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Wednesday, October 2, 2013

Podocarpus Leaf Scorch Update from Previous Question

Q. Thank you so much for responding to my letter. I did want some clarity on these trees because I was surprised you determined this was a watering and not sunscald. First off, these trees are in a courtyard about 5 ft apart and they are on the same run station.  The trees on the south wall are actually shaded by the wall.  The trees on the north wall are getting more direct sun, so I figured the leaves had sunscald.

If you don't mind my asking, why do you think it's a watering issue? I clearly diagnosed it incorrectly so I'd like to learn how to better diagnosis this issue if I come across this again.

Readers Podocarpus with leaf scorch on the north side.

Podocarpus showing no signs of leaf scorch and with rock mulch, a potential problem in the future.

A. I recently started a Yahoo group discussion page because I felt my blog did not give enough opportunities for discussion. It can be found in Yahoo Groups as desert_horticulture@yahoo.groups.com As long as you become a member (which is free but you have to be admitted by the Administrator) you can ask questions, post your thoughts about someone else's comments or add with your own experiences. It is meant for sharing information. To send a question for my blog you have to send it to me in an e-mail which is Extremehort@aol.com

            It is always difficult to assess a situation remotely. I have to rely on what I know about a particular plant, our climate and soils and my personal experience. I have these plants myself and they are located next to my home on the east side. They get a very small amount of water but it is regularly applied.

            First of all, we know they are not true desert plants so we have to add a lot of extra things to get them to grow well here. Soil improvement at planting times is one of them. They will do better with wood surface mulches as well as long as you keep them away from the trunk during the first five years. Besides that, the microclimate or their exposure to the elements can make a difference.

            I also know that these plants can suffer if they get watered too often or if they don't get enough water. The problem is, they look similar if they get watered too often or not enough. When they get watered too often, the roots begin to die. Once the roots begin to die they can't take up enough water and they look like they are drought stressed. Drought stress will be leaf tip burn like yours or even branch dieback if it is extreme. If it is a chronic lack of water in summer months they usually have leaf tip burn.

            I know that plants growing on the north side of the building, or the east side as in my case, are in a cooler location than they are on the South or West sides. High temperatures, wind and lots of sunlight drive plant water use up tremendously. So, plants on the north side and East side will not use as much water as they would on the South and West sides.

(As a side note, ideally, we should be irrigating plants on the south and west sides differently than the plants on the north and east sides. This means they should be on different valves.)

            You called it sunscald and in a way you are right. Usually the term sunscald has to do with burning of the limbs and trunk of a tree, not the leaves. But that is a technical issue and you would not necessarily know that as a layperson but I got what you meant. We would actually call this leaf scorch or tip burn.

Leaf scorch on mockorange

            Leaf scorch typically occurs around the margin of the leaf. Leaf scorch occurs because not enough water is being pulled by the roots of the plant and transported to the leaves. The margins of the leaves, or edges, are furthest from the veins and they are the first to show a lack of water, resulting in scorch.

            A lack of water can occur because not enough water is applied, or there is root damage so it can't take up enough water, or the plant is just is not suitable for a very hot and dry climate and it can't take up enough water in enough volume. We see leaf scorch in plants here like the really big sycamores (that always get cut down when they are about 15 years old because they look so bad) and a few others.

Sycamore with leaf scorch due to reflected heat from south facing wall

We will also see leaf scorch from plants that are stressed in other ways. For instance, if a plant is suffering in a lack of a nutrient, like iron in iron chlorosis, it will scorch when the same plant which is healthy will not. An unhealthy plant just cannot handle the extremes like a healthy one can.

Leaf scorch resulting from iron chlorosis in apricot
            Your plants have leaf scorch or the leaves are dying back on the north side but they are doing well on the south side, as you said. I am assuming that the plants on the north and south sides are getting similar amounts of water. If they are good on the south side, then it appears like they can handle that very hot and bright exposure okay (at least for now).

            These same plants should have no problem handling a north (less stressful) exposure ... but they ARE having trouble. So I ask myself, why do they look poorly on the north side when that should be where they look the best? The reason they look bad on the north side is because of leaf scorch, judging from your picture.

            Leaf scorch is a lack of water and my reasoning tells me that perhaps water is applied to often or the soil cannot drain adequately and roots of the plants are dying on the north side. This would result in leaf scorch in the areas where the plants are receiving watered too often or the soil cannot drain adequately. Whereas the plants on the south side are not. So I reasoned that the soil was staying too wet on the north side. On the south side, where it is hotter and water is lost more quickly, the roots are not dying and they are surviving.

            That is in a nutshell my reasoning for what I said. I could be very wrong but with that particular plant I would've guessed the opposite would've normally occurred; you would have seen leaf scorch on the plants on the south side, not the north side. I will be happy to talk with you further about this if you think I am wrong.

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