Float Tree Nuts To Test Viability – How Accurate Is It?

Anyone who has researched how to grow an Oak Tree from an acorn, or germinate a tree nut has probably encountered instructions telling them to place the nuts in water, and throw away anything that floats.  Well, I’m going to tell you how to do this test, why it is important, and provide you with some scientific research that backs it up.

What is the float test

The float test is a method for assessing the viability of acorns, walnuts, or other tree nuts.  Simply put, the nuts or acorns are immersed in water for sixty seconds.  Any nut or acorn that doesn’t sink to the bottom is discarded, as it is assumed that the acorn has too much air inside, indicating that it is undeveloped, aborted, or suffered insect damage.

The float test should be done quickly after harvesting acorns, walnuts, or hickory nuts.  If allowed to dry for several days, then more ‘good’ nuts will float as the outer shell will lose moisture, being replaced with air.  Plus, a nut that fully dries will not be viable at all.

Acorn float test

To perform the acorn float test to verify the viability of acorns, place them in a bucket or container of water for 60 seconds. Viable acorns that will germinate should sink after sixty seconds. Any acorn that floats, or is partially suspended should be discarded. Acorns that sink after 60 seconds are viable and can be planted^[1].

What pests can damage acorns or tree nuts?

There are several reasons for an acorn to lose viability by the time it ripens. 

• First, the acorn/tree nut could be diseased, which results in deformed embryos or infected cotyledons. 
• The acorn/tree nut could have not developed fully, resulting in a hollow interior
• Sometimes a tree may abort an acorn/tree nut, dislodging it early before it is fully formed
• Weevils can bore into the interior of an acorn/tree nut, and feed on the embryo
• Fly larvae, wasp galls, or Lepidopterous larvae can bore into the acorn/tree nut, and feed on the embryo

What problems can the float test detect?

The float test has been shown to be effective at detecting aborted or undeveloped acorns at a very high rate, gall wasp larvae, dipterous larva (~99%). It was also quite successful at diseased or weevil infested larvae (>80%).  The only issue is that it will overestimate the non-viable acorns.[1]

What do you take away from this?  Well, if you are trying to grow Oaks, Walnuts, Hickory, etc from seed, you should absolutely do the float test. 

Why should you discard any acorn, walnut, or tree nut that floats?

When the acorn or nut floats, it usually means that instead of the cavity being solidly packed with an embryo or cotyledons, it is deformed or shrunken resulting in air pockets.  Or, an insect has burrowed in and consumed parts of the nut, sufficiently lowering the density of the acorn/nut so that it now floats.

What tree nuts should be float tested?

Trees that produce large nuts should be float-tested.  These would include any Oak tree (Quercus sp.), Walnut, Butternut, Pecan trees (Juglans sp.), and any Hickory tree (Carya sp.).

What other tests should be done?

If collecting acorns, one should also remove the crown/cap to look for holes.  Any hole on the acorn, or in the husk of a hickory nut, walnut, etc indicates that a weevil or larvae has burrowed into the nut. 

In fact one study from the University of Mississippi found that careful visual inspections of acorns were more effective at detecting diseased nuts than the float test.  Although, the float test was still found to accurately predict viability.[2]

What science says

Acorns

Large studies of acorn float tests have been conducted.  Their results were that the float test was an absolutely great way to detect ‘bad’ acorns.  For example, a large study in West Virginia was conducted on Red Oak acorns that involved over 18000 specimens.  They float tested all of them three times over the course of a couple weeks to allow any larvae to continue eating away, hopefully resulting in a detection.

As reported earlier, it could detect deformed, aborted, or larvae infested acorns at a 99% rate.  While diseased or weevil infested acorns at an 80% rate.  What was interesting though was that the float test also resulted in the discarding of ‘good’ acorns at about 50%.[1]  So, what does that mean?

Well, for a backyard gardener, or even a nursery owner, you should collect a whole lot more acorns than trees you plant to grow.  And then float test them, and discard the floaters, knowing that some you discard will probably be ok.  Doing it this way makes it so that you will significantly reduce your chance of planting a bad acorn. 

Walnuts

The USDA has conducted investigations and experiments into the viability of Black Walnuts.  They have found that nuts that were not float tested germinated at roughly 48%.  But nuts that were float tested, and sank germinated above 70%. [2]

They have done so by float testing, and x-raying nuts to determine if they kernel is fully formed and in-tact.  What they found was that x-ray experiments weren’t really anymore effective than float testing the nuts, germinating at 80% for float tested along, and 75% for float tested followed by x-ray.  It doesn’t mean that x-ray was worse, it really just means there wasn’t a statistical difference. [2]

Conclusions

The float test is an effective method at detecting diseased or damaged acorns and tree nuts.  It will however give some false signals, resulting in the discarding of some viable nuts. It is therefore important to collect more nuts to test than you theoretically need to plant, as that way you will better ensure you are able to germinate enough to cover your needs.

The bottom line is that a careful visual inspection of the acorn, walnut, or hickory nut followed by a float test should allow you to detect and discard most non-viable seeds. This will save you time, container space, and give you more satisfaction in the long run.

Find more on native trees here