Greenhouses are having a moment. They have gone in and out of style for home gardeners, but right now, they are decidedly in, which is why my returning guest this week, Leslie Halleck, wrote her new book, “Home Greenhouse Revival: Growing Year-Round Foliage, Flowers & Food.”
Leslie is a certified professional horticulturist with a Bachelor of Science in biology from the University of North Texas, where she focused on botany and plant ecology, and with a Master of Science in horticulture from Michigan State University, where her graduate research focused on greenhouse plant production using environmental controls such as lighting, photoperiod, vernalization and temperature. She has more than 30 years of experience across several fields in the horticultural industry, and today she runs Halleck Horticultural, a consulting and marketing firm for the green industry.
Leslie Halleck’s new book is “Home Greenhouse Revival: Growing Year-Round Foliage, Flowers & Food.”
Leslie’s vast experience in horticulture has given her wings to write authoritative books on almost any subject, including one of my favorite books, “Gardening Under Lights.” I keep it on a shelf near my desk with the books that I refer back to most often because it is a great resource. Her new book on greenhouses for home gardeners is likewise a book you’ll keep going back to.
“ The approach I take to writing my books is, obviously, I want to educate,”
Leslie says. “I want to deliver some science. It can take a little time to percolate and sink in once you start using it and applying it, and then my intention is that there’s a lot of good reference material that you can go back to and reference as you continue to grow. … I write my books so that they’re not a one-time visit. They’re meant to be very dense and rich. I try to digest horticultural science down into palatable meals for you, but I don’t like to skimp on the science because I have faith in everyone’s intellectual capacity, and I feel like you need to know the science to really understand why plants are doing what they’re doing. Otherwise, you’re playing a constant guessing game.”
Before proceeding with my conversation with Leslie, I want to take a moment to let you know about two upcoming webinars I think will pique your interest.
On Monday, September 14, Melissa K. Norris, author of “Everything Worth Preserving,” will present “You Grew It. Now Keep It: 9 Ways to Preserve Your Harvest at Home” for the Online Gardening Academy community. Admission is $29, and the recording will be yours to keep. The program will begin at noon Eastern.
Then on Wednesday, September 30, I will host “Death in the Garden: Why Your Fall Cleanup Might Be Killing What You’re Trying to Protect” with landscape ecologist, researcher and author Shaun McCoshum, Ph.D. Shaun will show you how to read your garden’s dead and dying vegetation like a habitat map — and how to manage or redesign your space so it looks intentional, not neglected, while doing dramatically more for local wildlife. Registration is $25 plus taxes and fees, and the program will begin at noon Eastern.
Home Greenhouse Revival
Leslie’s publisher asked her to write “Home Greenhouse Revival” because she has the perfect résumé for it.
“I have a background in greenhouse production,” she notes. “My master’s degree is in floriculture, flowering physiology and controlled environments, greenhouse production.”
She decided to say yes to writing the book because before her publisher even approached her, she had the sense that greenhouses were starting to have another moment.
“ Gardening is an embedded trend in our culture. It never leaves us, but there are always activities within gardening that come and go with popularity and cycle back around,” she says.
And because greenhouse growing has challenging aspects, she knows it’s easy for folks to give up after a little while. But eventually, new people come to greenhouse growing, or gardeners who tried it before come back around to it.
That’s one reason she picked the word “revival” for the title. The second reason is she wants to help people who have tried before and not been successful “because they didn’t know what they didn’t know about greenhouse growing.”
“Let me teach you what you actually need to know first before you jump back into this so that you can take that greenhouse in the backyard you’re using as a storage shed right now and get it growing for you again,” she says.
She also notes that constant technological advances in the commercial greenhouse industry trickle down into the home gardening and consumer market.
“We benefit from those technological advances, and so I’d say in the last 10 years, there’s been a flood of new greenhouse manufacturers that are offering a whole lot more options to home gardeners than we ever had before beyond the rinky-dink little aluminum little kits,” she says.
Greenhouses for home gardeners have come a long way from aluminum kits. Photo Credit: Christina Salwitz
“ There’s a broad range of availability now to home growers that I just saw suddenly begin to expand.”
More advanced technology, such as sensors, is being digested down into accessible, more affordable products that home greenhouse growers can use. These devices can transform your greenhouse growing, she says.
One appeal of greenhouses is the constant resurgent desire to have access to your own homegrown food, according to Leslie. “Food security is a big issue, clearly, with what’s going on with food contamination in the country right now,” she says. “I think growing your own lettuce is on a lot of people’s minds. So I’m hoping this is great timing for this book, and that I can help a lot of people figure these things out.”
