MY HOUSEPLANTS are sulking, whispering among themselves, “Why doesn’t that woman get us some more light in here?” And then before I know it, seed-starting season will begin with leeks and onions, but what’s the right light to make those plants happiest indoors?

Leslie Halleck is author of “Gardening Under Lights: the Complete Guide for Indoor Growers.” Since her graduate research at Michigan State, where she explored greenhouse production, Leslie’s become an expert in the subject of light and plants. She shared some insights into what kind of light plants utilize, about short- and long-day plants, and more.

Read along as you listen to the Dec. 10, 2018 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).

Plus: Enter to win the book in the comments box at the very bottom of the page.

gardening indoors under lights, with leslie halleck

Q. So I just have to ask, when you visit someone’s home and you see their houseplants, can you immediately diagnosed how much or little light of the right or wrong kind they’re providing? [Laughter.] I bet you can.

A. Well let’s just say as a professional horticulturist that’s sort of a disease, a problem I experience when I visit anyone’s outdoor garden or indoor garden. I immediately start diagnosing issues, it’s a little consult-ive and I always have to apologize for it along with picking weeds or picking dead leaves off with other people’s plants, it’s a little bit of a problem.

Q. I know, I’m always weeding. [Laughter.]

A. So if you are brave enough to invite me over to your house and you have plants in your house, it’s probably where the conversation is going to steer. And so oftentimes the biggest cause of houseplants and an indoor edible growing frustration is a simple lack of light. And it’s very confusing for most home gardeners to grasp.

Q. Yes. You have in your … I think some of the pictures in the book or from your house, maybe not all of them but I think some of them are. And you seem to have lighted plants situations of some very creative kinds. Just before we kind of dig into what kind of light and how to figure it all out, give us some examples of some of the spots in your house that are lighted for plants that are a little different.

A. Sadly, as a horticulturist, I have an incredibly dark house, so it’s unfortunate for me. However that means I’ve had to get creative about a lot of different grow-lighting option. And I grow in all different areas in my home and my garage. And I’m also very particular about my style. And as you probably noticed a lot of good quality, powerful grow lamps aren’t necessarily aesthetically pleasing. You’re not going to want to stick a CMH or ceramic metal halide lamp with a big metal hood in your living room for your citrus trees, right? Not a great thing.

So I really had to experiment using all different sorts of lights and fixtures, and create and build some of my own, to integrate them into my living spaces to look the way that I want them to. And then I keep the utilitarian things, like the grow tents and the big grow racks and the big lamps, out in my garage.

Q. I see. [Laughter.]

A. I call that my plant lab. So I have all that.

Q.  But you have like spotlights [above].

A. Spotlights, yes. And grow tents, and everything. And then inside my house I have built wall pockets with recessed grow lighting so you don’t see the lamp itself, which is a nice way to keep herbs and small edibles and things near the kitchen [photo, top of page]. And then I built nicer fixtures in other open areas of my home. And then I have looked for nice looking spot lamps that are grow lamps that I can use with my attractive big healthy plants or little plant vignettes as I call them, that I like to create around the house.

Q. Yes. It was a very inspirational, and with the transcript of the show, we’ll show some of those pictures because some of them, like you said at the beginning of some of the fixtures you wouldn’t want to live within your living room, but you were able to ferret out ones that I would, and I thought, “Oh, now that could make a difference.” So very creative.

I have to confess that my head starts spinning when either I read on packages or even in the book, about all the different kinds of lights and the, the sort of–not chemistry, but almost like chemistry of light, the formulas and how it all works, the colors and the intensities. And uh-oh, I get like I want to shut down. But I was able to find in your book practical advice. And so I want to get to some of that and share that with people.

Many of us gardeners have heard about things like a short-day onion variety versus a long-day onion in the catalogs. Or we hear that poinsettias need a certain amount of darkness before they’re bloom, and it’s like in the book you mentioned it’s almost like plants have circadian rhythms or know how to tell time. Can you talk a little bit about that? And sort of demystify that for us.

A. Yes. So what you’re referring to are the physics of light, because technically that’s what it is, it’s physics. So I do include a little bit about it in the book because I feel like if you’re interested in the topic, all of you people are smart enough to absorb a little bit of science, right? That’s not going to hurt us.

Q. Right. [Laughter.]

A. You can skip around, but I also say you can skip around and piece out just the practical information I give you. You certainly don’t have to dig deep into that if you don’t want to, but yes, if you think about how plants evolve in nature, right? There are very different climates where plants evolved, and some of them are very cold and snow covered and some of them are very warm. And at those different latitudes, you have different lengths of night and length of day, so light versus darkness.

