 This is Jeremy Pickens with Auburn University's Department of Horticulture and the Alabama Cooperative Extension System, and I am the nursery and greenhouse specialist. In this video, we're going to discuss an overview of commercial hydroponic lettuce production. We're talking about leafy green production. We're talking about mainly leaf lettuce or bib lettuce. We're not really talking about the heart type, excuse me, the heading type lettuce such as iceberg or the large romaine. That's because they have a much longer crop cycle, typically around 50 to 60 days, whereas these leaf or bib type lettuce from transplant only take 30 to 35 days. And that's important because we want to push as many crops as possible through that greenhouse because we're selling them based on the head, not the weight. And so the faster we can move a crop through, the more crops we can have per year and the faster we can recover some of our expenses for this production system. There are two major cropping systems with greenhouse lettuce culture, at least on a commercial level. There's raft culture also called float beds or deep water culture, DWC, and also the nutrient film technique. The nutrient film technique or NFT is a series of plastic troughs that are positioned throughout the greenhouse. The plants are transplanted directly into these troughs where a thin film of nutrient solution is constantly recirculated through the system. It's not a large volume of water that is running through these NFT troughs, just a thin film that's probably just a fraction of an inch. This nutrient solution or fertilizer is constantly running through here. Once it reaches the end of the trough, it is going to be drained to a centralized collection sump, whereas then recirculated back to the troughs. Most commercial NFT systems are going to utilize some sort of automated control system to make adjustments to both pH and EC. EC stands for electrical conductivity. Fertilizer salts, the more that you add to water, the more conductive it is. You can set these controllers to specific ranges. If the fertilizer level is dropped too low, it's going to give you a lower EC reading outside of your range. That controller is going to send a signal to a dosing pump, which is in turn going to dose or inject fertilizer directly into the irrigation line. Same thing for pH. If the pH falls out of range, that controller is going to send a signal to a pump that's either going to inject acid or base to put that pH back in the appropriate range. The other major commercial lettuce production system would be deep water culture or raft culture. This system is a little bit less expensive than an NFT system, and a lot of people like to start off with this one. In these systems, rafts are used to float plants on top of a nutrient solution. The plants are planted directly into the rafts and the roots as they come in contact with that nutrient solution, they grow down into it. Most successful DWC systems are going to utilize aeration to deliver oxygen into the water column. That's important because roots need oxygen just as much as they need water or fertilizer. So to get that oxygen into the water column, we use regenerative blowers. Those blowers push air into pipes and from those pipes they get diffused through air stones. These air stones are trying to make very small bubbles. The smaller the bubble, the more surface area, the more surface area, the more opportunity for oxygen to dissolve in that water column. You really need to design your aeration system properly. What you don't want is a lot of bubbles bubbling up through the raft, creating a lot of splashing and splashing. It keeps the top of that raft saturated with highly nutrient laden film which grows algae radially. In doing that, you may create a situation where one thing that let us may become unmarketable and also could create some food safety hazards there. Now we're going to talk about some of the culture techniques with these systems. A really important aspect of greenhouse less production is being able to grow your own transplants. We don't sow seeds directly into the production system because it's using valuable time that we don't really want to waste by using transplants and transplanting seedlings into the system. We shave off about two weeks of production. Again we're looking for a 30 to 35 day window of production here. The most popular types of growing media for these seedlings would be oasis cubes or rock wool cubes here. You can see they come in pre-forated sheets with holes drilled in them. You can just pop off the cube once the seedling gets the appropriate size. Another option is net pots. Net pots are probably a little more common for smaller systems, not the large systems because they're a little more labor intensive. However, the benefit of a net pot is you can plant a much larger transplant. But again the disadvantage would be it's a little bit more labor dealing with those net pots potting the seedling up into that larger pot and then if you want to reuse the pot cleaning it later. So when you're choosing seeds make sure you use a reputable source and use greenhouse varieties. This is typically indicated in the seed catalogs or you can talk to the seed salesman about that. We also recommend pelletized seeds. You can see there's lettuce seeds in the bottom left hand corner here next to the penny. You can see they're very small and they're difficult to handle with your hands and so by pelletizing these seeds in a clay prill that makes it pretty easy to handle with commercial seeders or if you're doing it by hand with your fingertips. We definitely don't want too big of a transplant. Two weeks is pretty good size transplant