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As hot as it's been I've been worried that the tree was not getting enough water, and it seems with increasing the water amount the leaves have turned even more yellow.

How can I tell if I am over- or under-watering the tree and what else can cause the leaves to yellow?

The tree was planted last year, but was already 2-3 years old.

  • 1
    In addition to "Mancuniensis" answer you may wish to take a look at "Watering Guidelines by Arborist Don Gardner" found in this answer. Don said on his monthly tree talk radio show last weekend that a lot of places in the US are now publishing their own tree watering guidelines based off of his "original" work... – Mike Perry Aug 12 '11 at 19:11
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    Side note to above comment: Don was not criticising those who'd created their own tree watering guidelines based off of his "original" work, in fact he was encouraging people to-do-so. – Mike Perry Aug 12 '11 at 19:12
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You don' say whether your tree was planted recently or is a well-established one, but it sounds as though the problem is caused by over-watering, as you suspect. A mature Honey Locust is an extremely tough, drought-resistant tree, well fitted to survive long dry spells, but sensitive to over-watering.

If you planted your tree fairly recently. the following watering advice taken from the Draper City Street Tree Guide should help:

When To Water: Water frequently enough so the soil several inches below the surface is moist without being continually sodden. Soil with adequate moisture for root growth will form a ball when squeezed but will fall apart when bounced on your hand. Crumbling soil is too dry; sodden soil is too wet. Soil moisture can be assessed by poking a rod into the soil. Rod penetration indicates extent of moist soil. Resistance indicates overly dry soil… or a rock. A rod coated with mud indicates over watering.

Watering Frequency: Water every 2 to 4 days if you have planted during the summer in extremely sandy soil. Water less frequently if you have more clay in your soil, and it is poorly drained, or if you have planted during the early spring or late fall. You must not water so often that the tree’s roots become waterlogged. Over watering will kill a tree as surely as no water at all. Late fall and early winter water is essential for trees planted less than two years.

To preserve moisture, it would be a good idea to spread a 3 or 4 inch layer of mulch on the soil around your tree, but make sure that it doesn't come into contact with the trunk.

As yellowing leaves are the only symptom you mention, it's very unlikely that the tree is diseased or suffering from an in insect infestation; however, if its condition doesn't improve, it might be caused by a nutritional deficiency, and this could easily be corrected on the basis of a soil test.

  • Are yellow leaves also a symptom of under-watering? I expect you are right in that I over-watered. – JYelton Aug 12 '11 at 15:25
  • Yes, they can be a symptom of under-watering too, but unless you allowed the soil to remain completely dry for some time, or have been watering superficially rather than in depth (superficial watering often causes the tree to surface-root, undermining its health), I think the yellowing is more likely to be due to over-watering. – Mancuniensis Aug 12 '11 at 18:19

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