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 

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.

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