Leslie was the perfect person to write about home greenhouses.
A Greenhouse Will Never Feel Big Enough
When I was getting my greenhouse, I was talking to the owners of the greenhouse company about my choices. And I said, “I think I want your 12×24 greenhouse.” Right away, they said, “No you don’t.”
The owners talked me into the biggest one that could deliver to me, 12×32 feet. That’s as big as they can make them in the factory and still deliver them on a truck.
Going big was the right decision, as a greenhouse will quickly fill up with benches, equipment, a sink and supplies before even adding any plants. And if you plan on using water barrels to collect heat during the day and release it overnight, you’ll need to save some room for those barrels too, Leslie points out.
“ Bigger is usually better if you can manage it, but sometimes that doesn’t work with the space you have, and you have to get creative,” Leslie says.
Commercial greenhouse growing operations typically have a headhouse — it’s usually covered, and that’s where the potting soil, the potting tables, and all the labels go.
“In a home greenhouse, you’re usually not building in a whole separate headhouse, but you still have to account for that square footage of where all that stuff is going to go,” Leslie says. “ And I’ll tell you what, if you don’t consider those ergonomics and space issues, it can get real un-fun.”
You’ll be dragging water and potting soil, and maybe you’ll give up on gardening because it is too much hassle.
She adds that you don’t have to pay thousands to have someone come and install running water to your greenhouse. There are inexpensive DIY options for irrigation, Leslie says.
My greenhouse, which I have had for a few years now, is 12×32 feet. I’m glad the company talked me into the largest size that was available.
Where Do You Start?
In choosing a greenhouse, Leslie advises asking yourself many questions first.
“ I actually take a very opposite approach to this than pretty much everybody else, and that’s because I’m a horticulturist,” she says. “So my approach is plant-centric first. You don’t start with what kind of greenhouse you want — or you think you want — or what materials or any of those things. What you really need to do is sit down and think about your climate, your region, your weather.”
Ask yourself:
- What plants do you want to grow?
- When do you want to grow them?
- Are you expecting fruiting tomatoes in winter or are you just trying to jumpstart seedlings?
- How much do you want to grow?
- Do you intend on harvesting inside the greenhouse or are you only using it for cold protection?
- What species are you growing?
You need to get to know the plants and their environmental requirements first before you pick the kind of greenhouse or the size of the greenhouse or the height of the greenhouse or the shape of the greenhouse, she says.
“ Everybody starts with the aesthetics because we all have the greenhouse dream. We all think about the kind of greenhouse we’re supposed to have and wish we had, but you can spend a whole heck of a lot of money on a beautiful glass greenhouse only to discover it is not going to provide the environmental conditions you want for what you want to do in that space.”
So put plants first, and then reverse engineer what greenhouse you need.
“ You might find out that you end up with a very different greenhouse than you thought you were going to have once you really are thoughtful about the types of plants you want to grow,” Leslie says. “And if those plants require very different environmental conditions, because that might require zoning a greenhouse, that might require putting up two or three small greenhouses instead of one big greenhouse that you can create different temperature zones in.”
It is easier to control the temperature in a greenhouse that is full than in a greenhouse that is help empty. Photo Credit: Christina Salwitz
Your Growing Plans May Require Several Greenhouse ‘Zones’
If you want to grow vegetables, seedling starts and cut flowers in your greenhouse, you will need to make accommodations for each, because the growing conditions that work well for one activity won’t work best for others.
To achieve this, one side of your greenhouse may be kept warmer while the other is kept cooler. A propagation table with grow lights will have a different microzone inside the warm half of the greenhouse, with more warmth and light. Maybe another table is set for misting cuttings, which require more humidity.
How to Pick a Site for a Greenhouse
There are many considerations when picking a spot to place or build a greenhouse.
Light exposure, sun angle in the winter and obstructions are just a few considerations Leslie raises. Having a good foundational soil that is not sinking, slipping or collecting water is also a primary consideration.
In general, in the Northern Hemisphere, you want to maximize southern winter exposure.
“ You generally are going to want a spot that is far enough away from your house that your house is not shading it, not right under trees that are going to shade it all of the time. And you typically will run your greenhouse with the long side facing south, the pitch running east to west.”
Then there are also the government regulations and HOA bylaws. Are you allowed to build an accessory structure on your property? If so, how big can it be? And what are the required setbacks from neighboring properties?
