Getting the most from your solar greenhouse. In winter, every inch of growing space is precious.
The Solar Greenhouse
Remember how much you longed for your first bicycle–or car? That’s how much I wanted a solar greenhouse! Many a
winter I raised seedlings under fluorescent lights, always
wishing the abundant sunlight of our south-facing land
could reach my starts.
Well, dreaming, planning, and saving finally led to
building. And now there is a modest solar greenhouse
attached to our small Pennsylvania home. But, as always
happens when I get something I’ve longed for, the realized
dream brought with it a challenge: learning to use my
greenhouse well. So, over the past few years, I’ve studied,
experimented, and observed carefully, and–even though
I still consider myself a beginning solar
gardener–I’ve learned a lot I’d like to share.
A Room of Many Uses
Solar greenhouses can be much more than a place to start
spring seedlings. Mine provides such delights as crisp
cucumbers in December, green chard in January, and
succulent Chinese cabbage in February. It also
• protects tender herbs and geraniums over
• dries mint and such in summer,
• cures harvested onions and garlic,
• yields a fall harvest of potted peppers, tomatoes,
eggplants, cucumbers, and such, and
• easily roots cuttings of grapes, geraniums, and
herbs (I use the loose, moist soil of my large growing bed
Whatever design you choose for your greenhouse, I heartily
recommend that it include a large growing bed. The soil
will serve as an excellent, moisture-retaining plant home
and will provide mass for absorbing and holding the sun’s
When we built our greenhouse, we left the growing bed’s
foundation open for drainage. We shoveled 3 inch of gravel into
the bed, and we topped that with a layer of 4 inch-thick
“books” peeled from bales of spoiled hay. Next came 4 inches of
decomposing wood chips, 4 inches of compost, and 8 inches of loose
garden soil mixed with several large bagfuls of vermiculite
and commercial potting soil.
Each year we add another 2 inch to 3 inch layer of compost and
about three quarts of wood ashes to the bed. We dig in
clean, fine-textured kitchen scraps such as coffee and tea
grounds . . . and toss in a few earthworms from time to
time to help digest the scraps, aerate the bed, and add
While the design of a greenhouse sets the limits upon its
potential efficiency, you can juggle four other factors to
maximize output: light, temperature, humidity, and carbon
Solar Greenhouse Light
Most vegetables need at least eight hours of light a day to
thrive. Light-starved plants will show one or more of the
following symptoms: elongated, frail stems; sparse leaves;
slow growth; excess leaning toward the light.
To raise light levels, first check outside to be sure no
trees, woodpiles, or leaning snow shovels shade the
greenhouse. Next, paint all interior walls and shelves
white. (Use exterior-grade paint to withstand the high
humidity.) If you use heat-storing barrels that face the
sun on one side and plants on the other, slap some
reflective white paint on the sides toward the plants (but
make the sunny sides heat-absorbing black).
Shane Smith, whose The Bountiful Solar Greenhouse
has been one of my most helpful guides, suggests that when
light levels are low, it’s important to keep the greenhouse
as warm as you can and to avoid overcrowding the plants.
Solar Greenhouse Temperature
Many people, when raising the kind of cool-weather crops
best suited to winter greenhouse gardening, find that
overheating causes more problems than cold
temperatures (unless some dummy forgets, as I once did, to
latch the door; that was the end of my tomato plants!).
Greenhouse temperatures can hit the 90s when sunlight is
intense. A little overheating won’t hurt most crops, but
more than four to six hours of sweltering can make
cool-weather crops like Chinese cabbage tough and bitter.
To moderate swings from cool to hot, add more thermal mass:
another water-filled barrel, a brick retaining wall, or
some water-filled shelf jugs. You can also vent your
structure whenever it gets too warm.
Of course, don’t let your greenhouse get too cold, either.
But remember that soil temperature is more critical than
air temperature. (Use a soil thermometer–filled with
alcohol, not mercury, in case of breakage–to monitor
your bed’s temperature.) Chilly night air doesn’t seem to
hurt plants–as long as they don’t freeze. But roots
have trouble absorbing water and nutrients when the soil
temperature drops below 45 degrees. That’s another advantage
of a deep bed: It holds warmth better than do shallow beds
or pots. Watering with warm water can also help raise soil
Solar Greenhouse Humidity
Plants grow best in moderately moist air–45 to 60%
relative humidity. Most do fine at 7%, too. But molds and
diseases thrive as humidity rises toward 90%, and will
slow, often stop, plant growth.
