Willows are some of the fastest-growing trees you can plant, and in moist soil they rarely need help. Fertilizer is worth using only when a willow is genuinely short of a nutrient, which a soil test will tell you. Feeding a healthy willow mostly adds more soft, fast growth to a tree whose weak wood already breaks easily.

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The short answer: most willows do not need regular fertilizer. Get a soil test first, and fertilize only if it shows a deficiency. If you do feed, do it no more than once a year, in late fall after leaf drop or in spring as growth starts, using a balanced fertilizer such as 10-10-10 spread evenly over the root zone. Do not fertilize a newly planted willow for two or three years, do not use lawn fertilizer that contains herbicide, and avoid feeding in late summer.

Does your willow need fertilizer?

The Morton Arboretum’s guidance is clear: adding any soil nutrient is recommended only if a soil or foliage test indicates a deficiency. Before blaming nutrition, it says to rule out other causes of poor growth or pale, small leaves, such as lack of water, girdling roots and trunk damage, because fertilizer helps with nutrient deficiencies and nothing else. It is not a cure for other tree problems.

For willows, water is the usual suspect. NC State Extension says weeping willow grows best in very moist soils, and the University of Florida notes that in drier, open sites it needs regular watering to prevent leaf drop in drought. A willow dropping leaves in a dry summer usually needs water, not fertilizer. See why a weeping willow loses leaves for the common causes.

Signs that may point to a real deficiency include generally poor growth and pale or yellow-green leaves on a tree that is getting enough water. A soil test, available from your local extension office or a soil testing service, confirms it.

Why more is not better for willows

Willows already grow fast. NC State says weeping willow quickly reaches 30 to 40 feet tall and wide and warns that rapid growth comes with weak wood that cracks or breaks easily, especially under snow or ice.

The Morton Arboretum explains that too much nitrogen can make a plant grow faster than normal, with softer, more succulent leaves that are more susceptible to fungal infection. It also notes that applying too much nitrogen at once can damage roots. For a willow, that means more limbs to lose in a storm and more leaf disease, not a healthier tree.

When to fertilize

If a test shows a need, the Morton Arboretum recommends fertilizing trees no more than once a year, at one of two times:

Late fall, once the tree is dormant. Roots stay active longer than the branches, so they can absorb and store nutrients over winter and use them when the tree leafs out.
Spring, as new growth resumes.

Avoid late summer. The Morton Arboretum’s garden calendar advises against fertilizing trees then, because it can stimulate new growth that will not harden off before winter and adds stress to roots already coping with dry soil.

Older, established trees do not need feeding every year. Morton says every three to five years may be enough.

Newly planted willows: skip it

Fertilizer at planting is generally not recommended, according to the Morton Arboretum. It is ineffective until the root system has a chance to re-establish, and Morton advises waiting two or three years and then getting a soil test before fertilizing. For a new willow, steady watering is what drives root growth. The same applies to a tree whose roots were recently damaged by construction or trenching: water it, and let the roots recover before any fertilizer goes down.

How to fertilize a willow

Choose the right product. The Morton Arboretum says shade trees are well served by a balanced fertilizer with equal nitrogen, phosphorus and potassium, such as 10-10-10 or 12-12-12. Your soil test may recommend something more specific.
Avoid weed-and-feed products. Morton warns not to use fertilizers containing herbicides, such as lawn formulations, around trees.
Spread it evenly over the soil surface across the root zone, which on a willow extends well beyond the branch tips, rather than piling it at the trunk.
Follow the label rate. Do not exceed it, since too much nitrogen at once can damage roots.
Water it in, especially if the soil is dry.

Fertilizer and willow diseases

The University of Florida notes that many fungi cause cankers on willow, that cankered branches should be pruned out, and that a trunk girdled by canker will kill the tree. It recommends keeping trees healthy with regular fertilization as part of managing these problems. In practice, that means maintaining good nutrition where a soil test shows it is lacking, alongside watering and pruning out infected wood, rather than heavy feeding. For diagnosis, see how to treat willow tree fungus.

The basics matter more

A willow in moist soil, with a wide mulch ring and water during droughts, will usually grow vigorously without any fertilizer. The Morton Arboretum’s golden weeping willow profile lists moist, well-drained and wet soils among its preferences. Get those conditions right first. For pruning that keeps fast growth safe, see when to prune a willow tree.

Sources

How and When to Use Fertilizer on Plants, The Morton Arboretum
Caring for New Transplants, The Morton Arboretum
Garden Calendar, The Morton Arboretum
Golden Weeping Willow, The Morton Arboretum
Salix babylonica (Weeping Willow), NC State Extension Gardener Plant Toolbox
Salix babylonica: Weeping Willow, University of Florida IFAS Extension

Top photo: pam fray, CC BY-SA 2.0, via Wikimedia Commons

How to Fertilize a Willow Tree