In this week’s podcast, I’m talking hobby greenhouses with Sheri George. A lifetime master gardener, Sheri has been greenhouse gardening for over 20 years. Her experience has put her in a unique position on what to know before you buy (and after you do) when it comes to setup and growing conditions within the space that many gardeners (including me) are dreaming of – the greenhouse.
I first visited Sheri’s Atlanta, Georgia-area greenhouse in 2011 to film for an episode of my PBS series, Growing a Greener World®. Although she’s since upgraded her space (more on that in a minute), you can meet Sheri and tour the greenhouse she enjoyed for years.
Sheri began her foray into greenhouse gardening while living in Colorado. Having just moved from Texas, her outdoor garden was struggling in the high winds and extreme temperatures of the high desert; so Sheri’s husband, Lee, surprised her with a greenhouse kit as a Christmas gift. The greenhouse provided a controlled environment where Sheri was able to grow tomatoes all year, along with many other vegetables and aesthetic plants which struggled in the harsh Colorado conditions.
That said, things weren’t always so rosy within the greenhouse either. There was a steep learning curve. Temperatures within the small space could swing from 32 degrees F in the morning to 110 degrees F in the afternoon.
The purpose of a greenhouse is to magnify the light of the sun while protecting plants from freezing temperatures. Capturing the sun’s rays and trapping the resulting heat can quickly create an oven, so the greenhouse gardener must learn how to prevent temperature extremes – cooling the space down or keeping it warm as necessary for plant health.
The small, enclosed space of a greenhouse provides a sealed environment to protect plants from the pests and diseases which may ravage your outdoor garden, however once those issues make their way inside, they can spread quickly and be difficult to eradicate.
For all those reasons, gardening in a greenhouse is a time investment. Constantly monitoring the temperature, moisture levels and overall health of the plants you grow there is a necessity for success. Yet, having a greenhouse is on the wish list of many gardeners, and it can be incredibly rewarding once you learn the ropes.
Before you ever begin to plan to add a greenhouse structure to your property, it’s key to remember that it is – in fact – a structure. For that reason, there may be restrictions which prohibit or dictate where and how a greenhouse can be built.
If you live in an area with a Homeowners’ Association, check your covenants. Many HOAs don’t allow a greenhouse – even in a backyard. There may also be city or county ordinances which dictate where the structure can be placed on your property, what infrastructure – such as electricity – is required, and which permits are necessary.
All of that research is the not-so-fun part, but it is a necessary step.
If you are one of the many lucky gardeners allowed to have a greenhouse structure, and you’ve determined the placement and infrastructure requirements – the fun begins: Planning what and how to build your greenhouse.
Size and Structure
How large should your greenhouse be? Well, it’s a good idea to really think ahead to what you plant to grow in the space. If you will use it primarily to grow seedlings, a smaller space may suit you just fine. If you plan to overwinter tropical plants, you will need more space for those mature plants and larger containers.
In either case, most gardeners who have a greenhouse recommend that you determine the space you think you will need – then, build it the next larger size. Once the greenhouse is in place, it will be difficult to upgrade to a larger size. So, it’s a good idea to go with the largest structure your space and budget will allow.
When Sheri first moved to Atlanta, Lee built her an 8’x12’ greenhouse. It’s the space you’ll see in the episode of Growing a Greener World. It worked, but Sheri realized quickly that it was too small. Recently, Lee replaced that greenhouse with a larger 13’x20’ structure. 2019 will be Sheri’s first year in the larger space. She’s loving the extra room, but she knows it won’t take long before it, too, is full of plants.
For each of Sheri’s three greenhouses, her husband purchased kits. There are lots of affordable and well-made kits available, and they make building a greenhouse much easier than if you build from scratch.
Greenhouses built of wood are generally not a good idea – especially if you live in a humid area. Wood tends to warp, and it will eventually degrade. Avoid using wood if you can.
Greenhouses built with plastic PVC tubing are a more durable option. However, even UV-resistant PVC will become brittle and break down in the constant exposure to the elements. On the plus side, plastic is less-expensive and lightweight. If you will need to move your greenhouse, a plastic structure may be a good idea. Just remember that, if your area experiences high wind, you may step outside one day to find your lightweight, plastic greenhouse has toppled over.
A metal structure will be the most durable. Aluminum kits are affordable and commonly-available, and they come in unfinished or powder-coated options. A powder-coated kit will be a little more expensive, but Sheri strongly recommends it. She and Lee originally chose unfinished aluminum. They found that painting it themselves and the frequent upkeep, as paint chipped or wore off, was all more work than they had bargained for. A powder-coated finish will be durable and maintenance-free for years.