So don’t overwater (a common mistake). Most plants grow
slowly in winter and need only weekly waterings. Here
again, a humus-filled deep bed will serve you well: It
holds moisture and doles it out gradually.
If you do have humidity troubles, open a
greenhouse-to-house vent so the excess moisture can escape
into the drier house air.
Solar Greenhouse CO2
Light, temperature, and humidity problems are easy to spot,
and you can measure levels with common instruments. You
won’t, however, find a CO2 meter at your local hardware
store; indeed, you’re not likely to even notice specific
symptoms of a carbon dioxide shortage. It’s safe to assume
that just about any tightly closed greenhouse will
be short on CO2.
And that deficiency will slash output, because CO2 is a
plant’s only source of the element carbon, which makes up
half of its dry weight. CO2 is essential to
photosynthesis, the process by which plants make new
tissue. In winter, an abundance of CO2 can actually keep
plants growing when they get less than ideal doses of
Unfortunately, plants in a well-sealed greenhouse may use
up all the available carbon dioxide by early morning!
(According to researchers, a thin layer of CO2 -deficient
air hovers around the vegetation in an unvented
greenhouse.) To get more CO2 to your plants, vent the
greenhouse to break up stale air pockets and to let in a
fresh supply of CO2.
If the weather’s too cold for continuous venting, build a
fresh compost pile under a bench. It will eventually give
off between one-third and one-half of its dry weight in CO2. You might also run a fan.
Crops: A Full House
Greenhouse space is precious, so we cram as much as we can
into our growing bed. For instance, in late winter (see the greenhouse diagram in the image gallery), it yields radishes, parsley (or green
onion tops), Bibb lettuce, Chinese cabbage (spoon and
mizuna), and Swiss chard. Flowering tomato plants and eager
new cucumber vines promise May salad treats. Along the damp
edges of the bed, geranium, grapevine, forsythia, pussy
willow, and herb cuttings supply gift plants for visiting
friends. And a row of calendulas adds color.
Before long, plants carpet the room. There I stand,
juggling a flat of tomato seedlings in one hand and a pot
of petunias in the other, wondering where to put them.
That’s when I appreciate the area above the growing bed,
the high back shelves, even the windowsills. A determined
gardener makes creative use of every cranny.
Take that petunia–or New Zealand spinach, or sweet
potato, or one of a dozen other plants whose natural habit
is trailing or vining. Perch it on a shelf so the leaves
and flowers can cascade down into the light. What about
those tomatoes? If they’re a bush variety like Pixie Hybrid
or Basket King (Geo. W. Park Seed Co., Greenwood,
SC), they’ll do well in hanging baskets.
A trellis–or other support of string, poles, netting,
or fencing–also makes good use of vertical space. Our
cucumbers climb a fence on the front window framing,
leaving space for a small plantation of Bibb lettuce
Care and Feeding of Crops
When planting seeds in the greenhouse, I use plain
vermiculite or a loose, fluffy potting soil mix I buy in
bulk from a local greenhouse.
Seeds don’t need food–just moisture, warmth, and air.
Seedlings, though, need something to grow on. So I line
their flats with a layer of moss, some fine compost, then
potting soil. (I use a store-bought mix. For a good
homemade mix, combine equal parts of leaf mold, sphagnum or
peat moss, and perlite or sharp sand. Equal amounts of
compost, good loamy soil, and sharp sand, perlite, or
vermiculite also make a good mix.) I transplant seedlings
to flats when they develop their first true leaves, then
water them with a diluted solution of kelp (about two
tablespoons per gallon of water). This nourishment should
last at least two weeks. After that, I feed the
seedlings–and all my crops–every 10 to 14 days
with fish emulsion or compost tea.
During the worst of winter, when the soil is cold and the
light level low, plants don’t need much food, so I switch
to half-strength feedings. (Overfeeding crops, especially
with nitrogen-rich fertilizers, results in sappy growth
that attracts insects and invites disease.) As days
lengthen and the sun gets more assertive, I increase the
strength and frequency of feedings, especially for leaf
crops and my greedy but prolific cucumbers. (I don’t
fertilize overwintered herbs at all; I don’t want them to
outgrow their pots before spring.)
