Causes of Yellow Leaves on Roses: Heat, Water and Salt Stress and Chemical or Soil Toxicity

Leaf yellowing on roses can be symptomatic of a variety of problems. One major category is enviro-mechanical problems, such as heat, water or salt stress, phytotoxicity, and accidental root damage.

Heat Stress

This type of stress is frequently noticed as temperatures suddenly warm after wet and relatively cool spring weather. Cool weather promotes tender new growth. Exposure of this new growth to direct sun can cause scorching of leaf margins.

This type of stress is particularly noticeable on new roses. During periods of high temperatures, low humidity and rapid air circulation around leaf surfaces, water demand by the plant is most critical. Other symptoms:

• "Bull nosed" blooms, especially on dark colored roses.
• Scorching of leaf margins.
• Dark petal edges on blooms (on dark colored roses).

Treatments:

• Apply anti-transpirants/anti-desiccants (like Cloud Cover or WiltPruf).
• Provide sun screens for new plants.
• Plant roses in areas protected from the afternoon sun.
• Mulch heavily around root zones to stabilize soil and near-surface air temperatures.
• For new roses, concentrate on root development during their first year.
• Irrigate from above during the hottest period of the day to reduce air temperatures (this may cause other problems).
  

Water Stress

Yellow leaves may result from excessive irrigation or improper drainage. Water displaces air in the soil. As a result, roots cannot support the rapid transpiration required during hot summers. Conversely, too little water can cause drought, which can be equally devastating. Other symptoms:

• Failure of blooms to open completely (excessive water).
• Cane centers may appear soft and brown (excessive water).
• Drooping mature leaves.
• Scorched leaf margins and tips on new growth (drought).

Treatments:

• Improve drainage.
• Reduce irrigation schedule in excessive water conditions.
• Increase irrigation schedule in drought conditions.
  

Salt Stress

Applying excessive amounts of water-soluble fertilizers to roses frequently results in soil salt stress, especially in locations with heavy soil or poor irrigation. Fertilizers high in nitrate of soda, muriate of potash, potassium nitrate and ammonium nitrate can cause the highest accumulated salts.

As salt accumulates, it competes with rose roots for moisture, creating a drought condition. Yellowed leaves and plant death result. It is particularly critical with roses planted in containers.

Rain water and some water supplies can also contain dissolved salt ions. Common among these are bicarbonates, calcium, chlorides, fluorides, magnesium and sulfates - all of which can cause salt stress if found in excessive amounts. Other symptoms:

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