So plants evolved based on where they are endemic, to be able to reproduce successfully based on those environmental conditions. So you certainly wouldn’t want to, if you’re an onion or garlic, you certainly wouldn’t want to come up and flower while there’s still snow on the ground, right? So plants have evolved to be able to flower and reproduce when environmental conditions are favorable.

And so along with that has come what are called photoperiods—photoperiod responses for certain plants. So when you see those short-day or long-day onions in the catalog, a short-day onion is going to require a long night period, short day period in order to be able to flower, and garlic, for example, also requires a vernalization or exposure to cold, and long nights in order to trigger that bulbing and flowering.

So it’s a safety response for reproduction, and so some plants have very hard and fast requirements for certain lighting regimens. And so we call them short-day or long-day plants, but really what it is, it’s the plant responding to the length of darkness or nighttime, so it’s a little bit reversed.

Q. O.K. So it’s photoperiod, but it’s really about how much darkness there is.

A. Yes. Uninterrupted darkness. And I explain that a little bit in the book-

Q. I know you do.

A. It’s hormones and pigments that sort of biologically change in darkness and change in lightness. So that is a plant’s clock, if you will, their inner biological clock. That’s how they tell whether it’s light or dark, and then it also helps them measure the season, so they know when it’s the right time to flower and make seed and when they need to hold tight.

Q. So for instance, my grandmother a million years ago, my late grandmother, she was a garden club lady, she did single-stem or standard mums. She disbudded them and had single big flower at the top. I think she had some kind of place where she covered them all, or there was some rig going on as I remember it a million years ago at her house. And poinsettia I think is another one. So that’s about how many hours of darkness? And what about if it’s interrupted? Tell us a little bit about that.

A. Correct, right. So certain plants have requirements and certain plants are speeded up to flower with different periods of darkness. So she was black-clothing, so for those of us who were in graduate school before, we had automated black-cloth systems-

Q. [Laughter.]

A. … or you worked for a greenhouse grower that was producing a plant with a photoperiodic requirement, you basically had to cover or tent those plants with blackout cloth, black cloth to cut out all of the light exposure so that those dark periods were not interrupted for a certain amount of weeks. And then the plant is triggered to flower, so those mums, a short-day plant, meaning she had to cut out the light and keep it at a certain number of hours for a certain period of time in order to get those mums to create a flower bud.

Q. Another thing about plants’ reactions to light or dark that I loved reading about, in this case it was seeds. You talked about that mystery that sometimes happens when some packets say don’t cover this particular seed. And there’s a reason, you explained the reason in the book, why some seeds can’t be covered and they won’t germinate if they’re covered by soil.

A. Yes, some plants such as certain lettuce species won’t actually germinate unless the seed is exposed to some red light waves, while it’s going through that germination process, and so it’s a good thing to pay attention. So sometimes folks will say, “I started all these lettuce seeds and I can’t get them to germinate.” And I ask them, “Well did you cover them up with soil?” And they say, “Well yes, that’s what you’re supposed to do, right?”

Q.  [Laughter.]

A. Well, not for everything. And so definitely pay attention to the seed packet. It will tell you whether to cover them or not cover them and if it tells you not to cover them, simply press them into the soil, that means you need some light exposure or they’re not going to germinate or you’re going to have poor germination rates.

Q. Right, and as I said earlier, my head starts to spin a little bit when I try to grasp all the measurements of light and so forth. But what impresses me and what I kind of took away from the book is, what we see with our eyes, both the spectrum of the colors that we can see, the range of light that we see, and also our sense of how intense or not intense it is, and a plant’s sense of those things is very different.

So we say, “That’s a really bright light.” We couldn’t really look into it close up, but to a plant, it may be not much light at all.

A. Right. Our human eyeballs perceive light differently than the light that plants use for photosynthesis. So plants use light from the sun or light from grow lamps that we give them to drive photosynthesis. And the wavelengths of light, the colors of light, if you will, within a spectrum of visible light that are useful for photosynthesis are a little bit different than what we as people perceive as brightness. So humans perceive brightness, and which color of light do you think that our eye balls perceive as the brightest color of light? Do you remember?

Q. I don’t, no.

A. Green light, so we perceive green light to be bright. However, in the spectrum of what we call PAR, Photosynthetically Active Radiation—that’s a botanical acronym of the day—green light is actually one of the least efficient colors of light for driving photosynthesis.

Q. Even though most plants are green? [Laughter.]

A. Yes. So the plant does use green light for certain biological functions and a little bit for photosynthesis, but it’s not the most efficient light for photosynthesis. So what we perceive as a bright lamp might not be as useful to a plant. So simply looking at a grow lamp and saying, “Oh, that’s really bright.” That doesn’t mean anything to your plant. What that lamp needs to do is put out the spectrum of light that’s most efficient for photosynthesis in the volume that it needs to drive enough photosynthesis.

