Maples are unusual trees. Most trees barely move sap while they are dormant, but maples push sap out of any opening on mild days in late winter and early spring. A broken twig, a pruning cut or a crack can drip for days, which is exactly why maples are tapped for syrup. Most dripping is harmless. The two cases worth a closer look are rows of small holes, which point to sapsuckers, and dark, sour-smelling ooze running down the trunk, which points to slime flux.

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The short answer: in late winter and early spring, a dripping maple is usually reacting to the weather. Freezing nights followed by days around 40°F build pressure inside the wood that pushes sap out of any wound (Utah State University Extension). Sap from pruning cuts is not a health problem. Neat rows of quarter-inch holes mean a sapsucker is feeding. A slimy, foul-smelling streak down the bark in summer is slime flux, which is rarely fatal and has no cure.

The four usual causes

What you see	When	Likely cause	Worry level
Clear, watery drips from a cut, crack or broken twig	Late winter to bud break	Normal spring sap flow	Low
Drips from a fresh pruning cut	Late winter, early spring	Pruning while sap is running	Low
Rows of small, evenly spaced holes that weep	Spring and fall migration	Yellow-bellied sapsucker	Low to moderate
Dark, slimy, sour-smelling ooze and streaks	Mostly summer	Slime flux (bacterial wetwood)	Usually low

1. Spring sap flow driven by freeze and thaw

Maple sap flows because of temperature swings. Utah State University Extension explains that freezing nights and thawing days change the pressure inside the branches. At night the tree draws water up from its roots, and as the wood warms in the sun the sap is pushed out through any opening. It suggests watching for nights in the mid-20s °F and days around or above 40°F.

The University of Maine Cooperative Extension adds that maples store starch in the sapwood over winter. When the wood warms to about 40°F, enzymes turn that starch into sugar, which is why the sap tastes slightly sweet. In southern Maine these freeze-thaw cycles usually run from mid-February to early April.

So a maple that drips clear sap from a crack or a broken branch tip during a cold snap followed by a sunny afternoon is behaving normally. The flow stops by itself as the buds open and nights stay above freezing.

2. Sap from pruning cuts

Late winter is a good time to prune many trees, but on maples it means the cuts may bleed. Iowa State University Extension’s answer to this exact question is reassuring: the sap loss from pruning cuts does not harm the tree. If the dripping bothers you, our guide to the best time to prune a maple explains when to prune so cuts bleed less. Do not paint or seal the cuts to stop the flow.

3. Sapsucker holes

If the sap is coming from a pattern of small holes, look closely at how they are arranged. Mississippi State University Extension describes the yellow-bellied sapsucker as a woodpecker that drills sap wells about a quarter inch wide. It makes new holes in line with old ones, so you see neat horizontal or vertical rows. The bird eats the sap and any insects trapped in it. Thin-barked trees such as maple and birch are favorites.

The same publication notes that trees usually recover from light feeding. Heavy, repeated feeding can let in insects and decay fungi and, in severe cases, girdle a branch or trunk. Sapsuckers are protected by the federal Migratory Bird Treaty Act, so the usual fix is to discourage the bird. Wrap the damaged area with burlap or hang reflective tape during feeding season, then take the wrap off so it does not trap moisture against the bark.

Insect borer holes look different. They are fewer, smaller, scattered at random and often surrounded by fine sawdust (frass).

4. Slime flux (bacterial wetwood)

Slime flux looks worse than it usually is. University of Maryland Extension describes sap flowing from bark cracks or old wounds, leaving dark streaks that turn gray or white as they dry. The liquid often smells fermented. It is caused by common bacteria and yeasts living on sap inside the wood, which produce gases that force the liquid out. Maryland lists maple among the trees affected, although elm, oak and mulberry get it more often.

Penn State Extension notes that little damage is done in most cases. Trees that are severely stressed by things like soil compaction can show leaf yellowing, wilting and dieback. Neither source recommends a cure:

Do not drill holes or insert drain tubes. This is no longer recommended because the holes create new places for the slime to run out.
Avoid new wounds from mowers and string trimmers.
Reduce stress by mulching the root zone and watering in dry spells.
Have a certified arborist look at the tree if the crown is thinning or branches are dying.

When dripping sap is a sign of a bigger problem

Sap itself is rarely the problem. Look at the rest of the tree too. If the dripping comes with thin or scorched leaves, dead branches, cracked or missing bark, or mushrooms at the base, the sap is a symptom of something else. Start with our symptom-by-symptom maple guide, or use the Tree Symptom Checker to narrow it down by season.

Quick checklist

Note the date and weather. Clear drips during a late-winter thaw are normal.
Find where it comes from: a cut, a crack, a row of holes or a broad dark streak.
Smell it. A sour or alcoholic smell points to slime flux.
Check the canopy for dieback in summer.
Leave wounds unsealed, avoid drilling the trunk, and protect sapsucker targets with a temporary wrap.

Sources

045 – Maple Tapping in Utah, Utah State University Extension
How to Tap Maple Trees and Make Maple Syrup (Bulletin #7036), University of Maine Cooperative Extension
Sap is flowing from a pruning cut on my maple tree. Should I be concerned?, Iowa State University Extension
Recognizing Sapsucker Damage in Yard Trees, Mississippi State University Extension
Tree Trunks with Wet Wood or Slime Flux, University of Maryland Extension
Bacterial Wetwood or Slime Flux, Penn State Extension

Top photo: Jomegat, CC BY-SA 4.0, via Wikimedia Commons

Why Is My Maple Tree Dripping Sap?