UB's Plant Moisture Stress - Symptoms & Solutions
Quite often I hear groans from folks having leaf problems ->
“Help, my leaves are cupping and the leaf edges are turning brown!”, or,
“My plant's leaf tips are curling down and turning black ....what's wrong?”
Unless insect damage has occurred or the plant is suffering from a severe case of
calcium deficiency, the plant is trying to tell you that it is water stressed. It's hard to
tell *exactly* what the culprit is, and unfortunately the “solution” the grower
chooses many times is not the right one. A mis-diagnosis only serves to make matters
worse by promoting further decline. I’ll try to cover some of the more common causes
that can induce these common symptoms and try to offer a few simple solutions.
The ultimate and correct solution is in the hands of the grower.
1. Over-fertilizing - the most common cause of leaf cupping aka leaf margin rolling,
leaf margin burn, and leaf tip curl/burn is the overzealous use of too much plant food
in relationship to factors such as plant size, vigor and rate of growth. The first unit of
a plant to show moisture stress is the leaf at its margins and/or tips, reflected by margin
rolling (cupping) or burning. Sometimes copper colored necrotic spots show in the leaf also.
A hard, crispy feel to the leaf frequently occurs as well, as opposed to a soft and cool feel
of a happy leaf. When you have a high concentration of salts in solution (or in the root medium)
compared to lower salinity levels found in the plant’s tissue, water is actually drawn out of
the plant across the root gradient in order to fix the ppm imbalance. IOW, this is a natural,
osmotic response that serves to equalize salinity levels on both sides of the root’s epidermal
gradient. Back off on the amount and/or frequency of plant food. Too much plant food can also
burn the roots, especially the sensitive root tips and hairs, which then creates another set of
problems such as nutrient deficiencies. A note for the bio folks - as soil dries, the concentration
of the remaining salts rises further exacerbating the problem. Leach (flush) your pots once in a
while to get rid of excess salts.
2. High Heat - the plant is losing water via it’s leaves faster than what can be replaced by the
root system. The leaf responds by leaf margin cupping or rolling (up or down) in order to conserve
moisture. A good example is reflected by the appearance of broad-bladed turf grass on a hot
summer day, high noon, with low soil moisture levels - the leaf blade will roll in and the grass
will take on a dull, greyish-green appearance. Upon sunrise when moisture levels have returned
to normal, the leaf blade will be flat. Lower the heat and concentrate on developing a large, robust
root system by practicing sound plant culture. An efficient and effective root system will go a
long way to prevent heat induced leaf dessication and leaf margin curling by supplying sufficient
moisture for good plant health. One short episode of high heat is enough to permanently destroy
leaf tissue and cause a general decline in the leaves affected, which often occurs to leaves found
at the top of the plant located near HID lamps. The damaged leaf (usually) does not recover, no
matter what you do. Bummer in the summer. One can only look to new growth for indications that
the problem has been corrected.
3. High Light - yes, it’s true, you can give our faves too much light. Cannabis does not receive
full sun from sunrise to sunset in its natural state. It is shaded or given reduced light levels
because of adjacent plant material, cloudy conditions, rain, debris and dust collection on the leaf
surface, twilight periods of early morning and late afternoon, and light intensity changes caused
by a change in the seasons. Too much light mainly serves to bleach out and destroy chlorophyll
as opposed to causing leaf cupping, but it often goes hand-in-hand with high heat for indoor growers.
Again, back off on the light and concentrate on developing/maintaining an efficient and robust root
system. Keep in mind that all but equatorial material receive less light during flowering than during
the vegetative stage.
4. Overwatering - this practice only serves to weaken the root system by depriving the roots of
proper gas exchange. IOW, the roots are not getting enough oxygen which creates an anerobic
condition causing root decline and root rot with the end result showing up as leaf stress,
stunted growth, and in severe cases, death. <gasp!> Alot of times folks think the plant is not getting
enough plant food (which it can't under such adverse conditions), they add more nutes for a
"curative",and just add insult to injury.
5. Underwatering - not only is the plant now stressed due to a low supply of adequate moisture,
but carbohydrate production has been greatly compromised (screwed up). Step up the watering
frequency, and if need be, organic growers may need to soak the pot from the bottom up until
moisture levels reach an even consistency throughout the medium especially with mixes that are
heavy in peat. If severe, a little surfactant (liquid Ivory dish soap) added to the drench will help
return the organics back to a normal moisture retentive state. If the pot feels light to the lift - it’s
time to water. Don’t wait until the soil pulls away from the sides of the pot or leaves droop before
you water.
