Basil leaves turning black on stems after moving outside

The likely culprit: cold shock or transplant stress

Basil sulks hard below cool temps (anything under ~50°F/10°C at night will do it), and blackening on the stems is a classic cold-damage or transplant-shock symptom, especially with a sudden move from cozy indoor conditions to variable outdoor nights. Overwatering right after the move can also cause stem blackening (that's rot, not cold), so check the soil too.

Fix it

1. Check the soil moisture first. Squishy, black, mushy stems near the base = rot from wet feet. Let it dry out and don't water again until the top inch is dry.
2. Bring it in overnight if temps are dipping below the mid-50s°F. Basil is a tropical plant wearing a summer-herb costume; it wants warmth, no exceptions.
3. Trim off the blackened parts with clean scissors, cutting back to healthy green tissue. This stops the damage from spreading and pushes new growth from below the cut.
4. Ease it back outside gradually — a few hours of dappled light at first, working up to full sun over a week, rather than a hard switch.

Once it's stable, a light feed at the feeding-chart rate with Kelp It Real can help it bounce back and push fresh growth without stressing it further.

What to expect

If it's cold shock and you catch it early, new green growth should show up within a week or two once temps stabilize. If the stems are mushy and black all the way through, that section's not coming back — cut it out and let the plant redirect energy. Basil is forgiving as long as the roots are intact, so don't panic yet.