Magnolia care involves fertilizing in the spring with a slow-release fertilizer, so I do not see where you went wrong there. Magnolias are considered to be slow growing, achieving gains in height of 1-2 feet per year. While you are below that average you should be encouraged that you are still seeing growth. And keep in mind that the growth of a tree is not only judged by height but also growth in the circumference of the trunk and in the expanse of the branches. Does the tree otherwise look healthy? Growth can be influenced by environmental and cultural conditions. Was the tree planted in moist, rich, slightly acidic soil? One thing that could slow down growth rate is periods of drought. As part of your magnolia tree care, you’ll need to water the trees to keep the soil around the base of the tree moist. You may also want to consider mulching around your tree to conserve moisture.
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