The simplest and most reliable way to check whether an acorn will germinate is the float test: drop the acorn into a container of water, and if it sinks to the bottom, it is likely viable. Research on Oregon white oak found that the proportion of acorns that sank predicted roughly 84 to 89 percent of germination outcomes. But the float test is just one tool in a broader assessment, and understanding what makes an acorn fail in the first place will help you collect better seeds and store them properly.
The Float Test
Fill a bucket or bowl with room-temperature water and place your acorns in it. Give them a minute or two. Acorns that sink fully to the bottom are dense, well-hydrated, and most likely contain a healthy embryo. Acorns that float have lost internal mass, often because an insect larva has been eating the nutmeat, because the seed has dried out, or because fungal decay has hollowed part of the interior. In a study of Oregon white oak, the relationship between sinking and later germination was strong: float-test results explained about 84 percent of the variation in germination success during a non-masting year and about 89 percent during a mast year.1Forest Ecology and Management. The float test as a method to determine acorn viability and activity of insect herbivores during masting and non-masting years in Quercus garryana
A couple of caveats are worth mentioning. Some acorns that sink may still be damaged internally in ways the float test cannot catch, and a small percentage of floaters might germinate if the damage is minor. The test works best as a batch-screening tool rather than a guarantee for any single seed. If you are planting dozens or hundreds of acorns, it quickly separates the likely winners from the likely losers. If you are planting just a handful, supplement the float test with a visual inspection.
Visual and Physical Inspection
Before you even get to the water, your eyes and fingers can tell you a lot. A viable acorn generally has a firm shell with no cracks, holes, or soft spots. The cap should detach cleanly or already be missing (caps naturally fall off many species after ripening). Pick the acorn up and give it a gentle squeeze. A healthy acorn feels solid and heavy for its size. One that feels light, rattles when shaken, or yields under thumb pressure has likely lost moisture or been hollowed by larvae.
Look for small, perfectly round exit holes in the shell. These are the calling cards of acorn weevils, beetles whose larvae develop inside the nut and bore out when they are ready to pupate in the soil. A single exit hole means the larva has already left, but the damage to the cotyledon may be extensive enough to prevent germination. Also check for dark, wet-looking stains or a sour smell when you scratch the surface. These suggest fungal rot. Acorns that have already started to sprout a radicle (the first root tip poking out of the pointed end) are viable by definition, though they need to be planted quickly before the root dries.
Weevil Damage
Acorn weevils, primarily species in the genus Curculio, are by far the most common reason acorns fail. The adult female drills into developing acorns while they are still on the tree and deposits eggs inside. The larvae feed on the starchy cotyledon tissue, reducing the energy reserves the embryo needs to germinate and sustain early seedling growth. In a study of Japanese sawtooth oak, acorns with heavy larval infestations had significantly lower germination rates than those with few or no larvae.2Entomologia Experimentalis et Applicata. Prolonged diapause and seed predation by the acorn weevil, Curculio robustus, in relation to masting of the deciduous oak Quercus acutissima A separate experiment on holm oak showed that moderate physical damage to the cotyledon reduced final seedling emergence, even though damaged acorns sometimes germinated earlier than undamaged ones.3Forest Ecology and Management. The effect of simulated damage by weevils on Quercus ilex subsp. Ballota acorns germination, seedling growth and tolerance to experimentally induced drought
The practical takeaway is that a weevil hole does not automatically make an acorn worthless, but the more damage a larva has done, the worse the odds. If you cut an acorn open and see that less than half the cotyledon has been consumed and the embryo at the pointed end looks white and intact, you might still get a seedling, though it will likely be smaller and weaker than one from an undamaged acorn. If more than half the interior is brown mush or hollowed-out galleries, discard it.
Gray squirrels, interestingly, are remarkably good at this same assessment. In field trials, squirrels cached sound, intact red oak acorns for later use but preferentially ate infested acorns on the spot, sometimes consuming the weevil larvae as a protein bonus. They cached 20 to 35 percent more intact acorns than infested ones and dispersed those intact acorns farther before burying them.4Journal of Mammalogy. Caching and Feeding Decisions by Sciurus carolinensis: Responses to Weevil-Infested Acorns If the squirrels in your yard are ignoring certain acorns on the ground, those are probably not worth collecting.
Fungal Infections and Internal Rot
Weevils get most of the attention, but fungal pathogens quietly destroy huge numbers of acorns, especially in storage. One of the most damaging is a fungus called Ciboria batschiana, which causes a dark, spreading rot inside the cotyledons. In a study of English oak, the infection rate of untreated acorns climbed from about 14 percent at collection to 55 percent after 16 months in storage.5Forest Pathology. Effects of hot water treatment, biocontrol agents, disinfectants and a fungicide on storability of English oak acorns and control of the pathogen, Ciboria batschiana That is a staggering increase and explains why acorns that looked fine at collection can fail months later.
