Yellow Leaves on My Roses - Part 2

Yellow Leaves on My Roses - Part 2

by Mark Whitelaw
(Part 1 - Part 3)

 

Last time we explored the environmental and mechanical reasons for leaves turning yellow - especially this time of year. Now we will investigate the primary reason for leaves yellowing - nutrient deficiencies. Sometimes, however, too many nutrients can cause similar problems.


Nutrient deficiencies

Soil pH out of balance

Nutrients come in two forms - mobile ("translocatable") nutrients and immobile nutrients. The mobile nutrients are nitrogen, phosphorus, potassium and magnesium. That is, these nutrients can move quickly from the soil to the plant. The immobile nutrients are iron, sulfur, calcium, manganese, copper, zinc, boron, molybdenum, and chlorine. Their movement is determined by their electrical charge. If positive, the nutrients bind with the soil and become unavailable - an insoluble precipitate. The determining factor for this availability is the soil's pH.

Without a doubt, extremes in soil pH are the most common reason for yellow leaves. Although roses can survive a wide range of pH, they prefer their soil slightly acidic (6.5) to near neutral (7.1).

Other symptoms to look for:

  • Yellow leaves only on the bottom of the shrub
  • Irregular-shaped leaves
  • Burned leaf edges
  • Small blooms

Treatments:

  • A quality soil test, performed at least bi-annually, will help determine not only the soil's pH but the available nutrients it contains.
  • Check irrigation supply's pH
  • Add organic soil amendments if the pH is too high.
  • Add lime if the soil's pH is too low; autumn is the best time to apply.

Iron deficiency ("chlorosis")

Iron is one of the chief elements necessary for the production of chlorophyll - the molecule that makes green plants green. It also aids sugar burning enzymes which activate nitrogen fixation. And it regulates the respiration of the plant's cells. All-in-all, iron is an essential element to the rose.

Lack of iron (or its unavailability) is characterized by light yellowish-green leaves and dark green leaf ribs.

Other symptoms to look for:

  • Stems may turn yellow
  • Yellowed leaves appear near the top of new growth, but are generally distributed throughout the plant.

Treatments:

  • Perform a soil test; correct soil pH to 7.0 or lower
  • In iron-deficient soils, add bone meal or blood meal organic amendments, or
    Add iron sulfate or chelated iron liquid or granular inorganic amendments

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Magnesium deficiency

Like iron, magnesium promotes chlorophyll formation and vital to the photosynthetic process necessary to produce dark green foliage. It also promotes healthy, disease-resistant plants. It is generally available in acidic soils.

A lack of magnesium is characterized almost identically with iron deficiency, but the yellow leaves with dark leaf ribs are generally found only at the bottom of the plant.

Other symptoms to look for:

  • Sometimes purple discoloration on the leaves.
  • Lower leaves may fall prematurely from the plant.
  • Older leaves may have a mottled appearance.

Treatments:

  • Perform a soil test; correct soil pH to 7.0 or lower.
  • In magnesium-deficient soils, add fish meal, basic slag, greensand or dolomitic limestone, or
  • Add Epsom salt (magnesium sulfate).

Nitrogen deficiency

Nitrogen stimulates growth of the rose's canes, dark green foliage and good blooms. A lack of nitrogen manifests as small, light green leaves. On roses with yellow blooms, a lack of nitrogen frequently appears as yellow leaves. Nitrogen availability occurs in most soils with a pH above 4.5.

Other symptoms to look for:

  • Weak, spindly stems.
  • Small flowers.

Treatments:

  • Perform a soil test; correct as necessary.
  • If nitrogen is deficient, add blood meal, cottonseed meal and/or manure for organic amendments, or
  • Add nitrate of soda or ammonia sulfate for inorganic amendments.

Sulfur deficiency

Sulfur is the building block of amino acids and proteins (protoplasm) necessary for plant health. It promotes strong root development. Sulfur deficient soils produce rose leaves with both yellowish leaves and veins. Generally, these appear near the top of the plant on newer growth. It is widely available is soils with a pH above 4.5.

Other symptoms to look for:

None

Treatments:

  • Perform a soil test; correct as necessary.
  • Add sulfur, ammonium sulfate and/or potassium sulfate as necessary. Use caution when applying sulfur compounds, however. Too much sulfur ("sulfur toxicity") appears as veinal chlorosis followed by rapid defoliation of the lower leaves.

Manganese deficiency

Manganese, like magnesium, aids in the production of chlorophyll and activates enzymes to assist the photosynthetic process. A lack of manganese looks very similar to magnesium deficiency except that the smallest leaf ribs ("veins") remain green and take on a spider web or netted appearance. Manganese is generally available in soils with a pH below 7.0, but resumes its availability is soils above 8.5.

Other symptoms to look for:

  • Pale spots on new foliage.
  • Mottled older leaves
  • Symptoms appear mainly on new growth near the top of the plant.

Treatments:

  • Perform a soil pH test; correct to 6.5 or lower.
  • In deficient soils, add millorganite or houorganite treated sludge organic amendments, or
  • Add manganese sulfate inorganic amendments.

Nutrient toxicity

Zinc toxicity

Although it is rare, too much zinc can cause symptoms similar to iron or magnesium chlorosis. That is, the leaf tissues first turn yellow, then turn brown. The main leaf rib remains green, but the secondary veins lighten. New growth may become stunted and bloom buds may be distorted.

Zinc toxicity can occur when too much millorganite or houorganite treated sludge is used as a soil amendment or fertilizer.

Phosphorus toxicity

Phosphorus toxicity is becoming an increasing problem in areas where its overuse has accumulated in clay-based soils. This is particularly important to rosarians who frequently succumb to the media hype and apply water soluble, high phosphorus fertilizers in hopes of achieving bigger blooms.

Phosphorus toxicity displays the same symptoms of nutrient deficiencies - copper, iron and zinc. That is, leaf tissues yellow while the leaf ribs remain green. Leaves may become thicker than normal, but appear on shorter stems. Buds may become irregularly shaped.

The reason for this is because too much phosphorus makes these three elements unavailable to the plant.

Potassium toxicity

Potassium toxicity appears the same as phosphorus toxicity, and for the same reasons. The chief difference is that too much potassium also manifests in root loss and consequently the wilting of tender new growth.

Treatments for too much "love" - that is, too many nutrients - is to reduce the amount of fertilizer applied. In some areas of the country, many gardeners do not have to add any phosphorus to their fertilizer mix, for example.

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