From Exuberance to Dismay
We’ve gone from exuberance to dismay in just four days since planting our first lot of 4,600 CBD hemp seeds. By the second day after planting, about 40% of the seeds were sprouting; we were so excited. By the fourth day, nearly all of the seeds had sprung up. Unfortunately and to our dismay, they were shooting up to the sky out of control. In fact, they were so “leggy” that a few of them started falling over; they were all stem with little leaves. Gee, that didn’t take long to run into our first challenge growing hemp!
So it was back to the office to research more about cannabis. CBD hemp was derived from marijuana to have the medicinal benefits without the “high” and this makes the experience of cannabis growers helpful to us. Given their appearance, our first thought was that the seedlings weren’t getting enough light. However, this just didn’t make sense given that we’d planted later in the season (early June) in our southern facing greenhouse.
Wasn’t this enough light? Were CBD hemp seedlings so light thirsty that a few cloudy days during the beginning of their lives caused them to become so tall that they literally fell over or was it something else? In articles like Leggy Seedlings, we were reminded of a handful of other possible causes.
Causes of Leggy Plants
OK, so we went through the list. The first few days when the seeds were sprouting, we had rain. The skies were overcast. Also, we only had a couple T8 fluorescent light fixtures running a pair of “daylight” tubes each. In looking over lighting recommendations for indoor grow operations, clearly hemp seedlings require a lot of light. We’d naively figured that natural lighting along with our minimal supplemental lighting would be enough. Not true for our overcast Wisconsin weather.
In terms of nutrients, it was hard for us to imagine our leggy starts had a potassium deficiency as our grow medium consisted of organic potting soil supplemented with vermi-compost (worm compost). On the other hand, this mix certainly was higher in nitrogen. Too much nitrogen promotes vegetative growth but doesn’t make for strong stems. We don’t think our soil mix was a big contributor but it’s good to keep in mind.
In terms of temperature and wind, admittedly we did let the greenhouse get too hot and humid the first few days. It took a bit to figure out the correct settings on the large blower that drives excess heat into the rock thermal mass that makes up the greenhouse floor. As a consequence, when the sun came out and if we didn’t get the doors open in time, the temperature would rise up into the 90’s. And when it came to air movement, while we had two box fans moving the air about close to the ceiling of the greenhouse (to reduce the risk of mold), the seedlings themselves definitely were not being lightly jostled about by air movement – which is required to promote strong stems.
So in the end, we took a few measures to address our leggy hemp seedlings.
In terms of trial runs, we just didn’t think of starting a dozen or so seedlings ahead of time to see if there may be any issues. If we had, we could have avoided a lot of angst. From now on, as much as we can, we’re going to trial run before the “main event”.
In terms of supplemental lighting, there are three basic options.
In Best Lights for Growing, they state that “To get the most out of fluorescent tubes, you will need to keep them 6-8 inches away from your plant.”
Similarly, in CFL Grow Lights, they write that “The light emitted by CFLs won’t go far, so it’s usually a good idea to have more of them and keep them all within 4-6 inches from your plants.”
While this is all well and good for very small indoor grow operations where all the light is coming from fluorescent lights, if we were to install the recommended number of fluorescent lights in our greenhouse, the fixtures alone would completely block out the sun; there would be so much hardware hanging above the plants that the sunlight couldn’t get through. This clearly makes no sense for us; not to mention the impracticality of trying to water plants with lights dangling dangerously close above.
LED lights are the “new kid on the block”. They offer many advantages over HID lighting which is the cannabis industry standard. Specifically, they put off much less heat, cost less to run, last much longer, don’t contain mercury, and all the light is directed toward the plants whereas the light from an HID lamp projects in all directions and so it requires a reflector.
With these advantages, you may be wondering why we didn’t choose LED lights. There are two reasons: price and questionable results. When it comes to price, quality LED lighting can cost many times more than HID lighting. We purchased 1000W HID kits that included two lamps, the fixture, a transformer, and wiring for around $170 each. Depending on what source you trust on who makes a good LED grow light (there are a lot of junk LED lights out there), a 1200W LED light can cost $800 or more.
