Hemp Likes Water
Hemp plants need supplemental water to do their best. A University of Colorado study that found when industrial hemp is watered, yields increase 3 times or more. There are also various articles that state that hemp plants use between 1- 6 gallons per day – considerably more than is generally provided by rainfall. Large outdoor grow operations typically use plastic mulch and drip tape. To be confident that we would have sufficient water for a good yield, we would need to install an irrigation system.
Note: Hemp does not like “wet feet”. In Agriculture Industrial Hemp Production and Management they write, “Excessive rainfall and saturated soil conditions that persist for several days are very detrimental for hemp development especially in the early stages. Hemp plants turn yellow and cease development… After an extended wet period, some plants may resume growth, but the plants will have poor vigor, nutrient loss, chlorosis and plant development will be less than stellar throughout the growing season“.
How Much Water
After understanding that even with our average of 3” to 3.5” of rainfall in the months of May through August, the plants would probably need more water, we had to estimate water usage by plant to be able to size the pipe and the number of zones required for our irrigation system. Unfortunately, we couldn’t find any information for our area but did find information like the Humboldt County Cannabis Water Use Study which reported that on average, each plant was given 4.4 gallons of water a day. Although not specific to us in Wisconsin, it was at least a point of reference.
There are a number of variables that are different between grow operations in the sandy soils of the arid West and our wetter loamy soils in the Midwest. The altitudes, temperature, humidity, soil type, plant type, plant size, grower expertise, and so on, are all different. Furthermore, just because 3” of rain equates to roughly 1/2 of a gallon (4,000 cu-in) of water over a 3-foot by 3-foot area, we just never know if this is the year that’s going to be super dry. In the end, we guesstimated a maximum of 2 gallons of water per plant per day in the driest of conditions.
Incidentally, the reason there is water data out West in part has to do with the fact that cannabis has been legalized in western states for some time. The other reason has to do with concerns over the shear volume of water that is used by either indoor operations or outdoor operations in arid conditions. Clean water is such a precious resource; if you think about it, growing hemp in climates where rainfall is more abundant like Wisconsin makes sense from a resources standpoint – albeit with a set of its own challenges related to shorter growing seasons and more temperature variability. Note: Prior to being made illegal, Wisconsin was once the second-largest producer of hemp in the United States of America, according to a 1918 report from the University of Wisconsin.
Plastic Mulch and Drip Tape
There is a fair amount of labor required to grow hemp. Fields need to be frequently inspected for mold and pests. Excess foliage within plants needs to be cleared, foliar sprays (compost tea) need to be applied, plants need to be watered, fertilized, and inspected. Harvesting requires huge amounts of hand labor related to trimming, hanging to dry, curing, and packaging. It’s just a whole lot of work.
In light of this, we started looking into plastic “mulch”. Plastic mulch consists of using a thin sheet of plastic that is laid down the row with drip tape underneath. The edges are buried in dirt to hold it in place. We know, calling plastic “mulch” is a bit of a “stretch” that is used by the industry. Nonetheless, we needed a way to lower the manual labor while minimizing environmental impact.
Our choice to use plastic mulch was an environmentally conscious one.
Given the benefits of weed control, soil heating, and mold prevention, along with the relatively small environmental impact, led us to consciously choose plastic mulch. The idea of weeding around thousands of plants by hand simply isn’t economically feasible. Labor costs money and weeds grow like crazy in our fertile soils.
So we could use machinery to bury weeds but the costs for maintaining and using a tractor and equipment on a regular basis are greater than the plastic mulch – greater not only in terms of our out-of-pocket expenses but also in terms of environmental impact. Besides, constantly working the soil destroys the structure and life-sustaining microbial life in the soil.
When it comes to indoor grows with their sterile soil “mediums” and minimal weed growth, the costs are astronomical. The cost alone for lighting indoor grow operations in the USA is enough to power 2 million average homes! Given that we want to be able to continue to refine our work to grow premium CBD hemp and improve our soils, we need to be a viable business.
So for the next few years, we’ll be working with plastic mulch and drip irrigation. In terms of minimizing the environmental impact of this approach, we’ll look to heavy gauge drip tape that can be re-used and also we’ll use the thinnest mulch possible. Who knows; with time, maybe we’ll be able to plant directly into a cover crop or use organic mulches. We’ll see; no one is doing this yet. At least for now, we’ll be planting a cover crop between rows to prevent erosion and nourish the soil. As we learn more and get some success “under our belts”, the operation will certainly improve.
