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| California wild rose (Rosa californica) has lovely scented flowers |
There’s
something magical about a scented plant.
The heady fragrance of a wild rose, the musky-sweet scent of four-o-clocks
at dusk; or the spicy aroma of a native sage on a warm summer day. The list goes on and on. Californians are blessed with many native scented plants; these
make our gardens both unique and interesting.
But the scented plants may also be good for our health!
Plants
produce fragrant chemicals for a variety of reasons (most having little to do
with human beings). The sweet fragrances
of flowers are released to attract
pollinators. Some are quite
specific – like a perfume designed to attract the right admirer. Others attract all capable of detecting their
sweet scent (native bees, butterflies, moths - even humans).
Likewise,
the sweet aromas of ripe fruit attract creatures that can disperse a plant’s
seeds. Unripe fruits are usually
unscented – or have a repellant odor.
But ripe fruits can be irresistible, due in part to their attractive
fragrance. Plants have methods of
seducing animals to do their bidding, and fragrance is a key weapon in their
arsenal.
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| The scent of native Salvias is related to protection. |
The pungent and minty scents produced by some plant’s foliage have a different function: protection. Dry mediterranean climates support a number of plants with fragrant foliage. These include some of our favorite cooking and potpourri herbs: rosemary, sage, thyme, lavender, sagebrush and the mints. To the human nose, these scents are interesting, exciting and attractive. That’s why we use them extensively in foods, perfumes and other products.
But
many of the fragrant herbs have a repellant odor to herbivores. From larger herbivores like deer and rabbits
to the herbivorous insects, the pungent scents provide a cue that this plant is
‘not fit to eat’. In some cases, the
plant just tastes bad; in others, the plants are actually toxic to the
herbivores. It’s not surprising that scented foliage
releases its flavors when crushed. That’s
another example of plant thrift – using expensive chemical resources only when
needed.
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| Wooly bluecurls (Trichostema lanatum) has a complex aroma that's hard to describe |
But what actually produces the scents? Whether released automatically or when crushed, plant scents are due to small, aroma chemicals (odorants) that interact with the sense organs of an animal. Because they are carried by the air, most aroma chemicals are relatively small. And the plant-produced odorants have a limited number of basic structures (most are esters, turpenes, aromatics and amines). But the variants of these basic structures – and their combinations in individual plant species – can produce aromas as varied as the sweet scent of a rose or the putrid odor of the Voodoo lily (smells like rotting meat – attractive only to fly pollinators).
Aroma
chemicals interact directly with an animal’s olfactory receptors. In vertebrates, these receptors are located in
the nasal passages (the human nose and throat); in insects, the receptors may
be located in antennae or other sense organs. But in all cases, odorants bind
to and activate the olfactory receptors, resulting in a signal being sent to
the brain via olfactory neurons (see below; ref 1).
And
here’s where things get interesting. In
humans, the olfactory neurons (nerve cells) converge at the olfactory bulb, an area of the brain
rich in interconnecting neurons. Signals
from the olfactory bulb are sent to at least five different areas of the
brain. Some of these areas are responsible for
decoding the scent: does it smell like any other scent? Is it a known scent? But
there are also close, direct connections between the olfactory bulb and brain
structures important for memories and emotions: what is often called the limbic system.
The
limbic system is a series of evolutionarily old structures located deep within
the human brain. The limbic system supports a variety of functions including
emotion, behavior, motivation, long-term memory and olfaction. [2] It is
connected to the more modern, cerebral areas of the brain via the hippocampus.
There’s still much to learn about how the limbic system works – or even if the
concept of a limbic system is obsolete. But the ‘limbic system’ is involved in
some of the more interesting aspects of human interactions with aroma
chemicals: scent memories and the associations of particular scents with moods
and emotions.
Dr
Bryan Raudenbush, Professor of Psychology at Wheeling Jesuit University, notes:
‘Of our five senses, scent is the only one with a direct pathway to three
important areas of the brain: the orbito-frontal cortex, which presents you with
an awareness of scent, the hippocampus, which is associated with memory, and
the amygdala, which is associated with mood and emotions.’ [3]
Common
wisdom has long associated certain scents with human emotions or physiologic
reactions. For example, the scent of
lavender has long been used for relaxation and inducing sleep. In
aromatherapy, the scent of citrus is thought to be a ‘mood brightener’, along
with that of rose, heliotrope and peppermint.
But is there any scientific evidence for the efficacy of these scents to
alter moods?
