not flushing your medium regularly.
A nutrient deficiency does mean that your plants are lacking something, but it is not because you didn’t put it in the nutrient solution, rather it’s because the plants’ roots are having trouble absorbing the minerals down in the grow medium. This problem is especially common with synthetic or artificial nutrients as these nutrients are actually salts that, over time, build up in your medium and create what is known as “nutrient lock-up.” This is a big reason many growers switch over to organic nutrient lines.
To combat this before the problem starts, be sure to flush your plants by watering with fresh water only on every third day. To combat this problem after it has started is going to be a bit harder. In this case, you would need to flush your entire garden by watering with fresh water only for one whole week. However, even this may not work as the problem has likely already been ongoing for a couple weeks – way before the leaves began to discolor and your eyes caught it. This is why it is very important to be in your garden daily and keeping and eye on your plants – especially with synthetic nutrients.
Specific Nutrient Deficiencies
Sometimes nutrient deficiencies are, truly, just a simple glitch in the feeding program or a certain missing macro- or micro-nutrient. This can happen when using incomplete nutrient lines, or when using certain grow mediums, or when growing certain specialized genetics. It is rare when this occurs to just one or two plants out of many, though this may happen due to clogged irrigation lines, or plants in the way back of a garden not getting enough nutrient solution, etc.
Identifying specific nutrient deficiencies is not easy. The most common leaf issues include, but are not limited to: browning, spotting, yellowing, curling upwards, curling downwards, drying and crumbling (brittle leaves). Here are some common leaf issues related to macronutrient deficiencies:
Yellowing or pale leaves: Indicates a nitrogen (N) deficiency. A severe darkening occurs for an overdose of N. (Note: This should naturally occur due to flushing in the last couple weeks of flower.)
- Older leaf tips turn rusty brown /Leaf margins begin to pale/ Dark spotting develops: Indicates a potassium (K) deficiency.
- Small leaves and stunted growth/ Stems and leaf petioles (stems) begin to purple/ Necrotic or purple-black spotting occurs: Indicates a phosphorous (P) deficiency.
- Yellow patches on lower and mid-level leaves with dark veins/ Rusty spot on leaf tips and in leaf margins/ Leaf tips curl upward: Indicates a magnesium (Mg) deficiency.
- Paling leaves to lime-green/ Purple leaf petioles/ Leaf tips burn and curl downward” Indicates a sulfur (S) deficiency.
Those basic issues cover your N-P-K nutrient solutions and a few secondary macros. If you have one of these, it is usually due to nutrient lock-up as most reputable brands of nutrients have these macronutrients in wide abundance. Some of these problems may be caused by a lack of the lesser known/ used micronutrients. Let’s take a look at some of these problems:
- Younger, newer leaves turn yellow veins (chlorosis)/ Older leaves affected later/ Dead spots appear on leaves: Indicates a manganese (Mn) deficiency.
- Leaf tip discolor or burn/ Thin blades contort and wrinkle on new growth/ chlorosis between veins on newer leaves: Indicates a Zinc (Zn) deficiency.
- Green veins with yellow leaf on smaller leaves/ Yellowing starts near leaf stem, not tips/ Tips bend upward in later stages: Indicates an iron (Fe) deficiency.
Of these latter deficiencies, Zinc is usually the most common, so be sure to check your nutrient labels to make sure there is sufficient levels added. Iron is often an overlooked problem and can occur easily when pH levels rise above 6.4.
I’d also like to note that much of this information can be found (in much more depth) in the excellent horticultural quick-reference text known as The Indoor/Outdoor Medical Grower’s Bible by Mr. Jorge Cervantes, a longtime friend of HT and personal friend of mine. Be sure to pick up a copy when you can! Also, see Jorge’s latest book, The Marijuana Encyclopedia.
Thanks for reading everyone and remember: Grow… And help the world grow, too!
Got questions? Email ’em over to Nico at NicosNuggets@hightimes.com and be sure to put “Nico’s Nuggets” in the subject line!