It’s not just about color of light, it’s about amount of light. And the easiest way for me to explain this in layman’s terms is to think of individual photons of light like raindrops that are coming down-

Q. I loved that analogy in the book, yes, like rain drops.

A. Because again, we sort of incorrectly perceive it as a brightness factor, when in reality the red and blue light spectrum are the most efficient for photosynthesis. And that’s why you see all those LED grow lights out there that look pink, because they’re only blending red and blue light or red and blue with maybe a couple of other colors. So that’s not full-spectrum light, that is concentrated blue and red light because that’s going to be the most efficient way to drive the photosynthesis.

Q. Right.

A. So if you’re growing edible crops and for commercial reasons, and you want to grow them fast and full, you want to do that as efficiently as you can. So that’s why all of those vertical gardens you see, in the shipping containers they convert, are full of pink light. In your living room, you don’t want that spectrum.

Q. [Laughter.]

A. Unless what you’re going for is a disco vibe, that’s fine, but most of us are going to look for full-spectrum grow lamps that are going to provide all of the colors that plants can use. But then it has to produce enough, enough of those raindrops, to drive photosynthesis, and that’s where a lot of people run into trouble with their seedlings.

Q. So the photons are like the rain drops.

A. Yes. And you have to add up, in different plants… different species of plants need different amounts of light in their different biological functions. If you’re growing leafy greens and vegetables, you can get away with providing a less amount of light than if you’re trying to fruit tomatoes or citrus. Because making the flowers and fruits and seed are a lot of energy.

So just like if you grow lettuce outside, you can get away with sort of a part-sun, part-shade exposure, but that’s not going to work for tomatoes. Tomatoes have to have full sun, and it’s because they need that accumulation of volume of light throughout the day in order to have enough juice to make those flowers and fruits.

Q. I thought it’d be kind of fun to talk about some specific plants and different kinds of lights, kind of almost like matching as if we were thinking about shopping for the right kind of light for different uses.

And I will confess just a little sort of anecdote, years ago when I first started, and this is decades ago, I started growing my own seedlings, for instance, indoors, I made kind of a rack, you can buy them of course. And I had lights on it and they were shop lights, and they had one cool white and one warm white fluorescent bulb [laughter] and that seemed like a lot of light and it worked and whatever. Now fast forward decades and now I use T5, a different kind of fluorescent tube, narrower but more efficient, in a reflective hood, so that it really bounces more light down toward the plants and-

A. So you get more volume of light.

Q. Yes. And it’s so bright, but even that’s not as bright as a spring day in May in my region. Do, do you know what I mean? That’s not even as bright as outside.

A. Absolutely.

Q. So I carry my babies outside on fair days into a protected spot, not baking them right away in their young lives, because they get so much more light. It’s really different when you start studying what’s in your book, the measurements of lights.

Should we start with houseplants or should we start with seed starting? Should we start with some houseplants, because it’s that time of year? Like orchids, a lot of people love to grow orchids, but then they just kill them after one season, they get them in spike-

A. Yes, a lot of people bring sun-loving plants indoors for the winter or you they’re buying new houseplants and orchids. So again, flowering versus foliage production takes more energy, and so in winter, your ambient or natural light levels outdoors goes down because our sun angle goes down, right? So we have less light available to us coming in through our windows in our home.

And what else changes? The length of the day. So the number of hours of light that we get also gets cut off. So what changes? The volume of light decreases significantly indoors. So just at the time that you need to move all of these things inside or you want to flower your orchids, it gets that much dimmer indoors.

And again to your eye, it might not seem that different, but if you actually pulled out a light meter, you would be shocked at the difference between that light right outside your window and just a foot inside your window, drastically drops. And every inch you move away from that window or you go into a corner, you might have a completely different light zone from something that needs high light to a couple of feet away, all you can maintain is something that tolerates very low light conditions.

And that’s where a lot of people get in trouble and they think that they can maintain a lot of things just on their windowsill. It doesn’t matter if you have a bright southern windowsill, it’s still not bright enough for a lot of plants, especially if they’re blooming plants.

So supplemental lighting is what you’re looking to provide, and you may keep those plants in the windowsill and add a spotlight, add a grow light. So there’s some nice small LED bulbs that can fit into standard home lamp fixtures or lighting fixtures and those are going to be good options for individual plants, individual orchids or small groups of orchids or succulents. But there are also some small panels, little LED panels, that cover a couple of square feet that are good for orchids, a lot of the little self-contained growing units that have a couple of LED bars or eight or T5HO high-output florescence are good for orchids. So those units are going to be good for keeping orchids happier through the winter. You don’t need a big lighting rig for those, but you need to provide some supplemental light indoors.

