Why is my Pieris going yellow?

Native to the mountainous regions of eastern and southern Asia, eastern North America and Cuba, the Pieris genus could be considered to hold some of the most perfect of all ornamental garden plants. Hardy, evergreen and clothed in glossy and luxuriant leaves, its colourful new foliage and attractive racemes of bell-like blooms means that it kind of ticks all the boxes. Well at least it does when it's on display in the confines of a garden centre. However once it becomes established in your own garden these qualities can quickly deteriorate. One of the biggest issues with planting Pieris is that they become stunted and yellow, a condition that is generally related and known as chlorosis. So the question often asked by gardeners is this. Why is my Pieris going yellow?

The reason is twofold. Firstly, the Pieris genus is found within the family Ericaceae which is noted for its tolerance for acidic, infertile conditions. Secondly, many suburban gardens across northern Europe have an alkaline soil and herein lies the problem. The roots of ericaceous plants are unable to take up certain nutrients, notably iron, magnesium and zinc. The yellow that you see in the foliage of affected Pieris plants are the orange/yellow carotenoid pigments found and usually hidden below the green chlorophyll pigments. These chlorophyll pigments are the power drivers of the plant producing the sugars and energy with the plant needs to grow. Iron, magnesium and zinc are vital components of the chlorophyll molecule (iron being used in greater quantities) and without it the plant is unable to produce it. Therefore if the plant is unable to produce green chlorophyll pigments the green colour of the leaf will fade and reveal the yellow pigments beneath. This is why Pieris can go yellow. Without being able to produce the energy needed to grow the plant will become stunted. 

The good news is that this decline can be reversed. Also you need to do it provide conditions suitable for healthy growth and this can be achieved by either acidifying the soil around the root system or by supplying iron in a form which an be taken up by the root system.

Chelated iron is more readily absorbed by Pieris root systems and can be found in products such as sequestrien and Hydrangea blue colourant. Alternatively provide a liquid soluble plant fertiliser specially formulated for ericaceous plants such as Miracid by Sotts.

If you want to acidify the soil around the roots, but do so without disturbing the root system, dig in moss peat and/or apply flowers of sulfur to the soil surface and water in. One last option is to spray the foliage with iron sulphate (generally sold as known as ferrous sulphate) directly onto the leaves as a rate of 1 to 2 oz of ferrous sulfate per gallon of water.

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