It seems that spring is finally coming to Welkinweir: this
week there has been an explosion of birdsong, bulbs are pushing their leaves up
out of the ground, and a few of our early bloomers have come out in
show-stopping force.
So the question I asked is…why? How do plants in particular know when to wake
up in the spring?
From the Pavilion, looking over the ponds |
The answer has two parts.
Our climate is driven by temperature and precipitation; for the purposes
of my answer, I’m more interested in the former factor. And temperature as we all know is driven by the
sun. Because the Earth is spherical and
tilted on an axis, it doesn’t receive sunlight in equal amounts across all
areas and at all times; instead, as it rotates around the sun throughout the
year, different parts are exposed to the sun for longer and shorter
periods. During the winter, the northern
hemisphere (where we’re located) is tilted away from the sun, so the sun’s rays
reach us at a lower angle and with less intensity than during the summer, when
the northern hemisphere is tilted toward the sun[1]. This means that our days in the winter are
shorter and colder.
Plants are aware of the changes in light throughout the
year, and many use those changes to dictate when they flower. This awareness is called photoperiodism. Most plants are categorized as short-day,
long-day, or day-neutral plants, although these terms are misleading – plants actually
measure the length of night, not day. They
measure this time period using phytochrome, a light-reactive protein in their
cells[2]. Some well-known short-day plants include poinsettias,
chrysanthemums, camellias, and Christmas cactus; they prepare to flower as the
nights get longer (and the days shorter).
Long-day plants include asters, most sunflowers, hostas, snapdragons,
and purple coneflower; as the nights get shorter (and the days longer), they
set flower. Day-neutral plants flower
independently of night length[3]. Therefore, as the nights grow shorter as
spring approaches, the plants in our gardens take it as the cue to begin their flowering
cycles anew.
The other factor that influences plant cycles is temperature,
which drives when a plant goes into and comes out of dormancy. Most of our garden plants – from magnolias
and witch hazels, to coral bells and columbine – respond to specific
temperature changes, and each need their own unique period of cold temperatures
in order to flower. This is called
vernalization (another misleading term, as vernal means “of, relating to, or occurring in the spring”[4]),
and it varies widely among plants[5]. Some plant species require a combination of
the right temperature and light ranges; this has been studied in cereal crops
such as winter rye[6]. In summary, according to Steve Bogash, the Southeast
Regional Horticulture Educator for Penn State’s Cooperative Extension, “Plants
begin preparations for winter largely based on the increasing length of the
night (shorter days), but temperature is important to the dormancy and
hardening processes. They break dormancy
and resume growth in the spring based on increasing temperatures”[7].
A bit of a lecture, I know!
But I found it fascinating, if a bit confusing. In celebration of springtime, here are a few
photos of our witch hazels defying last week’s snowstorms, a Christmas rose in
the Barn Ruins, and snow drops. They’re
still lighting up the arboretum grounds, so come visit Welkinweir to enjoy the
year’s first flowers! And stay tuned,
because in the next week or two we should be seeing the blooms of our two
beautiful Persian Parrotia trees!
Hamamelis x intermedia 'Jelena' 'Jelena' Witch hazel |
'Arnold Promise' Witch hazel |
'Westerstede' Witch hazel |
'Diane' Witch hazel |
Christmas Rose |
Snow drops |
[2]
Bogash, Steve. “Timepieces in Our Plants”. Hort
Report. Pennsylvania State University Cooperative Extension, Southeast
Region. Page 2.
[4] http://www.merriam-webster.com/dictionary/vernal
[5]
Bogash, Steve. “Timepieces in Our Plants”. Page 4.
[6] Cathey,
H. Marc, Ph.D. “Unlocking the Secrets of Plant Dormancy”. The American Gardener. The American Horticultural Society. May/June
2004. Page 51. http://www.ahs.org/publications/the_american_gardener/pdf/0405/Everyday_Garden_Science_51.pdf
[7] “Timepieces
in Our Plants”. Page 5.