There is some natural variation in resistance. Research on a Japanese oak species found that acorns with higher tannin content in their cotyledons resisted Ciboria infection better than low-tannin acorns.6Forest Pathology. Effects of acorn tannin content on infection by the fungus Ciboria batschiana You cannot test tannin levels at home, but the finding hints at why some acorns from the same tree survive storage while others rot: individual seed chemistry varies. The best defense for a home grower is to plant or properly cold-store acorns as soon as possible after collection, limiting the window for fungal colonization.
Externally, fungal infection sometimes shows as a whitish or grayish fuzz at the cap scar or a blackened, sunken area on the shell. But many infected acorns look perfectly normal on the outside. The float test helps here: as rot progresses and tissue breaks down, the acorn loses density and is more likely to float. Cutting open a few representative acorns from your batch is the most direct way to check. Healthy cotyledon tissue is pale cream to yellowish; infected tissue is brown, black, or waterlogged-looking.
Why Acorns Are So Sensitive to Drying Out
Unlike most temperate-zone tree seeds, acorns are recalcitrant, meaning they cannot survive being dried to the low moisture levels that would allow easy long-term storage. They need to stay hydrated or they die. This is one of the most important facts about acorn viability that casual collectors miss. A seed that looks fine on the outside may already be dead if it has been sitting on a hot sidewalk for a few days.
Research on pedunculate oak established that even slight reductions in moisture content below the optimal high level decreased seed survival, especially when acorns were later exposed to temperatures below about minus 5°C.7Annals of Forest Science. Desiccation does not increase frost resistance of pedunculate oak (Quercus robur L.) seeds A study comparing three European oak species found that germination rates dropped dramatically once moisture content fell below species-specific critical thresholds. For pedunculate oak, that threshold was roughly in the 32 to 37 percent range; for sessile oak, about 31 to 36 percent; and for Turkey oak, about 28 to 31 percent. Germination plummeted from above 80 or even 97 percent to around 43 percent once acorns dried below those thresholds.8Šumarski list. Influence of desiccation sensitivity and critical moisture content on Quercus cerris, Quercus petraea and Quercus robur acorns
Research on English oak found that in intact seeds, the cotyledons are the weak link: they lose viability before the embryonic axis does, and once they lose all of their free cellular water, the seed is dead.9Journal of Experimental Botany. Embryo Water Status and Survival in the Recalcitrant Species Quercus robur L: Evidence for a Critical Moisture Content The practical lesson: collect acorns promptly after they fall, keep them moist (a zip-lock bag with a damp paper towel works), and either plant or cold-store them within a few days. An acorn on your kitchen counter for a week is likely no longer viable.
When to Collect
Timing your harvest affects viability in two overlapping ways: ripeness and pest pressure. Acorns collected before they are fully ripe will not germinate. Ripe acorns are typically brown, not green, and detach from the cap easily. Most species in temperate climates drop ripe acorns between September and November, though the window stretches longer in Mediterranean climates. Research on holm oak and cork oak in the Mediterranean found that both pre-dispersal insect predation and post-dispersal rodent predation shifted over the course of the extended dropping season.10European Journal of Forest Research. Effect of acorn fall phenology on seedling establishment success in two mediterranean oak species. Implications facing climate change
As a general rule, collecting early in the drop season gives you acorns that have spent less time on the ground exposed to moisture loss, rodent gnawing, and soil-borne fungi. But very early drops are sometimes aborted or insect-damaged acorns that the tree sheds before the main crop. The sweet spot is usually a few days after you notice a heavy fall under the tree. Gather from the ground rather than picking off the tree, since acorns that have naturally detached are more likely to be fully mature.
Mast years, when oaks produce an unusually heavy crop, are the best time to collect. Not only do you have more acorns to choose from, but the sheer abundance swamps insect predators. Over multi-year periods, researchers have documented a significant reduction in the proportion of acorns lost to predation when overall acorn production was high.11PubMed. Masting mediated by summer drought reduces acorn predation in Mediterranean oak forests In lean years, weevils and rodents can damage a much larger fraction of the crop, so expect more rejects when screening.
White Oaks Versus Red Oaks
If you are in North America, the two major groups of oaks behave quite differently after you collect them, and this affects how you assess and handle viability. White oaks (the group that includes white oak, bur oak, swamp white oak, and chinkapin oak) germinate in the fall, shortly after they drop. A healthy white oak acorn may already be sending out a radicle within days of hitting the ground. This is actually a good sign: if the root tip is pale and firm, the acorn is alive. But it also means white oak acorns are harder to store. They want to grow right now.
