Plant Growth Factors: Water

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questionThought Questions

Explain the science behind the following gardening questions:

1. Review how water stress impacts plant growth processes, then list common symptoms of drought stress.

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In Colorado, water availability and quality can be a limiting factor in plant growth. Quality issues generally deal with excessive sodium or other soluble salts.

Available water limits potential for crops and gardens in many areas of the west. In cities, the cost of the infrastructure to supply water drives the need for water conservation.

Water management is a topic of other Colorado Master Gardener training classes.  For additional information on water management, refer to CMG GardenNotes on Irrigation Management and Water-Wise Landscape Design.

Role of Water

Plants are over 90% water.  The role of water is summarized in Table 1.

Role of water in plants

Common Symptoms of Water Stress


  • Decreased growth
  • Small, off-colored leaves
  • Decline from top down
  • Early fall color
  • Reduced xylem growth = long-term growth reduction
  • Stress may show up five or more years later.

Water Logged Soils

  • Root activity slows or shuts down, and plants show symptoms of drought
  • Decline in root growth slows plant growth processes
  • Leaves may wilt from lack of water uptake
  • Root rots are common in some species
  • Lower interior leaves may yellow

Leaf scorch  (short-term water deficiency in leaves)

  • Marginal burning
  • Often from the top down, on southwest side, or from the side with root injury or root restrictions

Contributing factors to leaf scorch

  • Dry or overly wet soils
  • Compacted soils
  • Limited root spread
  • Root injury
  • Structural damage to xylem tissues
  • Trunk and branch injury
  • Excessive wind and heat
  • Excessive canopy growth (from heavy fertilization)

Relative Humidity

Water moves from areas of high relative humidity to areas of lower relative humidity. Inside a leaf, the relative humidity between cells approaches 100%. When the stomata open, water vapors inside the leaf rush out forming a bubble of higher humidity around the stomata on the outside of the leaf.

The difference in relative humidity around the stomata and adjacent air regulates transpiration rates and pulls water up through the xylem tissues. Transpiration peaks under hot dry and/or windy conditions. When the supply of water from the roots is inadequate, the stomata close, photosynthesis shuts down, and plants can wilt.

Water vapors move out and carbon dioxide move into the leave through the stomata.

Outdoors – In the arid climate of the west, low summer humidity helps manage some insect and disease problems and can aggravate others. The relative humidity returns to normal levels within a few minutes of watering/irrigation.

Indoors – With forced air heating, many homes have very low relative humidity in the winter. Some homes can have excessively high relative humidity due to a large number of houseplants, cooking and frequent long showers. Both extremely high and low indoor relative humidity are health concerns.

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Authors: David Whiting, Consumer Horticulture Specialist (retired), Colorado State University Extension; with Michael Roll and Larry Vickerman (former CSU Extension employees). Line drawings by Scott Johnson and David Whiting.

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Revised September 2015

Updated Thursday, January 14, 2016 by Mary Small

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