I have a
very tall, well-established Magnolia grandiflora. The tree is located on
the southeast corner of the lot. Every summer it has produced large white
flowers, but last summer that there weren’t as many blooms, almost
none. It’s left alone and watered spring through summer by a sprinkler system. I have also noticed yellowing of the leaves at the ends of the branches. Usually the leaves
have been dark and green. I’m wondering if it is a lack of nitrogen or food of some sort.
There are a number of reasons that plants may fail to flower, and it
would be difficult to pinpoint precisely why the Magnolia made such a
weak show this past summer. Sometimes, cold temperatures kill off flower
buds (and there were some cold snaps last winter). The tree is not
immature, and it sounds as if it is not pruned improperly, so those
potential causes can be excluded. You also indicate that it is not
fertilized, so it is probably not receiving excessive nitrogen which can
lead to lots of leafy growth at the expense of flowers. I wonder if
anything else in its environment has changed: has the amount of light
changed (any new construction obstructing sun?), or has anything happened
to the soil where it is planted? You might wait and see if flowering
returns to normal this year.
As for the yellowed leaves, that might be a result of winter injury
(desiccation) or drought stress. However, yellow leaves can also be a
symptom of sunburn or lack of light, or nutrient deficiency. See the
link here to University of California, Davis’s page on Magnolia problems. Excerpt:
Mineral deficiencies:
Certain nutrients, in relatively small amounts, are required for healthy
plant growth. Deficiencies can cause tip chlorosis or necrosis or cause
foliage to discolor, fade, distort, or become spotted, sometimes in a
characteristic pattern that can be recognized to identify the cause.
Fewer leaves, flowers, and fruit may be produced, and these can develop
later than normal and remain undersized. More severely deficient plants
become stunted and exhibit dieback. Commercial laboratories can conduct
foliage tests or soil analysis to verify deficiencies.
Identification/Solutions:
Nitrogen and iron are the only nutrients in which woody landscape plants
are commonly deficient. Poor root growth caused by water-logged soil,
root diseases, and nematodes can also cause iron deficiency symptoms.
Fertilize only as needed and only if other problems have been eliminated
as the cause of poor growth. Avoid overfertilization, especially with
high-nitrogen fertilizers. Slow-release formulations of nitrogen or
organic fertilizers reduce some risk of overfertilization. Correcting
deficiencies of minerals is tricky. Apply only the mineral found to be
deficient. In some cases, soil characteristics may exacerbate
deficiencies. Alkaline soil (high pH) often makes iron or manganese less
available; reducing alkalinity with sulfur or organic amendments (peat
moss) may be all that is needed. Some minerals such as iron, manganese,
and zinc are absorbed more rapidly as a foliar spray than a soil
application.
If the environmental causes don’t ring true with your tree’s situation,
you may want to do a soil test to see if there are nutrients which need
to be supplemented.