Selecting Drip Tape
There are two basic bits of information that need to be understood about drip tape before the correct type can be chosen. The first is that modern drip tape is Pressure Compensated (PC). What this means is that the water flows at the same rate out of the hole closest to the water source as the hole at the end of the tape a hundred feet or more away. This is done by using an “emitter” that is affixed to the inside of the drip tape at each hole.
An emitter is a piece of plastic with a series of tiny convoluted passageways that the water must snake through before exiting out of the hole in the tape. By forcing the water to travel through an emitter, the flow rate for any given hole is limited. As a consequence, there is no pressure drop as water moves down the tape flowing out of the holes. The typical 10-15 psi of pressure on drip systems remains uniform along the length of the drip tape. As long as the pressure on the header piping that feeds the tape is kept above a roughly 6 psi threshold, the drip tape will distribute water equally along its length. (Graph: Discharge Rates)
The second bit of information related to selecting drip tape has to do with how drip tape is designated. For the most part, the two specifications that matter most are the flow rate of the tape and the spacing of the emitters/holes in the tape as they relate to zoning and pump capacity. There are other factors too like the thickness, which relates to durability and whether the tape can be reused, the tape’s width, and the brand (Netafin is a respected brand).
Emitter Spacing
Drip tape comes with uniform hole/emitter spacing. The spacing ranges from 4” all the way up to 60”. Tape with spacing at 4”, 8”, 12”, or 16” is readily available. Tape with 48” or 60” is harder to find. In order to select the correct emitter spacing, one needs to consider the soil type, plant spacing, plant requirements and the system design.
To begin, looking at the charts below, we can see that if the soil has a lot of sand, and therefore drains well, a grower may want to tighten up the spacing – to prevent dry areas between emitters. Conversely, if the soil is poorly draining clay, then the emitter spacing may be widened some. Similarly, the closer the plants are spaced, the tighter the emitter spacing.
In addition, it’s important to know what level of water is right for the plant. Does CBD rich hemp like to be watered heavily and then left to dry out a bit, or does it like a slow and steady drip? From reading various references, both under-watering and over-watering hemp will cause serious problems. Furthermore, novices tend to over-water their plants. The goal is to have a lightly moist soil that never dries out.
For ourselves, we decided that the typical 12” spacing in combination with our loamy soil should give good coverage.
By the way, we initially wondered if the emitter spacing should be 48” to match our 48” plant spacing. Given what we’ve seen with other operations, it seems clear that this spacing would not uniformly wet the soil under the entire plant especially as it grew and its root system spread. For example, the massive Los Sueños Farms in arid Colorado uses two Netafim drip tapes with 12″ emitters that deliver 0.9 gph to each plant.
In fact, in Brookdale Farm – Hemp Irrigation, they do a nice job explaining how both flow rate and soil type effect the “lateral movement” of water in soil – how big of a circle and how deep the soil is moistened. In general, the faster the water is applied and the less porous the soil, the more the water spreads out and the less it penetrates below the surface. Relatively speaking, the lateral movement of course sand is 0.5-1.5 feet, fine sand is 1-3 feet, loam is 3-4.5 feet, and heavy clay is 4-6 feet.
| Soil Type | Soil Characteristics | Emitter Suggestion |
| Clay | Does Not Drain Well | Low Flow: 0.11 – 0.16 gph |
| Loam | Drains Well | Medium or High Flow: 0.18 – 0.46 gph |
| Sand | Drains the Fastest | High Flow: 0.33 – 0.46 gph |
| DripDepot Helpful Guides | ||
| Emitter Spacing | Crops or Application | Other Considerations |
| 4″ | Flowers, Peppers, Greenhouses | Good for Sandy Soil, Short Runs |
| 6″ | Germination, Onions, Garlic | Tight Plant Spacings |
| 8″ | Germination, Strawberries, Vegetables | High Flow for Sandy Soil |
| 12″ | Good All Around Choice | If Low Flow Emitter Used – Great for Long Runs |
| 60″ | Blueberries, Hops | Long Runs of Plants Spaced Far Apart |
| DripDepot Helpful Guides | ||
Flow Rate
The flow rate of drip tape tells a grower how much water the tape delivers over a given amount of time. This value is either given in gallons per hour (gph) or gallons per minute (gpm). If the value is in gph, this is the amount of water in gallons that will drip out of each emitter each hour. If the value is in gpm, this is the amount of water in gallons that will drip out of 100 feet of tape. Yeah, it’s a bit confusing.
