Download PDF - Penn State Extension

Timepieces in our Plants
Have you ever wondered how plants tell time?
Part 1: Light Reactions
In order for plants to flower at about the same time every year,
go dormant for winter and resume growth in the spring, it is
vital for a plant to know the time of year. Without this
awareness, plants could get caught fully leafed out, actively
growing and too tender to survive the cold. As the days get
shorter in the fall, plants get the cue from the change in
lighting that they need to begin winter preparations.
This awareness of the time of year results from the plants
ability to track daylength or photoperiod and is known as
photoperiodism. Plants are grouped roughly as long-day,
short-day and day neutral based on their response to
photoperiod. Photoperiod is defined as the time that plants are
exposed to light during a 24 hour period. A 10 hour
photoperiod has 10 hours of light and 14 hours of darkness
and would naturally occur twice each year, once in the late
winter in early February and again in the Fall at the beginning
of November. It's interesting enough just to know that plants
run on a 24 hour cycle. Many plants will go into a night cycle
with the lights fully on. Think of these plants as jet lagged.
Long day plants create blossoms as the days get longer.
Therefore, these plants set their flower buds as soon as the
days get long enough in the spring. Most sunflowers are long
day plants, so they bloom in the summer. Other common longday plants include: Petunia, Snapdragon, Black- eyed Susan,
Shasta Daisy, Purple coneflower, African marigold, and
Hosta.
Here's a great demonstration of photoperiod: Plant nearly any
common sunflower in the late summer after about mid-July.
They will grow to a very short height and make very tiny
blossoms and do it all very quickly as compared to the same
seed planted at the end of May. Since the daylength in
mid-late summer is correct for flower development, the small
young seedlings produce flowers almost immediately,
compared to the plant in the spring where weeks of vegetative
growth result in a much larger plant at flowering. To
complicate this scenario, there are some day- neutral
sunflowers grown for the cut flower trade that are much less
sensitive to day length and will look pretty much the same
assuming there is enough light and warmth to grow and
flower. Many bedding plants are photoperiodic, but have been
bred to have a reduced response to photoperiod.
Short-day plants have pretty much the opposite reaction to day
length as long- day plants. These plants set flower buds and
bloom as the days shorten in late summer going into fall.
Chrysanthemums (mums), Dahlia and Poinsettia are examples
of short-day plants. Generally, sometime after the longest day
of the year, these plants begin to set flower buds, but some can
set flower buds before if the conditions are right. This makes
dahlia a great choice for long-lasting summer through fall
color. If you dig sweet potatoes too early in the season, there
will be few if any of the swollen roots that we eat since the
plants are still actively making more leaves and shoots.
The mums and poinsettias that we purchase at garden centers
and greenhouses in the late summer and fall have been
manipulated during production to bloom late enough for a
great fall and winter display. Mum manipulation consists
chiefly of pinching the bloom buds back in summer making
the plants not only much thicker and more compact, but
delaying flowering. Many varieties of fall mums (there are
exceptions) will grow very tall and bloom in late June or early
July if planted in your garden and allowed to grow naturally
without pinching. In the same manner many varieties of
poinsettias are also day length manipulated through the use of
blackout curtains.
How do deciduous trees such as Red Maples know when to put
on their annual color show? As the days get shorter in the fall,
these plants begin to shut down the cellular chemistry that
creates the pigment chlorophyll, the green color we see all
season. As the chlorophyll breaks down, we can now see the
other color pigments that have been there all along, but were
masked by the overwhelming amount of green. This same cue
tells the maples to create the zipper or abscission layer that
eventually lets leaves fall from the trees.
So far, I've used the expression short and long day, this is an
easier way to talk about this subject, but is inaccurate. Plants
really tell time by how long the nights are by the reaction of
the red pigment phytochrome in their cells to the dark.
Day-neutral plants flower when they have grown large enough
to flower.
Why do trees near street lights still change color and lose
their leaves? Most street lights are of the wrong color
spectrum, direct their light downward, and are too weak to
affect trees. Drought and other stresses on plants can affect
when they begin going dormant.
Part 2: Temperature Reactions
In order for plants to flower at about the same time every year,
go dormant for winter and resume growth in the spring, it is
vital for a plant to “know” the time of year. Without this
awareness, plants could get caught fully leafed out, actively
growing and too tender to survive winter cold. As the days get
shorter in the fall, plants get their earliest cue from the change
in lighting that they need to begin winter preparations (part 1
explains this process in depth). Changes in temperature are the
other part of the process that plants use to mark the time of
year.