My Favorite Winter Vegetables
Although it may feel balmy on a sunny day, a solar
greenhouse is basically a cool greenhouse, so plan on
raising only vegetables that can take a night chill. And
remember that, under glass, leaf crops produce better than
Timing is also important. I start my fall greenhouse crops
in August or September so they’ll develop before the earth
turns away from the sun. Seedlings planted between October
and January–when sunlight is weakest–may need
twice as many days to produce. In spring, the growth rate
speeds up. Late winter seedlings may mark time during the
low days–January 1 to February 15 here in central
Pennsylvania. But once willow twigs turn yellow and snow
starts to melt, those starts will take off.
I’ve worked up a list of my favorite solar greenhouse
vegetables, in order of personal preference. My choices are
based partly on results and partly on out-of-season
desirability–fresh cucumbers and greens are hard to
come by in January!
Cucumbers are a zinger crop. They taste
great and make you feel pampered when trees are bare. The
new European types produce all-female, self-pollinating
blossoms, and their greenhouse-grown fruits taste much
better than garden cucumbers. Seeds are costly, but a
packet can last for two or three plantings. Dynasty (from
Otis Twilley Seed Co., Trevose, PA) and
Superator (Stokes Seeds, Fredonia, NY)
are two good varieties.
Greenhouse cukes like the good life–warm soil, weekly
feedings, and frequent warm waterings (every other day when
fruiting). They prefer soil temperatures above 55 degrees or
60 degrees Fahrenheit, but can tolerate cool greenhouse nights
if they get lots of light and warmth during the
I grow two indoor cucumber crops a year, planting in August
or September for a November-December harvest, and again in
February for May-June eating. Overproduction weakens the
plants and cuts fruit size, so Colleen Armstrong (staff
horticulturist at the New Alchemy Institute) suggests
removing every sucker from the six lowest leaf joints,
cutting the next eight shoots back to a single leaf, and
letting the rest develop two or three leaves.
Swiss chard has been my most reliable and
long-lasting leafy green. (The deep-rooted plant can
survive over a week without water in my deep bed.) Low in
calories and high in vitamins, chard tastes much milder
when grown indoors. I plant seeds in August or September,
start picking the succulent, seersuckered leaves in
December, and keep harvesting until outdoor greens take
When cloudy midwinter days make me keep the greenhouse
closed up, a plague of aphids or whiteflies sometimes hits
my chard. But once I start opening vents regularly again,
the bug pox clears, and the leaves become clean and green.
Chinese cabbage and other
Orientalgreens get two
plantings–fall and winter-like cucumbers. Loose-leaf
varieties do better than heading ones. I’ve had good luck
with Prize Choy (Johnny’s Selected Seeds, Albion,
ME), Shanghai Pak Choi, and Spoon Cabbage (both from
Kitazwa Seed Co., Santa Clara, CA).
Another Oriental green, kyona (also called mizuna, and also
from Kitazwa Seed Co.), provides many cuttings of tender,
mild-flavored, fringed leaves.
Bibb lettuce, with its delicate buttery
soft heads, is perfect for the greenhouse. I also like
Magnet, a forcing butterhead (The Cook’s Garden, Londonderry, VT) and Mescher, a wonderful heirloom
butterhead I got from a fellow member of the Seed Savers
Exchange. Lettuce likes rich soil, lots of moisture, and
good drainage. For the best harvest, plant the fall crop
early and replant often. (Don’t count on head lettuce in a
greenhouse; it takes too long to develop.)
Hothouse radishes are fun to grow and have
an unusually mild flavor. You can easily sneak them into a
pot or along the edge of a bed. They form roots best in the
lengthening days of spring. Plentiful moisture makes them
crisp and mild.
New Zealand spinach surprises everyone who
tries it with its luxuriant growth. A good space-saving
plant, it can either grow in a tub on a shelf–vines
spilling down into the sun–or climb a trellis or
string. Start picking leaves two months after you plant the
seed (which will germinate better if you file a little
notch in its tough coat, then soak it overnight).
