My plant is turning purple. Is it the cold, hunger, or just her genes?

Most purple is harmless. It usually comes from her genes, from cool nights, or from the fade at the end of flower. A shortage is the one to catch, and it brings other damage with it. So read the purple together with the rest of the plant, the overnight low and the stage, never on its own.

Purple is a color, not a diagnosis.

Where purple comes from

The purple is a pigment the plant makes in her stems, leaf stems and leaves. Several different things turn it up, so the color alone can't tell you which one you're looking at. These four cover most cases.

• Her genes. Some plants carry purple stems and leaf stems their whole life, in a warm room or a cool one. The same color on every new stem, healthy growth, nothing to fix.

• Cool nights. Cold nights bring purple out in many plants, starting in the stems and leaf stems. In winter it often shows up once the room starts dropping at night.

• The late-flower fade. In the last 2 to 3 weeks before harvest, the older leaves can fade to purple, red or near black as easily as yellow. That's her using up what's stored in the leaves to finish, not hunger.

• A shortage. Purple that comes with damage: older leaves going dark and dull, then spotted or crisp at the edges, with growth slowing down. Often there's enough food in the pot and she can't take it up. Cold roots and a pH outside its lane are the usual reasons.

Read it in this order

1. Look at the rest of the plant. Is the new growth healthy, green and the right size? Are the older leaves firm, with no spots or crisp edges? Then the purple on its own isn't a problem.

2. Check the overnight low. Read it on a sensor at canopy height, shaded and out of the fan's path. Aim for lights-off 5 to 10°F cooler than lights-on, and not below the mid-60s. Below about 55°F at the canopy is trouble for the whole plant, purple or not.

3. Check the stage. Purple that spreads from the oldest leaves in the last 2 to 3 weeks of flower is the fade. Purple that matches on every stem from early veg is usually her genes.

4. Check the roots. Cold roots take up food slowly, so a cold plant can look hungry when she's just cold. Keep the pot at 68 to 75°F, off a cold floor, and water at room temperature. Then check the pH going in: 6.2 to 6.8 in soil, 5.8 to 6.2 in coco and hydro.

5. Only then look at the bottle. If the damage is spreading on the older leaves and the room, the roots and the pH all check out, the feed is the next suspect.

Change one thing at a time. After a feed or pH change, the first signs show within a watering cycle, in the newest leaves. Judge the fix on the new growth about 7 to 10 days later; old damage doesn't heal.

Why winter brings it on

A cold room slows everything at once: the air at night, the pot on the floor, the water from the tap. That's when stems that stayed green all fall start to turn. Most of the time it's color only. When the purple comes with slow, dull growth, the fix is the room, not the feed. Warm the room, get the pots off the floor, and let the water come up to room temperature first. See How cold is too cold for my grow room at night?

Purple isn't a harvest clock

Late purple can make a plant look finished before she is. A cold finish can slow her down while the colors make her look done. Judge the harvest by the flowers themselves, not the leaves: see our Harvest Decision Guide.

What not to do

Don't chase purple with cold nights. Dropping the room for color usually costs more than it gives. Warm, happy roots beat a little purple.

Don't feed on the color alone. A bloom booster or an extra dose of cal-mag on a plant that's only purple adds salt to a pot that didn't need it.

Don't flush a healthy purple plant. If the growth is clean, there's nothing to wash out.

When it's something else

• Yellow leaves, not purple: see Yellow leaves: is it a deficiency, and is this too much?

• Fading in the last weeks of flower: see She's fading in late flower. Do I flush, feed or pull the leaves?

• She's barely growing: see Why is my plant growing so slowly, and what do I check first?

On our shelf in Worcester: EC and pH pens and calibration fluids are in pH, water & meters. Call (508) 459-9609 or get directions.

Keep going

Checking the pH going in: see our pH and EC meters shelf guide.

Read the plant before the bottle. The whole system for a steady grow is written down in the free Stability Playbook. Get the free Stability Playbook

More answers: all grow guides. Every shelf guide and maker feed chart: Grower Resources.