Steadily dropping temperatures during the fall into early
winter helps plants acclimate to the coming cold weather.
Before woody plants can go dormant their tissues harden as
they acclimate to the changes in conditions. In the hardening
process: buds dessicate partially by dropping from 80-90%
water to approximately 40% and sugars increase and soluble
proteins increase in plant storage tissues.Both fertilization and
pruning before plants are completely dormant will delay this
process since both practices stimulate new, tender, growth,
thus can cause winter damage.
Once a plant goes dormant, the buds will not respond to
mid-winter warm spells until they accumulate enough chilling
hours. Chilling hours are typically described as a range of
hours below a specific temperature. Most researchers agree to
a model that measures hours below 45°F, but above 32°F. If
buds do not receive enough chilling hours during winter, the
plant may experience one or more symptoms: 1) delayed leaf
growth, 2) reduced fruit set, and 3) reduced fruit quality.
Some common plants in our area that provide lessons in
chilling include: Saucer magnolias, Star magnolias, and
raspberries. The magnolias are at the very northern edge of
their hardiness zone in our area. They require very little
chilling, so break dormancy quite easily in even a short winter
warm spell. It is not unusual to see these magnolias in bloom
during February only to the see the flower petals, brown from
cold damage on the ground several days later. Raspberries, on
the other hand, are very well-suited to our highly variable
winter weather. They have chilling requirements of over 1,000
hours, so the buds stay dormant well into spring, thus sparing
raspberries the likelihood of cold damaged flowers from a few
warm days.
Many perennials require specific periods of cold in order to
flower the next season. This process is known as vernalization
and as in light response, varies considerably among plants.
This cold treatment generally requires weeks of temperatures
below 50°F. Some plants require no cold period and will
simply flower when the plant has grown enough or in response
to daylength. Others flower without chilling, but will flower
faster with more blossoms with enough cold. Yet another
group of plants require a specific period of chilling to produce
any flowers. Some of this group will flower immediately after
reaching the necessary cold period, while others will not
flower until the days are long enough.
Page 2
Examples of plants that will flower after vernalization, but are
not day length dependant include: False spirea, Columbine,
Coral Bells, Sea pink & Bugleweed. Once chilled, these plants
will flower as soon as it is warm enough and they have enough
vegetation to support flowering. Bulbs such as paperwhite
narcissus, freesia, and amaryllis are native to areas where
winter is warm so no chilling is required for flowering. Tulips,
Daffodils, Hyacinths, Crocus, Dutch iris and Scilla all require
vernalization before they can bloom. As soon as they receive
enough cold, they can begin growth and bloom. These bulbs
respond well to 'forced' blooming indoors by keeping them at
35°F to 45°F for 12 weeks or more, then planting them and
bringing them to room temperature so they can begin growing.
It will take at least 4 weeks for these 'forced' bulbs to flower.
The hardneck garlic that we commonly grow in PA is another
study in vernalization. Planting garlic outdoors on or about
October 15 will produce excellent quality bulbs by early to
mid-July. A single garlic clove planted in the fall will produce
a 6-8 (or more) clove bulb depending on type and variety in
the early summer. If on the other hand, you plant the same
clove in March or April, it will produce tasty greens, but will
not flower or produce a typical garlic bulb. Most softneck type
garlics are less sensitive to the chilling requirement as they are
more typically grown in warmer areas. There are garlic
varieties that can be grown in nearly every climate.
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.
For more detailed information on the acclimation process in
woody plants, read the North Carolina State University
publication “ Preparing Nursery Plants for Winter ”.
by Steve Bogash, former extension educator
Penn State College of Agricultural Sciences research and extension programs
are funded in part by Pennsylvania counties, the Commonwealth of
Pennsylvania, and the U.S. Department of Agriculture.
Where trade names appear, no discrimination is intended, and no endorsement
by Penn State Extension is implied.
This publication is available in alternative media on request.
Penn State is an equal opportunity, affirmative action employer, and is
committed to providing employment opportunities to all qualified applicants
without regard to race, color, religion, age, sex, sexual orientation, gender
identity, national origin, disability or protected veteran status.
© The Pennsylvania State University 2017
Timepieces in our Plants