Cold Shock

Cold shock is the plant shutting down because the air or the root zone dropped below the range it can work in. Cannabis stops functioning efficiently below about 60 F and takes real damage below 50 F.

What you are seeing#

This one shows up everywhere on the plant at once rather than starting at one end, which is itself a useful clue.

Stems and petioles turn red or deep purple. The leaf blades darken and take on a purple or blue-green cast, most visible on the undersides and edges. In a cold flowering tent whole colas may turn purple, which looks attractive and is still a stress response.

Growth slows or stops. New leaves come in small. The plant droops and looks deflated without the pot being dry.

After a hard cold snap you may see curled leaf edges and lower leaves dying off. Roots stop drinking, so the medium stays wet far longer than usual.

Why it happens#

Below about 60 F, root uptake of phosphorus and water slows sharply and enzyme activity falls off. The purpling is anthocyanin pigment becoming visible as chlorophyll production drops, often alongside a real cold-induced phosphorus lockout at the root.

Common causes:

• Lights-off temperature dropping far below lights-on. A gap over 15 F is a problem even if neither number is extreme.
• Pots sitting on a cold basement or garage floor.
• An exhaust fan running all night pulling cold outside air through the tent.
• A tent in an unheated space in winter.
• Watering with very cold tap water, which chills the root ball directly.

How to fix it#

1. Put a thermometer at canopy height and a second against the side of the pot, and record tonight's overnight low. Guessing is how this goes unfixed for weeks.
2. Target 70 to 82 F lights on and no lower than 64 F lights off. Keep the day to night gap under 15 F.
3. Get the pots off the floor. A piece of 1 inch foam board or a wooden shelf is the cheapest fix on this page.
4. Put the exhaust fan on a temperature controller or a timer so it is not running full speed all night.
5. Water with room temperature water, 65 to 72 F. Fill the jug the day before and let it sit.
6. Over the next week, hold off on feeding. A cold plant is not taking up nutrients, so adding more just raises salt. Resume once overnight temperatures are stable.

How to tell it apart#

Versus phosphorus deficiency. These overlap almost completely, because cold causes a phosphorus lockout at the root. The discriminator is distribution and timing. Phosphorus deficiency concentrates in older lower leaves and produces dull bronze blotches. Cold shock purples stems and leaf undersides across the whole plant including new top growth, within a day or two of a cold night. If you can tie the onset to a temperature drop, fix the temperature and leave the feed alone.

Versus magnesium deficiency. Magnesium yellows between the veins on lower leaves and adds rust spots. Cold shock darkens and purples rather than yellowing, and it does not produce the green vein skeleton pattern.

Versus overwatering. Both produce drooping and a pot that stays wet. Overwatering does not purple the stems and is not tied to the overnight low. If the medium is staying wet and the stems are purple, the cold is causing the wetness, not the other way around.

Versus root rot. Root rot smells. Pull the pot and check for a sour, swampy odor and brown slimy roots. Cold shock leaves roots white and firm, just inactive.

How long until it looks better#

Purple stems often stay purple for the rest of the grow. Some strains purple in the cold and never go back, and that is cosmetic rather than harmful once temperatures are corrected.

Function recovers faster than color. With overnight temperatures back above 64 F, drooping should resolve within a day and the plant should drink normally again within two or three. New growth takes five to seven days to return to normal rate. If you were in late flower during the cold snap, expect a few extra days to ripen.