Why do plants need water?

Plants need light, water, air, nutrients, and a comfortable temperature to survive and thrive. We all know that plants need water, but WHY do they need water? It turns out there are at least six key reasons that plants need water to grow:

  1. Seeds depend on water to germinate. Water is one of the primary signals telling a plant seed that it’s time to germinate and grow.
  2. Plants are mostly made of water. Their cells are 80-90% water! The water fills the cells and holds plants upright, giving them their shape/volume.
  3. Water is a main ingredient in photosynthesis, the process by which plants create their energy.
  4. Plants use water to regulate their temperature. Plants release water from their stems and leaves to cool down (plants “sweat”).
  5. Water delivers essential nutrients to plants. Plants can only absorb the nutrients they need when the nutrients are in a liquid solution with water.
  6. Plant “helpers” depend on water. The beneficial soil organisms that help plants grow all depend on access to water for survival.

So there are multiple reasons that plants need water to live! Read on to learn more about each way plants use and depend on water to grow.

“Although most people realize that plants need water, not all of them are clear about why. Until you understand this, you won’t be clear about how much water plants need, how often they need it, and which kind of container will keep plants as hydrated as they need to be.”

The Vegetable Gardener’s Container Bible: How to Grow a Bounty of Food in Pots, Tubs, and Other Containers, by Edward C. Smith
Waterfall and rainbow in front of mossy stone cliff

Why plants need water

Plants are almost entirely composed of water and they require water for the processes vital to their survival. Water gives them their volume, their shape, their food, their temperature control, and their overall environment. Let’s look at each of the ways plants use water to grow and live.

“Water is the most limiting abiotic (non-living) factor to plant growth and productivity, and a principal determinant of vegetation distributions worldwide.”

Water Uptake and Transport in Vascular Plants, by Andrew J. McElrone, Brendan Choat, Greg A. Gambetta, & Craig R. Brodersen, Nature Education
Watering grass seeds so they can germinate and sprout

1. To germinate plant seeds

Most seeds require water in order to activate enzymes that orchestrate the process of germination. Seeds absorb water, and the hard shell of the seed starts to soften. The seed swells and cracks open, making it possible for the seed to break through and sprout. Water is therefore key for encouraging the germination of plant seeds. Even after sprouting, the first thing the newly germinated seed does is to look for more water!

“After a seed imbibes with water and commits to germination, the embryo mobilizes nutrient reserves in the seed and thus gains sufficient osmotic pressure for bursting through the seed coat. The first organ to appear from a germinating seed is the primary root because a seedlings primary requirement is to establish external sources of nutrients and water.”

Mineral Nutrition of Plants: Principles and Perspectives, by Emanuel Epstein and Arnold J. Bloom
The cells of plants are almost all made of water - mainly h2o

2. To fill growing plant cells and hold the plant upright

Water is the main component of plant cells created through mitosis, making up 80 – 90% of cellular volume. The water fills each cell with enough fluid to pressurize the cell wall and make the cell stiff (keeps plant cells “turgid”). On a grand scale, all these water-filled cells are what make the plant stand up straight instead of wilt over on the ground.

Water enters the plant via its roots and travels all the way up to the leaves. So when you properly hydrate your plant, it gets sufficient water pressure to make the stem strong and sturdy. Inadequate hydration makes the internal water pressure drop, causing the plant to wilt. Any fruits that the plant manages to grow will also be smaller and not as juicy.

“Plant cells will not divide or elongate without water. No water = no growth.”

The New Gardener’s Handbook: Grow a Beautiful & Bountiful Garden, by Daryl Beyers

“Like other living things, plants are made up almost totally of water, at least 80 to 90 percent. Water provides the firmness we observe in healthy plant tissue. Conversely, the sign that a plant has too little water is wilting: drooping leaves and stems. Plants that are starved for water produce fewer, smaller, and less juicy and tasty fruits.”

The Vegetable Gardener’s Container Bible: How to Grow a Bounty of Food in Pots, Tubs, and Other Containers, by Edward C. Smith
Still reflective water pond surrounded by trees

3. As an ingredient for energy creation through photosynthesis

Photosynthesis is the process by which plants use energy from sunlight to generate glucose from water and carbon dioxide. Without water, plants couldn’t photosynthesize and would not be able to survive. So, plants not only are made mostly of water, but they also need water as an ingredient to make their energy. Plants need water just as they need sunlight.

