Yel­low aza­leas caused by de­fi­cien­cies in soil

The Standard Journal - - LIFESTYLE - / By Ricky Ens­ley Polk County Ex­ten­sion Co­or­di­na­tor

Why are the leaves on my aza­leas yel­low?

Aza­leas can turn yel­low in re­sponse to de­fi­cien­cies of iron or ni­tro­gen. How do you know the dif­fer­ence? Look at the part of the plant af­fected and the way the leaf looks. Ni­tro­gen de­fi­cien­cies show up on the older leaves first. The en­tire leaf turns yel­low and the leaf may fall off. Ni­tro­gen de­fi­cien­cies are more com­mon in the fall and win­ter.

Iron de­fi­cien­cies show up on the younger leaves first. The leaf turns yel­low but the veins of­ten re­main green. The leaf may be­gin to look white if the de­fi­ciency be­comes se­vere.

These prob­lems can be cor­rected by adding fer­til­izer con­tain­ing this nu­tri­ent but the prob­lem may be caused by more than a short­age of the nu­tri­ent in ques­tion. In­ves­ti­gate a lit­tle far­ther to see if there is an un­der­ly­ing prob­lem caus­ing the de­fi­ciency.

Ei­ther one of these de­fi­cien­cies can be caused by root in­jury to the plant. Has the plant been trans­planted re­cently? Has it been kept too wet or too dry? If the roots are not strong, the plant can­not take up nu­tri­ents and will be de­fi­cient. Find the prob­lem and cor­rect it if you can.

Ni­tro­gen de­fi­cien­cies are more com­mon in the win­ter. The plant did not have enough stored ni­tro­gen to last through the win­ter. Wait un­til after the plant blooms and then fer­til­ize. Re­mem­ber to re­peat the fer­til­izer ap­pli­ca­tion in Au­gust.

Iron de­fi­cien­cies are of­ten the re­sult of high soil pH. Aza­leas pre­fer a lower pH (5.0 – 5.5) than many other plants (5.5 – 6.0). If the pH is too high and you do not cor­rect the prob­lem, ex­pect the iron de­fi­ciency to re­turn. Take a soil sam­ple. The re­sults should tell you how much sul­fur or am­mo­nium sul­fate to add to the soil to lower the pH. Be pa­tient. These chem­i­cals take a while to work.

Add iron to the plant with fo­liar iron sprays and soil ap­plied iron prod­ucts as well as chang­ing the pH. Plants prob­a­bly re­spond to iron com­pounds we spray on the leaves faster than soil ap­plied prod­ucts. Soil ap­plied prod­ucts last much longer. Ap­ply the soil ap­plied prod­ucts first and the sprays if you can af­ford both.

Some times near build­ings we find soils with high pH due to buried mor­tar or con­crete. These prob­lems are dif­fi­cult to solve. First, dig up the buried mor­tar. Then soil sam­ple and ad­just the pH if needed. One year after plant­ing the shrubs, take an­other soil sam­ple and ad­just pH again if needed. In these sit­u­a­tions, you may want to use a plant other than aza­lea that would not be so sen­si­tive to high pH.

Good Luck!

Ricky Ens­ley

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