Dew, condensation and plant bags: possibilities and limits

Air always contains some invisible water vapour. How much it can hold depends on temperature: warm air can carry more water vapour than cold air. When moist air cools on a cold surface, the vapour turns back into small droplets – this is called condensation. On clear, still nights, grass, tarps or car windows cool down a lot, and dew forms.

Plants draw water from the ground with their roots and release part of it as vapour through their leaves. This is called transpiration. A tightly closed, transparent bag around a leafy branch catches this vapour. Sunlight warms the bag, the vapour condenses on the inside and runs into the lowest corner.

How much collects depends on humidity, temperature, sun, the plant, soil moisture and how well the bag is sealed. In very dry air hardly any dew forms, and a plant in dried-out soil has little water to spare. That is why this atlas deliberately gives no amounts – the yield is usually small and hard to predict.

It can make sense if you are waiting in one place for a long time, for example after a vehicle breakdown, and no real water source can be reached. Then a few bags or wiping up dew in the morning cost little effort, and every small amount adds to your supply.

It does not make sense as your main supply. Never plan to cover your daily needs this way. If you have a choice, it is almost always more worthwhile to catch rainwater, melt snow or look for a real source. And in great heat you should not spend a lot of energy and sweat on a few sips.

Collect dew early in the morning, before the sun evaporates it. Drag a clean, absorbent cloth over grass or clean surfaces and wring it out into a container. Condensation also collects overnight on tarps, sheets or vehicle windows. The surfaces must be clean, or you will collect dust, bird droppings or residues as well.

For a plant bag you need a clean, clear bag – never a used bin bag or packaging from chemicals. Put it over a leafy, sunlit branch, seal the opening tightly and let one corner hang lower. Later, pour off the water carefully without taking leaves or plant material with it.

If the water tastes bitter or unusual, pour it away. Treat it if possible, as dew and plant water can be contaminated by animals, dust and surfaces.

The biggest danger is poisonous plants. Toxins can get into the collected water with the vapour or via plant material. Use a plant bag only on plants you know for certain are non-toxic. This atlas deliberately names no plants that are supposedly suitable, because plant identification is a specialist field with dangerous risks of confusion.

Do not drink plant saps, milky latex or liquids from cacti or unknown plants. Such liquids can contain substances that cause vomiting or diarrhoea – and so cost you even more water.

If nausea, vomiting or other symptoms appear after drinking, get help via the local emergency number (112 in the EU) or a poison centre, if you can reach them.

Military survival manuals such as FM 3-05.70 describe collecting dew and vegetation or transpiration bags as stopgaps for long waits – not as a replacement for water points or carried supplies. They explicitly warn against using poisonous plants.

In films and videos such methods often look more productive than they are in reality. The footage usually shows good conditions and does not say how long it took. If you have realistic expectations, you will not waste energy and will rely on the things that really help: carried water, known sources and less sweat loss.