Onions won’t bulb up in the depths of
winter, but will yield a modest supply of savory green
spears. Chives make a good greenhouse crop, too. (Let
perennials freeze outdoors for two weeks before you bring
them into the greenhouse. They need a rest after producing
Tomatoes don’t always earn their space,
but, ah, the satisfaction of raising even a few
real tomatoes in the off season! Small-fruited
kinds like Presto, Pixie Hybrid, and Sweet 100 (Geo. W.
Park Seed Co.) will bear more and ripen
faster than full-sized varieties. If you insist on large
fruit, Vendor (Southern Exposure Seed Exchange, North Garden, VA) and Park’s
Greenhouse Hybrid are good to try.
Start with potted tomatoes in the fall, either summer
seedlings or rooted cuttings from established plants. They
should start to bear by New Year’s. You can also bring
potted adult plants inside for a few extra weeks of fall
fruits. Then start more seeds in December for a May
harvest. I often use cold-tolerant Siberia and Santa
(Siberia Seeds, Sweetgrass, MT) varieties
for spring plantings. Wind normally helps tomatoes
self-pollinate, but with few breezes indoors, jiggle
blossoming plants several times a week to disperse pollen.
Greenhouse peppers tend to be small but
are nonetheless welcome salad material when the outdoor
garden is frozen. I often use extra starts from the
previous spring–kept in two-gallon pots on our sunny
patio over summer–for fall indoor plants. You can
also sow seeds in May, grow them in pots until August, then
transplant them to the greenhouse growing bed. Perennials
in their native Tropics, peppers will keep leafing and
bearing as long as they’re healthy. Whitefly and aphid
infestations, though, often result in disease.
Both beets and turnips grow well in
greenhouses. You can plant them in fall, winter, or
spring–directly in the growing bed or in 8 inch-deep pots
or tubs. Eat them while the roots are small, and cook the
Green beans make a fun spring crop:
They’re self-pollinating and like warm soil. Sow them in
early March and harvest in June. You can even plant
climbing beans in summer to shade the greenhouse or as an
off-season green manure crop to add nitrogen to the soil.
Grow to It
I think of my solar greenhouse as a little resort: I go
there whenever I need a renewing breath of balmy, leaf- and
earth-scented air or a few relaxing minutes of pruning and
transplanting. But I also want to raise food and flowers,
start seedlings, dry herbs, cure onions, and propagate
cuttings. Getting the most from a greenhouse–bounty
as well as peacefulness–takes both time and practice,
but the results are worth the effort.
Nancy Bubel is a long-time garden and country-living writer
and the author of The Seed Starter’s Handbook and–with her husband, Mike–Root Cellaring.
They wrote our root-cellaring handbook in MOTHER EARTH NEWS NO. 95.
Greenhouse Supplies and Books
• Charley’s Greenhouse Supplies, Mt. Vernon, WA. Catalog $2.
• Gro-Tek Greenhouse Supplies, South
Berwick, ME. Catalog free.
• Necessary Trading Co., New Castle, VA. Catalog free.
• Abraham, George and Kay, Organic Gardening Under
Glass, $11.95 postpaid, Simon and Schuster Inc., Old Tappan, NJ.
• Smith, Shane, The Bountiful Solar
Greenhouse, $9.75 postpaid, John Muir Publications,
Inc., Santa Fe, NM.
Design and Construction Rules of Thumb
Dealing With Greenhouse Crop Disease
You can prevent greenhouse diseases more easily than you
can cure them. The following prevention tips are important.
In fact, you may want to print them up on a little poster,
especially if several people use or visit your greenhouse.
1. Do all you can to raise healthy plants.
2. Grow disease-resistant varieties.
3. Avoid overcrowding plants.
4. Keep the space well-ventilated. Venting dissipates
excess heat, lets in carbon dioxide, and sets up a lively
pattern of circulating air that discourages fungus disease
5. Never smoke in the greenhouse, or handle tobacco-family
plants–like peppers or tomatoes–after you’ve
been smoking. Tobacco mosaic virus is highly
6. Don’t use old potting soil for new seedlings–it
may bring in damping-off disease.
7. Remove and compost (or burn) all dead leaves and