Guide

Look closerPlants

Why dry moss opens up when it gets wet

March and November to December

True mosses grow on rocks, logs, stumps and tree bases, and when they dry out the leaves of many kinds curl, twist or press flat against the stem. Add water and they spread open again, because a moss dries out and rehydrates with the wetness around it.

Why this time of year

Moss grows on rocks, logs and tree bases, and moss on exposed rocks dries out between rains, so there is often a dried-out patch to test.

How to notice it

  1. Walk past a boulder, a log, a stump or the base of a tree and look for green cushions and mats of moss.
  2. Find a patch that looks dull or grayish, often on an exposed rock, and look at the leaves with a hand lens or your phone camera. On many mosses the dry leaves are curled, twisted or pressed to the stem.
  3. Drip some water on that dry patch, then watch for a few minutes. On many kinds the leaves spread back out. Species differ in how fast.
  4. Leave the moss where it is. If it is frozen or covered in ice, skip the water test and just look.

Try this with kids: Can you find a moss that looks dead? Give it three drops of water and watch what happens. Did it wake up?

Safety

  • Rocks and logs can be slick with frost or ice. Do not climb stone walls, and do not pull moss off rocks or trees.

Sources

  • webarchive.library.unt.eduUSDA Forest Service Northeastern Area, 'Learning About Mosses in Our Northeastern Woodlots' (2010, archived copy): mosses grow on tree trunks, stumps, downed wood, soil, rocks and boulders; several species have leaves curled and twisted when dry and spreading when moist; Hedwigia ciliata on dry granitic boulders in open woods, pastures and roadsides.
  • production.wordpress.uconn.eduUConn bryologists, 'A Visit to the Miniature Forest': bryophyte water content tracks the air; they dry into dormancy and when it rains rehydrate and resume photosynthesis within minutes or hours.
  • pmc.ncbi.nlm.nih.govPeer-reviewed study (Plants, 2025) on Hedwigia ciliata: defines poikilohydry; dried plants rehydrated within 15 minutes to close to 90% relative water content (lab).
  • etheses.durham.ac.ukDurham University MSc thesis abstract: many bryophytes are adapted to photosynthesis at low temperatures (under 10 C); repeated freeze-thaw cuts carbon uptake.
  • fs.usda.govUSDA Forest Service (Oregon) panel: bryophytes can dry out completely then fully recover when moistened, and usually photosynthesize more efficiently at lower temperatures and light than most flowering plants.

Checked against these sources on October 6, 2026.

In the guide for March, November, December

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