Kits built of steel are harder to find and will be the most expensive of the options out there. However, a steel-built greenhouse will be sturdy and durable for a long time.
Most kit suppliers offer choices of glass, single polycarbonate (twinwall) or double polycarbonate (triplewall) pane options.
Glass is risky and not recommended. No matter where you live, hail or other falling or blowing debris can knock out glass panes in an instant.
Sheri recommends triplewall panes. They allow great light to pass through the panel, and they do a better job of retaining heat than twinwall or glass. If you can’t afford triplewall, twinwall can be a good choice, but opt for triplewall if you can swing it.
The surface of the floor within your greenhouse has more impact on your success than you might think. Bare ground is easy, but it also creates a ready path for pests to infiltrate to your plants. As you water, the bare floor gets muddy and uneven, and the bare earth is a potential harbinger of disease pathogens too.
Some gardeners use bark mulch on the surface, but this is another option best avoided. Bark mulch is great in the garden, but in the enclosed greenhouse environment, it often introduces pest issues. Fungal problems also become much more difficult to manage in the humid greenhouse air when bark mulch is present.
Pea gravel is a good and affordable option. It’s comfortable to stand on, and during hot days, you can spray the gravel with water to help cool the space. Critters prefer not to dig in pea gravel, so you’ll also battle fewer pest problems.
Sheri’s personal favorite for surface material is crushed marble. It costs more, but the small pieces mesh together for a smoother surface. Like pea gravel, it will reduce pest and disease risk, but the white marble will also reflect the sun to improve light during the winter months. If you can afford it, look into crushed marble for your greenhouse.
The best spot to place a greenhouse is the sunniest spot. It’s important that the structure (and therefore the plants within it) receive at least six hours of winter sun exposure. Situate the structure with the front or the end facing east. That allows the longer side to receive the full southern exposure of the sun’s rays through the winter. If you can place the greenhouse to receive afternoon shade in summer, all the better.
Regardless of the materials you use, it’s a good idea to mount the greenhouse on a base. It will provide better stability and increase the life of the structure.
No matter the size or materials, whether you’ve used a kit or built it from scratch – there are some non-negotiables for a successful hobby greenhouse. Here are the features that are necessities:
This is the big one. It’s vital to keep the air within the greenhouse moving in order to balance the temperature and prevent fungus and other diseases. In every greenhouse, there are inevitably pockets of hotter and cooler air. Those pockets will impact the health of your plants, but fans will keep all the air moving and eliminate that issue as well.
Depending on the size of your space, you will need two to five fans for proper ventilation. Oscillating fans move air most efficiently, so you can get by with fewer of those.
Sheri recommends a large fan mounted to the greenhouse gable. She adds two oscillating floor fans and has found that combination serves her greenhouse well.
Another key element for ventilation: The ability to open the upper panels to allow hot air to escape. Most kits include automatic ventilation devices. These are filled with wax, and as the temperatures in the greenhouse rise, the wax expands – pushing the panel open and releasing hot air. If you build your own structure, you can also buy these devices to add to your roof panels.
Don’t rely on opening the roof panels manually on warm days. We all get busy and temperatures will rise quickly. Your plants will be quickly decimated the first time you are distracted and forget to open the panels and allow the excess heat to escape.
There are a number of monitors on the market to help you keep a watch on the air temperature within the greenhouse. The best models have an alarm which is wirelessly connected to a second device in your home or to your smartphone or other device. When temperatures rise or fall outside the safe range, the monitor will trigger your device to sound the alarm. So no matter how busy you may be indoors, you’ll be alerted that things outdoors need your immediate attention.
If you plan to grow seedlings in the greenhouse, you will need heat mats. Seedling trays sit on these warming mats to increase the soil temperature. Warm soil is key to seedling health, and even when the air dips as low as 30 degrees F, your seedlings will continue to thrive in warm soil. Just bear in mind that not all heat mats are created equally.
Propagation mats generally do not include a thermostat. These mats will increase the soil temperature approximately ten degrees over the ambient (surrounding air) temperature of the space. The thermostat included on germination mats allows you to set a specific desired soil temperature. You can dial in the temp you want to maintain, and the thermostat will ensure that the soil remains at that temperature – even if the surrounding air dips significantly.
If you plan to overwinter tropical plants or grow crops, like tomatoes, in the greenhouse, heating mats won’t do the job. Those plants will require warmer air to survive and produce fruit, so you will need a heater. Heaters are also available with a thermostat to allow you to set a desired temperature and keep it there, regardless of conditions outside.
Most of these must-haves require electricity in order to run. Whether you wire the greenhouse with outlets (and many government codes will require that you do) or run an extension cord from the house, you will somehow need to supply the structure with electricity.