“Even those of us out on agricultural land with a lot of space, there are still regulations we have to abide by because all of these things have impacts on your neighbors,” Leslie says.
Check government regulations and setback requirements before committing to a spot for your greenhouse, especially if you will be pouring a concrete slab.
A Greenhouse Still Needs Light
Does having a greenhouse mean you can grow summer crops in winter?
“ People have a misconception that because a greenhouse is transparent, you don’t need grow lights in a greenhouse,” Leslie says. “That couldn’t be farther from the truth. In fact, pretty much every single commercial production greenhouse is using grow lights. In the winter, your days get shorter. Your photo period shrinks. Your sun angle drops. You can lose about 50% of that ambient light. So your DLI (daily light integral), your overall quantity of light you’re getting in the day, drops significantly.
“So even if you’re getting a good sun angle in the winter, if it’s cloudy all the time, that’s even going to further reduce it. So if you’re starting seedlings or you’re trying to get things to fruit to harvest, you’re generally going to need grow lights, and that means power. So you need to think about having power.”
You might be able to use an extension cord for small sets of LED lights.
“ If you’re putting in HID, high-intensity discharge, more powerful grow lights, you’re going to need a separate circuit for that greenhouse, and you’re probably going to need a permit for that new, for that electrical line that’s going to go in.”
But if you just intend to use your greenhouse in winter for hardening off, overwinter protection and starting early crops that can handle the cold or germinate at low temperatures, like spinach. If your needs are simple, it will simplify how complicated your greenhouse setup will be.
Greenhouses Are Harder to Cool than to Heat
“ I always say gardening is zip code local. So your conditions and your unique microclimate or obstructions, all of those things on your property are really going to dictate when and how long you can grow inside the greenhouse, along with whatever systems you have to heat and cool,” Leslie says. “People don’t realize it’s a lot harder to cool a greenhouse than it is to heat a greenhouse. And the summer months is where a lot of people run into a lot of disappointment because you’ve put up this greenhouse, and then in your mind, this is a 12-month growing operation. I should be able to grow, and everything’s going to be beautiful and happy in here 12 months out of the year, and that is not the case. That’s not reality.”
If you live in a climate where the summer temperatures max out at about 80°, you can use ventilation and air conditioning to get the greenhouse’s internal temperature down in combination with shadecloth or whitewash.
An air conditioner can only bring the temperature down 15-20°. So it can be very difficult if not impossible to keep the temperature in a greenhouse below 90° in summer.
“ Once the inside temperature of that greenhouse hits about 90 degrees Fahrenheit, plants are just going to go downhill in it,” Leslie notes. “ The 24-hour temperature helps, so if your nights are getting very cool and it’s averaging out lower, that’s a big bonus. But for example, in a very hot climate like Texas where I was for the last almost 30 years, night temperatures could be still close to 100 degrees at night, so you ain’t getting no good 24-hour day and night average. It’s all hot all the time, and that greenhouse will be blazing even with air conditioners running in it.”
The shape and height of a greenhouse also influence how easy or difficult it is to cool. In a taller greenhouse, it’s easier to manage the environment and easier to cool, Leslie says. A Venlo-type greenhouse, for instance, is very tall and easier to cool.
“It’s harder to control your environment in a smaller space. The heating and cooling is harder. Ventilation has to be done much more frequently,” she notes.
A geodesic dome loses less ambient heat than most other shapes. Glass will lose more heat than plastic glazing.
She also points out that it’s easier to control the environment in a full greenhouse than a half-empty greenhouse.
Vents help cool a greenhouse on warm days, but vents can only cool a greenhouse so much.
Understanding DIF
I couldn’t understand why plants growing in my greenhouse were getting long, with lots of growth between internodes, until I learned about DIF.
DIF is the difference between daytime and nighttime temperatures.
“To make this really simple,” Leslie explains, “you have a differential between your day and your nighttime temperature.”
That difference can be 10, 20, 30 degrees, if not more.
“That differential matters. It also compounds to create your 24-hour average temperature or your day and night average temperature. So when you have a very big drop like that, that can influence the physiology and elongation of plants.”
A greenhouse growing technique called negative DIF involves warming the greenhouse at night or dropping the temperature a little bit in the middle of the night or in the morning, and then warming it back up again.
“What that does is trick plants into a reduced internode development. It keeps them shorter and stockier,” Leslie says. “… Plants very generally benefit from about a 10-degree drop at night. But if that differential is too big, you can run into physiological developmental problems.”
