Leaves of Crepe Myrtle Yellowing

Q. The leaves of my crepe myrtle are turning yellow some show signs I think of a specific mold on underside of the leaves. Please advise in your usual expert advice.

A. When yellowing occurs at the tips first it usually indicates a salt problem. This is possibly due to a lack of water or not enough water applied during an irrigation. It is also possible water is not delivered often enough so the soil becomes too dry between waterings. It is also possible it might be watering too often and not letting the soil dry between waterings.

Crape Myrtle growing at the research center with wood mulch, annual applications of complete fertilizer, iron EDDHA and foliar application of Miracle Gro

Look closely at your watering habits. If you are watering by drip make sure you put lots of gallons down every time you water. For trees this is gallons of water applied each time. The number of gallons depends on the size of the tree. As the tree gets bigger it needs more water applied at each irrigation. NOT watered more often.

Make sure when you water that it is not daily or even every other day. This time of year every three days is adequate but put enough gallons down, not a small amount.

Estimate how much water is delivered each time you water. You can use the chart below if you do not know the gph (gallons per hour) of your drip emitters. For instance if your drip emitter delivers 10 ml of water in 9 seconds then you have a 1 gph emitter. Increasing the total water applied will flush salts from the tree.

Number of seconds to deliver gph (3600 seconds/hr) (3785ml/gal, approx. 4000)

Gph     10ml     20 ml    30ml    40ml     50ml    60ml    70ml    80ml    90ml    100ml
¼           36 sec. 72           -           -            -           -           -           -          -            -
½           18       36        54         72
1            9         18        27         36           45       54         63         72
2            4.5       9         13.5      18           22.5
3            3          6           9         12           15
4            2.25     4.5        6.75     9            11
5             -
6             -
7             -
8             -
9
10           -            -            -           -           4.5      5.4         6.3       7.6        8.1       9.0

If you suspect you are watering too often then increase the number of emitters (this way you do not need to increase the minutes). Increasing the minutes means everything else will get more (or too much) water.

If you suspect you are not giving enough water each time then flush the soil with water several times to remove excess salts and rewet the rootzone deeply. Put a hose at the base of the tree and let the hose run very slowly around the tree near the emitters for several hours. Or build a donut around the tree and fill it several times a few days apart. Or buy a small inexpensive sprinkler that goes on the end of the hose and let it run for 15 to 20 minutes. Repeat every other day three or four times to flush salts.

This could also be from a lack of organic matter in the soil IF the tree is surrounded by rock mulch. Buy some decent compost and apply it to the rocks under the tree and water it in thoroughly. Do this two or three times this fall and repeat it in the spring.