TOW: Dormancy, Part II Bulbs
Roy M. Sachs (Tue, 13 Jan 2004 10:20:14 PST)

BULBS. Dormancy in many geophytes is an annual occurrence, even in
those native to equable climes that have much relatively little
seasonal variation in temperature. The growth cycles can be quite
complex in some species. For example, the mother bulb may be in full
growth and yet the daughter bulbs (or corms or rhizomes) remain
dormant. Then as the mother bulb completes its growth cycle and
senesces the daughter bulbs may begin growing even while attached to
the mother bulb (acidanthera definitely follow this pattern and I
believe that the same holds true for many amaryllids that I grow).

Chilling and dormancy.

Tulips. Some tulip selections are well known to require over 1000
hours below 50 F (10C) to get proper shoot development of the mother
bulb (the one purchased from the nursery). Four out of the last 6
winters in Davis (mid November through February) we have been shy of
that number and consequently I have given up on the beautiful, tall
cut-flower tulips. The thing to do is to refrigerate bulbs for 6
weeks shortly after purchase and then plant them. This works for
relatively small quantities of bulbs but not the large numbers I had
in mind for cut-flower purposes.

Narcissus. It's clear that most selections of narcissus require
considerably less chilling than tulips to develop excellent shoot
systems. My first Earlicheer this year in Davis, a tazetta variety
that I obtained from Bill Welch, were at anthesis in late November
when we had little more than a hundred hours of chilling. Some of
Rees' data for 'Golden Harvest' narcissus (The Growth of Bulbs, A. R.
Rees, Academic Press, 1972) show that as few as 600 hours of chilling
at or somewhat below 48 F (ca 9 C) suffice for early and high quality
flowering.

I've been hard-pressed to find exact chilling data for other species
and would appreciate help from the forum.

Ethylene and ethylene generating treatments and dormancy.

Ethylene gas at low concentrations (20 ppm) reduces the length of or
quickly breaks dormancy in a number of geophytes. Ethephon (2
chloroethane phosphonic acid), a water-soluble ethylene releasing
compound can substitute for ethylene gas. Immersing bulbs, corms,
rhizomes, etc in ethephon solutions, with say 0.1 active ingredient,
would be the preferred treatment technique for most of us. I cannot
find a source for ethephon on the web so this may no longer be a
useful tool to gardeners and nurserymen.

Triteleia laxa (Queen Fabiola), responded to a 7 day treatment of
corms with 20 ppm ethylene with a 30 day advance in corm sprouting
and an 80 % increase in number of florets per inflorescence (and a 25
% increase in daughter corm weight).

Smoke from burning foliage releases ethylene and promotes earlier
sprouting of the bulbs and earlier flowering. This practice was
developed by iris growers in Japan to control disease, then found to
reduce the dormancy period, and then the research was done revealing
the release of ethylene during burning.

Other problems.

In adequate chilling is not the only thing that goes awry when I have
attempted to naturalize the large, cut-flower tulips. Excessively
high spring temperatures, a frequent problem in the Davis area and
much of inland California, cause early foliar senescence of some
selections that I've grown, preventing adequate photosynthesis during
the post-anthesis daughter bulb-filling stage. Small daughter bulbs
produce small or no flowering shoots the following spring.

High temperature after-ripening and dormancy. Many geophyte species
are stored by growers at temperatures in excess of 90 F ( 30+ C)
immediately after lifting to promote sealing of base plate or other
wounds incurred on lifting. For some species this period is as short
as a few days, but in freesia the optimum storage time is on the
order of 10 to 13 weeks, much longer than required for healing of
tissues. Something else is changing in the corms but I find no
literature describing what is happening, physiologically speaking.

I've encountered high temperature-induced dormancy frequently in
alstroemeria hybrids and some species. When soil temperatures are at
or near 80 F (26 C) most alstroemeria cease growth. For some
resumption of growth is almost immediate when temperatures are
reduced to 60 F (14 C), but in A. pelegrina and other species (see
below) growth did not resume for months after the soil temperatures
were lowered.

Also, in A. ligtu, A. magnifica, A. aurea, A. diluta, A. kingii and
others the rhizomes become dormant shortly after anthesis or if the
soil temperatures are elevated. Chilling the rhizomes or cooling the
soil does not break this kind of dormancy; but two to three months of
patient waiting works wonders.