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Acclimatising plants to light

Also known as Light acclimation, Hardening off, Sun and shade leaves

Growing know-howDepends
Plants on a bright windowsill, where light is far stronger than a few steps into the room Photo: Lucas Alexander lucasalexander · CC0

In short

Leaves are built for the light they grew in. Move a plant suddenly into much stronger light and its leaves can scorch; move it suddenly into much less light and it sheds leaves. Change light levels gradually over days to weeks.

How it looks

Plants grown in bright light have smaller, thicker sun leaves; plants adapted to dim rooms make larger, thinner shade leaves with a lower light compensation point. Light levels vary hugely: the University of Florida describes low light as about 25–100 foot-candles, medium 100–500, high 500–1000 and direct sun over 1000 at midday. Our eyes adjust so well that a spot by a window can be ten times brighter than one a little further in, yet look similar; a light meter or phone app gives a truer reading.

Signs

Too much light too quickly: bleached, pale or brown scorched patches on the most exposed leaves, leaf drop. Too little light: yellowing and drop of older leaves, slow growth, pale, stretched new growth, no flowers.

Life cycle

Iowa State suggests building up outdoor light over 10–14 days; University of Missouri IPM notes that leaf drop after a move to lower light shows up several weeks later.

What favours it

Sharp increases in light (moving indoors to outdoors, a dim room to an unshaded window, the start of a sunny season) cause scorch; sharp decreases (greenhouse to home, outdoors to indoors) cause leaf drop. In the tropics, midday sun through unshaded glass or outdoors is intense.

Prevention

  • Never move a houseplant straight from indoors into full sun; start in full shade and add a little more light every few days.
  • Moving a new plant from a bright nursery to a dim room: start in a bright spot and move it to its final position in steps.
  • Measure light with a meter or app rather than guessing.
  • Use sheer curtains or shade cloth to soften strong direct sun during the transition.
  • Turn plants occasionally so all sides get light.

Treatment

Start with the first step and move down only if the problem persists.

  1. If leaves scorch, move the plant back to softer light; scorched patches will not recover, but new leaves will be adapted.
  2. If leaves drop after moving to lower light, give the plant a brighter spot if possible and wait for new, better-adapted growth.
  3. Water less and feed lightly while the plant adjusts to lower light, since it is growing more slowly.

Pros and cons

Pros

  • Gradual change lets the plant build leaves suited to the new light, avoiding scorch or heavy leaf loss.

Cons

  • Takes days to weeks and a little planning.

Look-alikes

Sunburn
Sunburn is the damage itself: bleached or brown patches on the leaves that faced the sun.
Etiolation
Etiolation is the long-term result of too little light: pale, stretched growth with wide gaps between leaves.
New-purchase shock
Leaf drop after bringing a plant home is largely a light-acclimation response.

Typical uses

All plants moved between light levels; most sensitive are shade-grown foliage plants moved into sun and high-light plants (succulents, cacti, citrus, weeping fig) moved into dim rooms.

Sources

  1. Houseplant Acclimatization — Missouri Environment and Garden, University of Missouri IPM
  2. Moving Indoor Plants Outside for Summer — Iowa State University Extension
  3. Light for Houseplants — UF/IFAS Gardening Solutions

Written in our own words from these sources. Check a product label before using any treatment.