Given the cost along with the fact that there is still a lot of debate on what constitutes a good LED lighting system, and whether the quality of the bud is equal to that grown under HID lighting, we chose HID lighting. When it came down to it, trying to select and tweak particular frequencies of light using LED to mirror metal halide HID lighting just didn’t make sense to us at this stage of our farming experience. Our simplistic view was that it would be better to use an HID lamp that burns white hot like the sun (6,000K) for vegetative growth putting off a wider array of frequencies than using LED lighting to try to target specific bands in the spectrum. It’s all about the entourage effect and the study, UV-B Radiation Effects On Photosynthesis, supports the contention that lighting that includes all spectrums produces the best cannabinoids.
When it comes to HID lighting, growers tend to use Metal Halide (MH) lamps for vegetative growth and High Pressure Sodium (HPS) during the flowering phase. The difference between them is that MH lamps run whiter/bluer at 6,000 Kelvin while HPS lamps produce a yellowish light at around 2,200 Kelvin. Yes, yes, there are lots of other nuances in terms of the quality of light various HID setups produce, but for us, we just needed some basic supplemental lighting in the 6K range, and this meant purchasing a relatively inexpensive HID kit.
When it came to lamp size and spacing, we referred to How Many Plants to Maximize Grow Space for guidance. Not wanting a lot of fixtures that would block the natural sunlight, we choose four 1000W lamps and spaced them about 30” away from the seedlings. Given that our greenhouse is roughly 9 feet by 15 feet inside and using the 5’x5’ area of coverage listed in the article, we guestimated that this would be enough supplemental lighting. Since we’re only concerned with seedlings in our greenhouse, we probably could have even gotten away with four 600W lamps as the light doesn’t need to penetrate through the more dense foliage of adult plants.
In terms of heat, the lamps are really hot. If you brush up against a lamp, you’re going to pay the price. On top of that, if you spend too much time under the lamp, you’ll get “sunburned”. Trust us, we found this out the hard way.
On the other hand, all that extra heat was an advantage for us as the nights have been quite cool. Our solar greenhouse uses a rock solar mass that we warm using a blower to drive extra heat into the rocks during the day and slowly release this heat at night – so the greenhouse remains nice and warm through the night. Note: Picture shows light difference between a 6,000K metal halide lamp and a 2,100K high pressure sodium lamp.
Fans in the greenhouse are important for two reasons. First, the air movement helps to keep mold away. Granted, we use lime to “whitewash” our greenhouse at the beginning of every growing season but the extra air movement helps too. Mold likes to fester away unperturbed. Lime mixed with salt to make the whitewash stick to the walls is an old standard. Lime kills mold, bacteria, and viruses. We always notice the effect of whitewashing; the air in the barn and greenhouse is notably fresher.
The second very important reason for fans, (that we learned the hard way), is that by gently tossing seedlings about using fans, the stems are strengthened. After just a week of adding small oscillating fans in the greenhouse, the stems on our hemp seedling were noticeably thicker. You could feel they were a lot stiffer when brushing over them by hand.
Seedlings that fall over because they’re too tall are more at risk of fungal infections like “damping off” where the stem shrivels up and the plant dies – not to mention being difficult to transplant in the field. For these reasons, we ended up repotting several hundred of the leggiest seedlings. Burying the stem deeper isn’t a problem as the stem will sprout roots.
In the end, with a little ingenuity and about 6 hours of time, the tallest plants were set about an inch lower into their cells. During the process, we tried hard to avoid disturbing the roots or exposing them to air too much. Any time “feminized” seedlings are stressed, the risk that they revert to being male plants increases.
One other important lessons we learned when it comes to HID lighting is that if the outer protective glass globe is damaged, these lamps put off wicked levels of UV. One of our metal halide lamps inadvertently got sprayed with water and not surprisingly, shattered. Thankfully, the inner lamp containing mercury was still intact and the lamp continued to work. However, we mistakenly left the lamp on while we cleaned up the glass and went to grab a replacement lamp.
It only took the few minutes to clean up the glass under the unshielded lamp but the result was a serious “sunburn”. And it only took about 30 minutes of exposure to the very high levels of UV for our poor little seedlings to get burned. All of the outer leaves on about 1,000 seedlings took on a dark green hue and cupped up at the edges. The damaged leaves didn’t fall off but they also never recovered. Over time, they were replaced with new growth.
Ugh. We just didn’t know any better. In hindsight, we should have read the packaging that clearly states the risk of UV when the outer glass is damaged. Lesson learned.