Here at Gardening Know How we get
lots of questions, and this includes greenhouses.
Our goal is to provide answers to these inquiries to the best of our knowledge.
The following information includes the 10 most commonly asked questions relating
Surprisingly, even in a greenhouse environment, insect pests
are inevitable. These greenhouse
pests may range from aphids and mealybugs to mites, whiteflies and
more. One of the easiest ways to monitor them is by using sticky pads, which
keep many insects under control or at least signal you when their numbers are
growing for further control. The use of insecticidal soap and neem oil are
additional methods of control and normally safe to use on edibles.
Most plants prefer humidity between 50 and 80 percent.
Foliage loses water and wilts rapidly when levels are below 50 percent, but
high humidity can cause fungal diseases such as damping
off, which causes tiny stems to rot at the base. If high humidity is
a problem, you can provide a drier environment by watering plants from the
bottom, and by providing adequate heat
If humidity is too low, improve conditions by misting plants, with cool mist
vaporizers or small humidifiers, or by placing pots on trays of wet pebbles.
often pop up in potting mix, which indicates the substance is high
in organic material. Although the fungi are harmless, they tend to be
unsightly. If you notice tiny mushrooms, pull them up by their base and discard
them to prevent development of spores. Water plants from the bottom and be
careful not to water excessively. Provide proper ventilation to keep the air
moving. If air circulation is limited, a small rotating fan will help.
stuff on the surface of potting mix is probably algae or moss. While
the substance is generally harmless, a thick layer can prevent proper
absorption of moisture and nutrients. Often, you can remove the green by
breaking up the surface of the soil with a pencil or toothpick. Always start
seeds in fresh, good quality seed starting mix. Allow the surface of the
potting mix to dry slightly, and don’t water to the point of sogginess. Be sure
your greenhouse is properly ventilated to prevent excess humidity.
A greenhouse provides sunlight that warms the soil and
protects plants from frost. However, an air gap is essential, as too much heat
in an airtight greenhouse can quickly cook tender greenhouse plants. Be sure to
close vents at the end of the day when cool temperatures are expected, then
open them in the morning while the air is still cool. Automatic vent systems
are available. The same applies when using
cloches in the garden, either by tilting it to allow for some air to
enter or removing altogether when warmer day temps are expected. Just remember
to reposition the cloche in the evening.
Greenhouse experts recommend that exterior shade is more
effective than interior screen systems because light is blocked before it
enters the greenhouse. Once light is trapped inside, it becomes difficult to
remove the excess heat. However, if you want to install interior shade
cloth, it’s best to invest in a retractable system that allows
sunlight to enter on cloudy days. Mechanized systems, consisting of support
cables, motor and shade, can be opened and closed as needed. You can also
install a less expensive indoor system consisting of ropes and pulleys.
If you live in a cool northern climate, it will be difficult
herbs in a greenhouse. Most herbs will struggle when temps drop below
40 to 50 F. (4.5-10 C.), while many prefer consistently warm temperatures of 65
to 70 F. (18-21 C.) during the day and 55 to 60 F. (13-16 F.) at night. You
can, however, get a jump on spring growing by starting many herb seeds in an
unheated greenhouse, but you may need a heat mat for germination. Herbs that
tend to be cold hardy include lavender, garlic, rosemary, winter savory, catnip
If you want to grow tropical plants, you’ll need a
greenhouse capable of maintaining steady temperatures of 65 to 70 F. (18-21 C.)
or higher. Most flowers, vegetables and herbs need temps of around 55 F. (13
C.), while a “cool” greenhouse is suitable for germinating seeds and raising
starter plants in spring. A greenhouse with clear panels provides warmth and
light for sprouting seeds, while a diffused or glazed covering provides
balanced levels of light heat for growing plants to maturity. Additionally, look
for a greenhouse
that is adequately ventilated and insulated to prevent plants from excess heat
cloth, either internal or external, will help block the intense rays
of summer sunlight. Make sure shade is in place before the heat of the day,
then remove the cloth on cooler days. Ventilation,
in the form of roof or side vents, keeps air moving and prevents buildup of
heat. If ventilation is limited, you may need to run a fan (or two) to
circulate the air. Be sure to
keep greenhouse plants well watered during hot weather. A light
misting can cool plants on hot days, but mist judiciously, as too much moisture
can promote disease.
can grow nearly any plant in a greenhouse, but it’s all about
determining the needs of particular plants. For instance, you can germinate
seeds and grow starter plants in an unheated greenhouse, while tropical plants
need consistently warm temperatures year round. You can easily start most
flower seeds and cool season vegetables such as peas, broccoli and lettuce in
an unheated greenhouse, but tomatoes and peppers need plenty of sunlight and
warm temperatures between 55 and 85 F. (12-29 C.). Some flowers, such as impatiens and ferns,
thrive in shade, while geraniums and daisies need bright sunlight.
We all have questions now and then, whether long-time
gardeners or those just starting out. So if you have a gardening question, get a
gardening answer. We’re always here to help.