Both Nitrogen and Iron Fertilizers Contribute to Dark Green Plant Color

Q. Thanks to your previous advice I used an iron chelate, applied it to the soil in March, and got my plants to develop a dark green color rather than a yellowish green color.

Shrubs that the reader was speaking about. Not sure but they do look like desert natives.

A. There are many things that develop a yellowish color in plants besides needing an iron fertilizer or chelate. The plant you questioned appears to be native desert plant, a Texas sage. That is odd needing an iron fertilizer for native desert plants. They are usually accustomed to our soils and don’t need iron. Watering or a nitrogen fertilizer may be the issue.

Not sure but I do recommend iron chelates that contain EDDHA as the chelating agent. It works in all the different alkalinities of desert soils. EDTA and DTPA iron chelates don't.

Regardless, the two fertilizers that can create dark green leaves are nitrogen and iron. If that plant is native to the Western US, then yellowing leaves is more likely issues involving either nitrogen or watering too often. Nitrogen causes stem growth as well as dark green leaf color. Adding only an iron fertilizer or chelate causes the new growth to become green but does not stimulate new growth that much. When iron is involved, the yellowing occurs on newer growth. Yellowing due to a need for nitrogen occurs all over the plant. Also, the yellowing of leaves due to iron may be a yellow leaf color while the veins of the leaves stay a darker green.

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Winter yellowing I call bronzing of a shrub due to very cold temperatures for Las Vegas.

Two types of “overwatering” can occur; watering too often or giving the plant too much water all at once. It is easier for the plant to resist “overwatering” from the second kind than the first kind. It is easy to water desert native plants too often when placed on the same irrigation line as non-desert plants.

Mesquite leaf yellowing and leaf drop during winter cold temperatures.

Another reason for yellowing of plant leaves are cold temperatures. This type of yellowing is more of a “bronzish yellow” leaf color and happens during cold weather. Cold weather damage to mesquite leaves (yellowing or bronzing) is a common occurrence during cold weather just before the leaves may fall from the tree if it gets cold enough.