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Tumbling For Seed Cleaning and Conditioning1
By: David Dreesen 2
Small rock tumblers can be used to clean and condition seeds both in an aqueous and a
dry mode. During the process, grit and gravel remove fruit pulp and abrade seed coats.
Wet tumbling of seed aids imbibition, leaches water-soluble germination inhibitors, and
may partially substitute for cold stratification for some shrub seed lots.
Key Words:
Oleaceae, Forestiera pubescens var. pubescens, New Mexico olive, Plantanaceae,
Platanus wrightii, Arizona sycamore, Grossulariaceae, Ribes aureum, Ribes cereum,
Solanaceae, Lycium torreyii, wolfberry, Cornaceae, Cornus sericea ssp. sericea, redosier
A reformatted version of the published article from Native Plants Journal, Spring 2004, Volume 5, Number 1.
David Dreesen, Agronomist, USDA-NRCS Los Lunas Plant Materials Center, 1036 Miller St. SW, Los Lunas, NM 87031
[email protected]
At the Los Lunas Plant Materials Center in New Mexico, we use small, hobby-size rock
tumblers to accomplish a number of seed cleaning and seed conditioning treatments. The
principal application of the tumbler has been maceration of dried or hydrated fruit pulp.
We commonly use it to remove pulp from dried New Mexico olive (Forestiera pubescens
Nutt. var. pubescens [Oleaceae]) fruits. The fruits collected in late summer or fall after
the pulp has dehydrate and adheres tenaciously to seeds. A wet tumbling procedure
employing pea gravel/crushed stone and water in a rubber lined tumbler vessel allows the
rehydration of the pulp an slow abrasion of pulp from seeds. The amount of water is
minimized so that the gravel and fruit makes a slurry. This method is not quick, but the
tumbler can be run overnight and check the following day. After a course of tumbling,
the contents dumped into a sieve and the pulp is washed off, leaving clean seeds. The
tumbling process is repeated until clean seeds achieved (Figure 1).
Another cleaning application involves
removal of fine hairs attached to achenes of
Arizona sycamore (Platanus wrightii S.
Wats. [Platanaceae]). The dry fruiting heads
are crushed under water to partially liberate
the achenes while preventing and fine hairs
from becoming airborne (Figure 2). A slurry
achenes with pea gravel is tumbled and the
hairs detach time and can be separated using
sieves and strong sprays of water. In
addition, the wet tumbling thoroughly
imbibes seeds and may leach out water
soluble germination inhibitors. After
cleaning and imbibition, seeds are typically
cold stratified.
Figure 1: The pulp of naturally dehydrated
fruits (top) of New Mexico olive can be Dry tumbling to scarify legume seeds has
removed using a rock tumbler, leaving been investigated, (Bonner and others 1974;
extremely clean seeds (bottom).
Dreesen and Harrington 1997). The rationale
for dry tumbling is to avoid seed destruction
that can readily occur with sulfuric acid, boiling water, and high energy impact
mechanical scarification treatments. Dry tumbling is a slow process taking several days
to a week, but we often use it when we have small seed lots we do not want to risk with
other scarification treatments. The procedure uses carborundum grit (sold by rock
tumbler dealers), pea gravel, and seeds. After tumbling, scarified seeds are separated
from the grit and gravel with sieves. The grit can also be reused by washing the seed coat
debris through a fine sieve or by floating off the debris and then drying the grit. Different
size grits are available and we typically use fairly coarse material. Coarse grit size is still
much smaller than most legume seeds, allowing the easy sieve separation of grit, seeds,
and gravel.
Wet tumbling can be used for
scarification if an abrasive (typically
pea gravel) is incorporated in the seed
and water slurry (Dreesen and others
2002). The force imparted to the grit
by the tumbling gravel facilitates
abrasion. Although this treatment
method may result in some seed coat
degradation, other effects may be
more important, such as assuring
com- plete imbibition in well-aerated
water and the leaching of water
soluble germination inhibitors in Figure 2: At the Los Lunas Plant Materials Center, dry
the seed coat. A typical treatment fruiting heads of Arizona sycamore, seen lower left, are
would involve wet tumbling for crushed under water in a large pan. The hairs agglomerate
several days to a week with daily into balls (gray sieve in foreground). A slurry of achenes
changes of water. and pea gravel are tumbled in the rock tumbler to dislodge
the hairs. Finally, the achenes, hairs, and pea gravel are
For a few species, wet tumbling separated with soil sieves with the cleaned achenes visible
may partially substitute for a cold in the brass sieve (background).
stratification requirement. Two
currant species (Ribes aureum Pursh and R. cereum Dougl. [Grossulariaceael]) and
wolfberry (Lycium torreyii Gray [Solanaceae]) generally require 2 to 3 m of cold
stratification to achieve acceptable germination. Wet tumbling followed by 1 to 2 wk of
storage in a warm moist environment has resulted in germination without cold
stratification. The dry seeds of another important riparian species, redosier dogwood
(Cornus sericea L. ssp. sericea [Cornaceae]), generally require 1 h scarification in
concentrated sulfuric acid and then 2 to 3 mo of cold stratification for acceptable
germination. Using fresh fruit with hydrated pulp, rapid germination has been achieved
by wet tumbling the fruit with 1 to 2 cm (0.5 to 0.75 in) gravel. Most of the pulp is
removed in the first day of tumbling and separated by screening and float/sink
manipulations in water. After pulp removal, seeds are wet tumbled for several more days
with daily water changes. The imbibed seed is then stored in a warm moist environment;
germination starts in about 7 to 10 d and continues for several weeks. Although a limited
number of species have been tested with wet tumbling for seed conditioning, additional
species may benefit from this treatment.
Bonner FT, McLernore BF, Barnett, JP. 1974. Presowing treatment of seed to speed
germination. In: Schopmeyer CS, technical coordinator. Seeds of woody plants in the
United States. Washington (DC): USDA Forest Service. Agriculture Handbook No.
450. p 126-135.
Dreesen DR, Harrington JT 1997. Propagation of native plants for restoration projects in
the Southwestern U.S.–preliminary investigations. In: Landis TD; Thompson JR,
technical coordinators. National proceedings, forest and conservation nursery
associations. Portland (OR): USDA Forest Service, Pacific Northwest Research
Station. General Technical Report PNW-GTR-419. p 77-88.
Dreesen DR, Harrington J, Subirge T, Stewart P, Fenchel G. 2002. Riparian restoration in
the southwest: species selection, propagation, planting methods, and case studies. In:
Dumroese RK; Riley LE; Landis TD, technical coordinators. National proceedings:
forest and conservation nursery associations–1999, 2000, and 2001. Ogden (UT):
USDA Forest Service Rocky Mountain Research Station. Proceedings RMRS-P-24. p
USDA NRCS. 2002. The PLANTS database, version 3.5. URL: http://plants.usda.gov
(accessed 10 Jun 2003). Baton Rouge (LA): The National Plant Data Center.

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