Alas,
olfaction is the least-researched of the senses. But there’s good reason to think that scents do
affect our moods and physiology. The
‘limbic system’ can directly activate the hypothalamus, an important hormonal
control center in the brain. The
hypothalamus is responsible for the production of growth hormones, sex
hormones, thyroid hormones, and neurotransmitters such as serotonin. These
hormones affect energy levels and many other physiologic responses. So it’s reasonable to hypothesize a
physiologic role for hormones in the human response to scents. And scientific studies are beginning to provide
the evidence. [4]
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| Each Salvia has a slightly different scent |
But there’s even more theoretical reason to suspect a direct role for scent and mood. When we smell something nice, receptors in the amygdala release dopamine and serotonin. [3, 4] These are powerful neurotransmitters; the former is linked with the high you feel when taking drugs such as cocoaine, the latter with mood. ‘In a nutshell,’ says Dr Raudenbush, ‘You smell something ‘good’, and those scent chemicals stimulate the amygdala, which in turn prompts the release of dopamine and serotonin, and we feel ‘good’.’ [3]
So
scents may directly modulate our moods, via the effects of neurotransmitter and/or
hormone levels. But the picture is
further complicated by the close relationship between the olfactory neurons and
the hippocampus, which is associated with memory. This part of the brain helps us ‘remember’ a
scent – and its likely consequences (for example: ‘this is the scent of White
sage’; or ‘smoke = fire’).
The
ability to identify scents correctly was key to survival in our evolutionary
past. It helped us find food and avoid
harm. Pleasant plant scents may also
have played a role in the human-plant interactions leading ultimately to
agriculture. It should come as no
surprise that a good sense of smell developed early in our ancestral past. And that scent memories are different from
other types of memories.
Scent
memories are among the most vivid, and are often associated with a person or
place. We’ve all smelled a particular brand
of perfume and been instantly reminded of a person who used to wear it. Or smelled a scent and been transported back
to the chaparral or woods where we played as children. Such scent memories are often vivid, specific
and long-lasting (even into advanced old age).
And the most vivid scent
memories seem to be those associated either with novel scents or with intensely
positive or negative feelings coupled with a particular scent. There’s a reason why the scent of pancakes
brings us back to our grandmother’s kitchen!
So,
the effect of specific scents on our moods may in part be due to their
associations with ‘vivid’ people and events – good and bad – in the past. And it’s very difficult to tease out the direct effects of scents on human
physiology from the indirect effects
on moods and emotions tied to scent
memories. In fact, this is an area
of active research and current debate. [see ref. 5 & 6, below, for more]
But
it really doesn’t matter how scents affect our moods. In fact, one way or another, they do. The
makers of perfumes and the aromatherapy industry have known this all
along. There’s a reason that scented
plants have been used to enhance bodies and abodes, in most cultures, for as far
back as we know. Plant fragrances can be used to disguise body odors and
other unpleasant smells. But fragrance
also plays a role in human moods, emotions and even social interactions. The fact that science is beginning to back up
these claims is just icing on the cake.
So
how do we apply all this to our gardens?
First, we should remember that the most
interest gardens are those that engage all of our five senses. Most of us think a lot about color and form –
the visual elements - when designing our gardens. And gardens that sooth and heal should be
pleasant to the eye. But the sense of
smell is at least as important. So we should think about scent when choosing plants
for our gardens.
Each
person experiences a given scent uniquely.
This is due both to physiologic differences and differences in scent
memories. When choosing scent plants,
choose ones that make you feel good.
Visit a native plant nursery and ‘try out’ the different fragrances
of the native sages (Salvias). Each cultivar and species is a bit different,
so be sure to plant one you like. Visit
Mother Nature’s Backyard. We have a
number of scented plants, and encourage you to experience them fully. For a list of fragrant native flowers see: https://www.slideshare.net/cvadheim/california-plants-with-fragrant-flowers.
For a list of scented native foliage
plants: https://www.slideshare.net/cvadheim/califoria-plants-with-fragrant-foliage.
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| Mother Nature's Backyard & Garden of Health: a great place to 'try out' plant scents |
Place
scented plants where you can best enjoy them: near a window, along a path or near
an outdoor seating area. Grow scented herbs in your kitchen garden, in
pots on your porch or near the back door.
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| Fragrant pitchersage (Lepechinia fragrans) has fragrant foliage |
Take
time to really enjoy the fragrances in your garden.
Remember to ‘stop and smell the roses’. Leave your electronic
devices inside, close your eyes, and truly experience the fragrance of a
rose. Crush the leaves of a mint, inhale
its aroma and be aware of its effect on your breathing, heart rate and
mood. Do you feel relaxed? Invigorated? Does your mind feel clearer?
Share
your scent garden with others. Invite your friends to enjoy your
garden. The scents will enhance your
interactions like nothing else. Dry
your scented flowers and foliage for use in cooking and potpourri. Be sure to share some of your scented wealth. For more ideas see: http://mother-natures-backyard.blogspot.com/2016/04/california-gourmet-making-flavored.html
and http://mother-natures-backyard.blogspot.com/2013/11/garden-crafts-making-potpourri-from.html.
These
are challenging times! It’s no wonder
we’re feeling a little stressed! In
fact, we’re in serious need of stress relievers – and the scents of our gardens
can play an important role. So resolve
to add some scented plants to your garden this year. You’ll be amazed at the
results!
For more
posts in this series see : http://mother-natures-backyard.blogspot.com/2018/01/gardening-for-health-1-go-hug-tree-for.html
For several
talks on the use of native scented plants, see:
- http://www.slideshare.net/cvadheim/extraordinary-scents-2012
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We
welcome your comments (below). You can
also send your questions to: mothernaturesbackyard10@gmail.com