depending on temperatures and that's going to be two to four true leaves. You can see at the upper top left hand corner there's several sizes of transplants. You'd be surprised the second from the right actually will grow faster than the other three to the left that are bigger. That's because you'll notice all the roots around these larger transplants. Those roots grow into other cubes surrounding that transplant if it or that seedling if it grows too big and when you break that cube off to go plant it you're breaking off a lot of those roots and it subsequently you'll get a lot of stress for that plant and it takes a little time for it to recover. So here's a few tips if you're building your own DWC system. You can use water depths of anywhere from six to 16 inches deep. I like it at the minimum at least 10 inches deep that just helps with the bubble size with your aeration. You can use two inch lumber. I like to use two by 12's but you can use smaller lumber if you need to or you can use concrete blocks. I definitely recommend at the minimum a 20 mil HDPE liner. I like liners with a white on black with the white side pointed up. That way I can see if there's any tears in the system after having it in place. They sell a repair tape that you can use if you find any tears. Those liners are relatively inexpensive about $600 a liner. You can have the corners pre-molded by the manufacturer or you can fold them yourself. I prefer to fold them myself just in case the manufacturer and my numbers are off a little bit. You definitely want to make these in four foot increments. That's because the rafts you're going to use are four by eight. You can just cut those in half for ease of handling. By making them in four foot increments you're not going to have a lot of waste in cutting that foam. You also want to install an oversized drain. I can't stress to you enough how important it is to have a lot of the ability to drain that system fast. If you have something go wrong and you need to swap out your water quick you definitely want to be able to drain that and fill it fast. You can use tank adapters or bulkhead fittings to plumb a PVC pipe through your liner. Also you really only need to support that tank from the outside. Any kind of supports on the inside could be potential for tears or rips in the future. One other thing before you tack the sides of your liner down you want to fill that tank up and let it sit for 24 to 48 hours just so that ground settles a little bit. It's a lot of weight. If you were to tack it down and it settled later it might tear your liner. Obviously an important component to the deep water culture systems that are utilizing rafts is the actual raft itself and building that rafts can take a little time. A lot of people are using this blue construction type foam board. I can't recommend that because it's not food grade. However a lot of people are using it if you're going to be cutting holes for the one inch cube media a seven eighths inch speed or bore or auger type drill bit works very well. Butterfly bits or spade bits do not work that well. Neither do hole saws. So using this type of bit with high rpms and applying low pressure very slowly works really well cutting these one inch holes. If you do end up using the blue board you definitely want to get this plastic sheet off of it before you drill the holes or put it in the system. What I would recommend is getting a food grade polystyrene sheet like you see on the far right. The thicker the board the longer it's going to last because again this at a minimum you definitely want to use at least one inch thick foam boards because you get a lot of weight on there and if you're going to be picking these boards up with all those lettuce heads that it can crack so the thicker the board the longer it's going to last. I like to cut my rafts in half to make four by four units that just makes it easier to handle. There's a lot of options for plant spacing however most everyone is using eight inch on center spacing that'll give you about 36 heads per raft. That's good for most leaf type lettuce. If you're going to be growing basil you might switch down to a six inch to get a little bit more higher plant density and I wouldn't recommend going any larger than eight inch because just again you don't have a lot of those crops from take longer to grow and you get a lot less heads per area. What's great about the DWC system is that you can make it any size you want. You can make it hobby size system or you can make it a commercial large system. You can make them to fit just about anywhere you can see this one on the far left. It's kind of made a little bit more ornate and is probably about a four by 20 foot system whereas on the the system in the middle will hold about 4,000 heads of lettuce in a 30 by 96 greenhouse. It's got two 88 by 12 foot beds and you can get really small. You can grow them again just about anything that'll hold that nutrient solution. Just a little small two square foot River May tote can work just fine as a hobby system. If you're interested in learning more about greenhouse lettuce production please check out our latest fact sheet ANR2903. You can access it via the ACES website that's aces.edu. Once you get there just type in ANR-2903 in the search bar and it should come up pretty easily. If you have any further questions you can also contact your regional extension agent for commercial horticulture. You can access them through your local county office and through them we'll try to answer any questions you might have. A special thank you to the Alabama Department of Agriculture and Industries for a 2019 specialty crop block grant that funded this program.