Q. And people also bring … so for instance, succulents have become such a thing, and a lot of people bring home succulents. And let’s just be real, they didn’t evolve and adapt to a place any anywhere like my northern garden, my northern house- [Laughter.]

A. No they are full-sun plants, and that’s why most people kill them. They bring them inside into what are very dim conditions to succulents, and then they do what? They water them. That’s the kiss of death, because a lot of people don’t realize that when light levels go down, so does the rate of photosynthesis and water uptake.

So the plant can only use those resources like water you give it if it has enough light to keep using it. So when the light levels go down and you add a little bit of water to that succulent, it’s going to turn to mush, because there’s not enough light.

So LED spotlights again, the little self-contained grow units with a couple of grow lamps, a couple of bars, are good for that. That will provide you with sort of a medium light level. So your succulent, your orchids and things like that will all be happy with those, but take care not to use incandescent bulbs. There are still some incandescent bulbs being sold as grow lamps, but don’t do that.

Q. Hot.

A. They’re very inefficient, and they produce a lot more heat than they produce the light. And that’s what grow lamps do, they produced two things. They pull that energy out of the wall, right, and then they convert it to either usable light or heat. So the more efficient the grow light, the more light it produces versus heat, does that make sense?

Q. Yes, absolutely. That’s one of the things with my seed starting and so forth, it was great to get to expose those little tender babies to the least amount of heat with the most amount of light, that combination. Do you know what I mean? Let them not grow in a hot, baking environment stretching toward a weak light source anymore.

A. Right? That’s the trick with seedlings, is that you really have to… so those, those two lamps fixtures I was talking about that are good for maintaining some of your houseplants and orchids, you’ve got to double that for seedlings.

So what most people don’t realize is that seedlings need a large volume of light, and so when you see the recommendations to keep your seedlings lighted 14 to 16 hours a day, that’s not because they’re long-day plants—and I see a lot of misinformation online about that. They need 14 to 16 hours of light because they need the accumulation of volume of light; it’s cumulative. So they’ve got to keep absorbing all of those light photons in order to have enough to successfully germinate, and that means putting those lamps pretty close, 3 or 4 inches above your seedlings.

The new retrofit LED T5 bars that go into your fluorescent fixtures are a great option for that, and I like to have four of those lamps over my trays of seedling.

Q. Oh, so we can retrofit? Is it just for the high output T5 fixtures that can take those bars, those LED bars?

A. Yes.

If you have a standard HOT5 fluorescent lamp fixture that holds four to eight lamps, you can actually take out those fluorescent tubes—which by the way, HOT5s are still a good, efficient option. Those are not obsolete yet, so if you’re using those, those are still a good option for you. The LED retrofit bars have the same little fixture on them and you pop them into that fixture just like you would a fluorescent, and it allows you to provide that bright light for seedlings. But they generate a little less heat, so you don’t have as much heat output right next to your seedlings.

Q. Are you optimistic about the sort of pricing on the LEDs continuing to come down and then become more in reach? Because I’ve hesitated, I’ve stuck with my T5s, like I said, with my reflective hood, because it just was crazy to, they work well like you also said. Are the LEDs starting to really come down in price, be more affordable?

A. Yes, so what you’re paying for with LEDs is still some of that innovation, there is a lot of innovation going on with LED grow lamps right now and the options are getting better and better. They did not start out being very good grow light option, they did not and it was very expensive, and they just didn’t put out enough light volume. And be careful because there’s a lot of really cheap, low quality LEDs on the market coming out of China, and a lot of them will burn out quickly or just not create as much light as they should.

So you’re still going to want to buy a good price and quality LED grow lamp in whatever configuration they come in, but I do think in the next couple of years prices are going to continue to get better, and the light output is going to increase on LEDs. You’re going to get a better value for what you spend with LEDs.

Q. Well that is your professional evaluation and prediction. There’s so much to learn; I have to dig back in now that we’ve spoken before I go off shopping for anything new. And those spotlights for some of the possible living room plants really appealed to me, so thank you for that, too.

enter to win ‘gardening under lights’

I’LL GIVE A COPY OF ‘Gardening Under Lights’ by Leslie Halleck to one lucky reader. All you have to do to enter is answer this question in the comments box at the very bottom of the page:

What kind of indoor plant lights do you have, if any, and what do you use them for?

No answer, or feeling shy? Just say something like “count me in” and I will, but a reply is even better. I’ll pick a random winner after entries close at midnight Tuesday, December 18, 2018. Good luck to all.

prefer the podcast version of the show?

MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its ninth year in March 2018. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play Dec. 10, 2018 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).