Recently, new heating products have been introduced to provide solar energy. If you don’t want to bother with running electricity to the greenhouse, these solar options might be just the ticket for you, and I would love to hear about your results. Share your experiences in the Comments section below.
There will likely be times when the intensity of light streaming into the greenhouse will be too intense. It can scorch plant foliage and raise the air temperature to levels beyond what good ventilation can offset. Invest in some shade cloth. It comes in varying densities, is easy to use, and can be a great tool in your arsenal to manage the temperatures within your greenhouse space.
While it’s not a necessity, plumbing a water supply to the greenhouse is very worthwhile. If this isn’t an option, you can haul water from indoors or with a garden hose. Just don’t leave the garden hose outside during freezing temperatures, of course.
The greenhouse, when properly managed, creates an ideal environment for your plants. Unfortunately, it also creates an ideal environment for pests and diseases to rapidly multiply and thrive. It’s relatively easy to keep these issues out of the enclosed space of the greenhouse, but once invited inside, pests and diseases are very difficult to eliminate.
Sheri once broke her own greenhouse rule and agreed to shelter a friend’s sick plant. She quickly regretted that decision. Hidden within the friend’s plant were whiteflies, and soon, every plant in the greenhouse was infested.
One of your most effective proactive steps is to avoid taking in the plants of friends and family. New plants from a nursery should also be closely scrutinized before bringing them into the greenhouse. If you can, isolate any new plant and keep a close eye on it for several days before you add it to the greenhouse. It takes a little extra time, but trust me, it will take much more time to eliminate a problem which could have been avoided.
Use only new pot and soil materials to prevent the spread of any disease pathogens lurking. Don’t be afraid to incorporate used soil into your garden and landscape beds, but stick with fresh soil in the greenhouse. If you do choose to re-use pots or trays, give them a thorough cleaning with a solution of 1 part bleach to 9 parts water first.
A Few More Sage Greenhouse Tips
Whenever you find might yourself with a pest problem, be as careful with pest management in the greenhouse as you are in the garden. For example, yellow sticky traps are a common recommendation for flying greenhouse pests. Unfortunately, those can be lethal to more than just pests.
The adhesive on yellow sticky traps also snags ladybugs, spiders, butterflies, and other beneficials which are good to have in the enclosed space of the greenhouse. When Sheri used these traps to manage her whitefly infestation, she found even a wren had become stuck and unable to free itself.
These traps are yellow because many insects are attracted to that color. A safer alternative, which Sheri has used successfully, is an index card covered in petroleum jelly (aka Vaseline). She buys yellow index cards – or colors white index cards – and covers it with a thin layer of Vaseline. The goo captures the tiny pests, like gnats, but larger creatures, like ladybugs, are able to free themselves.
In her early days of greenhouse gardening in Colorado, Sheri used an ingenious method to help warm the space. She painted barrels black and filled them with water. During the day, the water was warmed by the sun. Then as temperatures dipped overnight, the warmth from the barrels was slowly released into the greenhouse to increase the enclosed air temperature enough that it helped plants make it through to the next day.
If you plan to grow seedlings and your greenhouse receives at least six hours of full sun, you won’t necessarily need to provide the seedlings with supplemental light. Your seedlings will germinate and grow on only the light they receive from the sun. However, it’s important to keep in mind that you will be dependent on the weather. If your area experiences several cloudy days, seedlings won’t receive enough light for their needs. As with all things garden-related, observation and mindfulness is key.
This season, I’ve found my seedlings have thrived under 24 hours of light. Check out a recent episode on the science of light to get a better understanding of why light duration and quality are important.
Some gardeners worry that the air in a greenhouse can become too humid. Don’t let that be a concern. As long as you have good air flow, humidity won’t be an issue. Dry air can be detrimental to plants, but they will love the humid air – as long as that air keeps moving to prevent disease.
Once you do invest in a greenhouse, consider your first year an experiment. The environment in each structure will be different – based on your weather conditions, the placement of your greenhouse, etc. Sheri recommends keeping a journal. She continues to journal her greenhouse activities twenty years on, but it’s especially important during the first year.
Sheri records high and low temperatures, what she did and didn’t do to warm or cool the space, how long it took for her seeds to germinate, etc. In general, journalling is a great way to look back and reflect on what worked and what didn’t, so you can improve on your successes year after year.
So, what’s next in your greenhouse pursuit? As I often say, we learn from each other’s experiences, so I hope you’ll share yours in Comments below.
Check out my conversation with Sheri by scrolling to the top of the page and clicking the Play icon in the green bar just under the page title. She shares more stories from her hobby greenhouse experiences, and you will likely pick up another nugget or two to ponder for your greenhouse gardening.
Links & Resources
Beginning Gardener Fundamentals – Online course details
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