Red oaks (including northern red oak, pin oak, scarlet oak, and black oak) have a built-in dormancy period. They drop in autumn but do not germinate until the following spring after a period of cold stratification. This makes them more forgiving for storage. Red oak acorns can be kept in cold storage for a year or more without catastrophic viability loss, provided they stay moist and cool.
Research comparing the two groups found that white oak seedlings tolerated cotyledon damage better than red oak seedlings. When cotyledons were removed after germination, white oak seedlings survived and grew better, likely because they had already transferred more of the cotyledon’s energy reserves into the developing root and shoot before the damage occurred.12Journal of Ecology. Responses of seedling growth and survival to post‐germination cotyledon removal: An investigation among seven oak species For a collector, this means that a white oak acorn with moderate weevil damage may still produce a usable seedling, while a red oak acorn with the same level of damage is less likely to succeed. Set a higher quality bar when selecting red oak acorns.
Storing Acorns After You Have Tested Them
Once you have floated, inspected, and sorted your acorns, storage conditions determine whether they stay viable. The core requirements are cold temperatures and high humidity. For red oaks, research on northern red oak acorn storage found that viability actually peaked about 6 to 12 months into cold storage, then declined with continued storage. After 18 months, at least 60 percent of acorns still germinated in most seedlots tested, and even after 30 months, more than half germinated in several lots. The preferred long-term storage temperature was around minus 2°C, because warmer temperatures above that threshold sometimes allowed acorns to germinate prematurely inside the storage container.13The Forestry Chronicle. The effect of storage temperature and duration on northern red oak acorn viability and vigour
For home growers without precision temperature control, a standard refrigerator set to about 1 to 4°C works well for a season. Place acorns in a sealed plastic bag with barely damp peat moss, vermiculite, or a paper towel. Check every couple of weeks for mold growth (remove any fuzzy acorns) and for premature sprouting (plant those immediately). White oak acorns, given their eagerness to germinate, are best planted within a few weeks of collection rather than stored at all.
Fire and Heat Exposure
If you are collecting acorns in an area that recently burned, or if your property has had a controlled burn, the heat exposure matters more than you might expect. Experimental burns showed that even a single pass of fire through normal leaf-litter fuel loads reduced acorn germination to about 40 percent, compared to roughly 88 percent for unburned acorns. When fuel loads were doubled or tripled, germination dropped below 5 percent for both white and red oak species tested.14Fire Ecology. Impacts of increasing fine fuel loads on acorn germination and early growth of oak seedlings Acorns on the soil surface during a fire are essentially cooked, and there is no external sign that reliably distinguishes a heat-killed acorn from a healthy one. If you know the area burned recently, either dig below the litter layer to find buried acorns that were insulated from the heat, or collect from unburned zones.
The Cut Test
When you want certainty for a small batch, sacrifice a few acorns by cutting them in half lengthwise with a sharp knife or pruning shears. This gives you a direct look at the embryo and cotyledons. A viable acorn has creamy-white to pale-yellow cotyledon tissue that is firm and moist. The embryo, located at the pointed end, should be a slightly differentiated pale structure. Signs of trouble include brown or black discoloration (fungal rot), hollow or crumbly areas (weevil feeding galleries), a dried-out and shrunken interior (desiccation), or a foul smell. The cut test is destructive, so use it on a random sample from your collection to estimate the overall quality of the batch rather than on every individual acorn.
X-Ray Analysis for Large-Scale Projects
Nurseries and forestry programs that handle thousands of acorns sometimes use X-ray imaging to assess viability without destroying the seed. Research on northern red oak demonstrated that X-ray image analysis of cotyledon damage in dried acorns could predict both germination capacity and seedling vigor, offering a rapid and nondestructive alternative to the cut test.15Canadian Journal of Forest Research. Evaluating desiccation sensitivity of Quercus rubra acorns using X-ray image analysis The X-ray reveals internal voids from insect feeding, cracks in the cotyledon, and areas of tissue collapse from desiccation. This is not practical for a backyard planter, but if you are sourcing acorns from a commercial seed supplier, it is worth asking whether the lot was screened this way.
Putting It All Together
A reasonable workflow for a home collector looks like this:
- Collect early: Gather acorns within a day or two of a heavy fall, preferably during a mast year.
- Visual screen: Discard any with exit holes, cracks, soft spots, or visible mold.
- Float test: Submerge in water and remove all floaters.
- Sample cut test: Slice open five to ten acorns from the batch to check internal quality.
- Keep them moist: Bag with damp material and refrigerate if not planting immediately.
No single test catches every bad acorn, and no batch will have a 100 percent germination rate even under ideal conditions. Planting more acorns than you need and thinning later is standard practice for a reason. But a few minutes of screening can dramatically raise your success rate, turning a pile of random nuts into a batch where the large majority will sprout.