Helpful Tools for Greenhouse Owners
Leslie shared the important tools that she doesn’t skip out on.
Hygrometer – You need to, at a bare minimum, understand what your relative humidity is, and that’s the humidity level relative to temperature, ’cause that’s always relative to the temperature. Warm air can hold more water vapor. Cold air holds less. So you have to know these things because it affects plant transpiration.
Soil moisture meter – A soil moisture meter will help you learn better watering habits. “ Over time you won’t rely on those anymore because they’re pretty rudimentary, but initially as a new greenhouse grower it can help you figure out if you’re watering adequately or not,” Leslie says.
Quantum flux meter – To measure light, a conventional light meter is not adequate. A quantum flux meter, or photosynthetically active radiation (PAR) meter. “ You cannot rely on human eyeballs to determine whether there is enough usable light for the plants that you’re growing,” Leslie says. A quantum flux meter can tell you the photosynthetic photon flux density in the greenhouse — the intensity of the light. And you can use it to calculate the greenhouse’s daily light integral.
Vapor pressure deficit sensor – A vapor pressure deficit, or VPD, sensor will let you know when the heater needs to be kicked on or if the vents need to be opened more often. “If the vapor pressure deficit is bigger, that means your relative humidity is lower. You have a bigger deficit of water vapor, which means you can add more water vapor to that space, but it also means your plants are going to transpire faster, they’re going to lose water faster.”
Vents – A greenhouse kit doesn’t necessarily come with vents. “Always add on the vents,” Leslie says. It’s also a good option to get a solar-power-controlled opener and closer for those vents for less labor and healthier plants.
Grow lights – Lighting is needed to supplement sunlight when your greenhouse doesn’t get enough light exposure or the day lengths have gotten shorter.
Heat mats – Heated propagation mats can help your seeds start when the soil temperature would otherwise not be high enough.
These tomato vines benefit from the natural light volume provided by the greenhouse windows as well as supplemental volume from grow lights. (photo: Leslie F. Halleck)
I hope you enjoyed my conversation with Leslie Halleck on “Home Greenhouse Revival.” If you haven’t listened yet, you can do so now by clicking the Play button on the green bar near the top of this post.
What has your experience with greenhouses taught you? Let us know in the comments below.
Links & Resources
Episode 083: Gardening Indoors: The Science of Light, with Leslie Halleck
Episode 093: Hobby Greenhouse Considerations: What to Know Before You Buy (and After You Do)
Episode 108: Easy Ways to Make More Houseplants, Vegetables and Flowers
Episode 350: What to Know About Buying and Owning a Hobby Greenhouse
Episode 352: Greenhouse Hacks and Tips: Lessons Learned In My First Year
joegardenerTV YouTube: Seed Starting in the New Greenhouse | Getting Warmed Up
September 14th webinar: “You Grew It. Now Keep It: 9 Ways to Preserve Your Harvest at Home” with Melissa K. Norris, author of “Everything Worth Preserving”
September 30th webinar: “Death in the Garden: Why Your Fall Cleanup Might Be Killing What You’re Trying to Protect”
Fall Gardening Masterclass – Get access to the webinar replay for $20.
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Organic Vegetable Gardening: My new premium online course. The course is designed to be a comprehensive guide to starting, growing, nurturing and harvesting your favorite vegetables, no matter what you love to eat, no matter where you live, no matter your level of gardening experience.
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier.
joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
Halleck Horticultural on Facebook
Leslie Halleck on Instagram | @lesliehalleck
“Home Greenhouse Revival: Growing Year-Round Foliage, Flowers & Food” by Leslie F. Halleck (You can also pre-order the book in Leslie’s shop)
“Gardening Under Lights: The Complete Guide for Indoor Growers” by Leslie F. Halleck
“Plant Parenting: Easy Ways to Make More Houseplants, Vegetables, and Flowers” by Leslie F. Halleck
“Tiny Plants: Discover the joys of growing and collecting itty-bitty houseplants” by Leslie F. Halleck
Milorganite® – Our podcast episode sponsor and Brand Partner of joegardener.com
Soil3 – Our podcast episode sponsor and Brand Partner of joegardener.com – Use code JoeGardener26 for $5 off
Disclosure: As an Amazon Associate, I earn from qualifying purchases. I receive a commission every time you purchase a product through an Amazon affiliate link. Some product links in this guide are affiliate links. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast was based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Milorganite, Soil3, Territorial Seed Company, Cool Springs Press / Quarto Group and Dramm. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.