“As long as plant leaves are exposed to sunlight, the plant requires a continuous stream of water to us in cooling, nutrient transport, turgor maintenance, and photosynthesis.”

The Nature and Properties of Soils, by Nyle C. Brady and Ray R. Weil
Water dripping down stones in tropical forest

4. To moderate the temperature of the plant through transpiration

Transpiration is the evaporation of water from the surface of the leaves and stems of a plant. Transpiration of water helps the plant cool off on a hot day. When water evaporates from the plants during the transpiration process, it cools the entire plant (the plant “sweats”).

This flow of water from the roots and out through the shoots provides the plant with continuous cooling and a supply of nutrients. There is no active “pump” moving this water – it is a passive flow dependent on chemical and pressure gradients that are part of the transpiration process.

“Water’s importance to plants stems from its central role in growth and photosynthesis, and the distribution of organic and inorganic molecules. Despite this dependence, plants retain less than 5% of the water absorbed by roots for cell expansion and plant growth. The remainder passes through plants directly into the atmosphere, a process referred to as transpiration.”

Water Uptake and Transport in Vascular Plants, by Andrew J. McElrone, Brendan Choat, Greg A. Gambetta, & Craig R. Brodersen, Nature Education
Why do plants need water

5. To deliver essential nutrients to plant roots

Water is an essential medium for the transportation of nutrients from the soil into the plant. Plants get their nutrients from the liquid solution in the soil. The water in garden soil “delivers” their food – nutrients must be dissolved in water for the plant to absorb them. Without water, soil nutrients would be completely immobile and couldn’t be absorbed by plants.

“Roots and other plant structures (e.g., rhizomes) that are embedded in the soil absorb most of the water and mineral nutrients for the growth of terrestrial plants.”

Mineral Nutrition of Plants: Principles and Perspectives, by Emanuel Epstein and Arnold J. Bloom
Stream flowing among trees and moss in west coast rainforest

6. To support the overall soil ecosystem

Plants have evolved to live their best lives in soil that is alive with beneficial microorganisms. Plants and the other members of the soil ecosystem all have a complementary role to play in keeping the system healthy and sustainable. Beneficial organisms in the soil include bacteria, fungi, and other small creatures. These soil organisms need water to survive and thrive.

Some plants also establish symbiotic relationships with beneficial organisms present in the soil to improve water uptake. So the soil microorganisms require water to live, but they pass on the benefit to the plants by increasing the effectiveness of their root water absorption.

“Water is of vital importance in the ecological functioning of soils. The presence of water in soils is essential for the survival and growth of plants and other soil organisms. The soil moisture regime, often reflective of climatic factors, is a major determinant of the productivity of terrestrial ecosystems, including agricultural systems.”

The Nature and Properties of Soils, by Nyle C. Brady and Ray R. Weil
River with many waterfalls surrounded by forest trees

Why plants require water to germinate, grow, and thrive

Plants depend on water from the very beginning when those initial drops of water allow the seed to germinate. The sprout immediately looks for water to establish the supply that will take in the water that will make up the majority of its structure.

The establishing plant uses water to make its energy, regulate its temperature, and to deliver essential nutrients. Lastly, water supports the overall symbiotic ecosystem upon which the plant depends.

Pacific ocean shoreline with green plants, palm trees, volcanic rocks, and blue sky
Mary Jane Duford
Mary Jane Duford

Mary Jane Duford is a quintessential Canadian gardener. An engineer by trade, she tends to an ever-expanding collection of plants. In her world, laughter blooms as freely as her flowers, and every plant is raised with a dash of Canadian grit.

Mary Jane is a certified Master Gardener and also holds a Permaculture Design Certificate. She's also a proud mom of three, teaching her little sprouts the crucial difference between a garden friend and foe.

When she's not playing in the dirt, Mary Jane revels in her love for Taylor Swift, Gilmore Girls, ice hockey, and the surprisingly soothing sounds of bluegrass covers of classic hip-hop songs. She invites you to join her garden party, a place where you can share in the joy of growing and where every day is a new opportunity to find the perfect spot for yet another plant.

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