Let’s look at our case. As discussed above, we’ve decided to shoot for a maximum of 2 gallons per plant per day in the driest of conditions for our loamy Wisconsin soil. With our tentatively selected 12” emitter spacing, the question then becomes, “What flow rate do we need?” Time for some math.
At one our grow locations, there is a high capacity well pump rated at 45 gallons per minute. Furthermore, water is currently being delivered to the field through a 3/4″ garden hose – we’ll upsize this later if needed. Using the chart to the side, it can be seen that a 3/4″ line will flow 30 gallons per minute at 50psi. When we checked this by timing how long it took to fill a 5 gallon bucket, it took 12 seconds. This works out to 5 x (60/12) = 25 gallons per minute. The actual number was less than the theoretical value in the chart due to bends in the piping and other factors. (Graph: Pipe Flow)
Given the relatively high capacity of the pump along with the discussion above about flow rates and lateral water movement, we tentatively selected a 0.34 gpm tape with 12” emitters, and then looked to see if this met our 2 gallons per day target. Here’s how the math worked out.
First, we assumed that when the plants were fully grown, their roots would occupy the entire 4 feet between plants. In other words, that they would take water from 4 emitters (4 x 12” emitter spacing = 48” plant spacing).
Second, we calculated the time it would take to deliver 2 gallons and checked to see if the number seemed reasonable. To do the math, we first converted the gpm rating for 100 feet of tape into a gph rating for each emitter. We used the formula below.
flow rate per emitter in gallons per hour (gph) = (flow rate in gpm) / (100 x 12 / emitter spacing in inches) x 60
The “100” stands for the 100 feet of drip tape, the 12 stands for 12” in a foot, and the 60 stands for the number of minutes in an hour. Using this formula gave us (0.34) / (100 x 12 / 12) x 60 = 0.204 gph for each emitter. Note: If your drip tape is rated in gph, you can skip this step.
Third, knowing the gph for each emitter and the number of emitters feeding each plant, we then used the formula below. Specifically, we came up with (2/0.204)/4 = 1.85 hours to deliver 2 gallons of water to each plant. For our loamy soil, 2 gallons in roughly 2 hours should be no problem for that area of soil to absorb 2 gallons of water. Furthermore, the short 2.45 hours of time means we can add more zones later on should we decide to expand while still have enough hours in the day to deliver the water. So far everything looks good.
number of hours to deliver water = (gallons per day / flow rate in gph) / (number of emitters feeding plant)
Zoning and Pump Capacity
After we’d tentatively selected 0.34gpm (0.204gph) drip tape with 12″ emitters, the next question to answer was how many plants could the pump water at once. Knowing this would determine if we needed to subdivide the watering system into smaller zone. A simple measurement and a bit of math answered the question.
As noted, from filling a 5-gallon bucket, we found that the pump could deliver 25 gallons per minute. Given that we weren’t going to have a very long “header” pipe that feeds the runs of drip tape and assuming we’d sized the header large enough so it didn’t act as a “bottleneck”, the question of how much trip tape can the pump feed is a simple one.
number of feet of tape fed = (capacity of header pipe in gpm) / (drip tape flow rate in gpm per 100 feet of tape) x (100)
For us, this worked out to (25) / (0.34) x (100) = 7,353 feet of drip tape.
Note: If your drip tape is rated in gph, you need to convert the gph per emitter rating into gpm for 100 feet of drip tape. For example, using 0.204 gph and a 12” emitter spacing gives (0.204) x (100 x 12 / 12) / 60 = 0.34 gpm and this checks with the number crunching we’ve already done.
flow rate in gpm = (flow rate per emitter in gph) x (100 x 12 / emitter spacing in inches) / 60
Given that we’re spacing plants every 4 feet, this means the pump can water (7,353/4)= 1,838 plants at a time. Since we may plant up to 3,500 plants in that field, we would need 2 zones. And from the previous math, it will take about 2.45 hours to water each zone. Everything checked out OK so far but we needed to look at the capacity of the header pipe that fed the drip tape too – see Header Pipe below.
Drip Irrigation Design
Filter
The holes in the drip tape are really small. If you don’t want your drip tape plugging up and plants withering, you have to install a water filter. The recommendation is to install a 150 mesh (100 micron) or smaller filter.
The type of filter you’ll select depends on the level of sandy particulates and organic matter that’s in your water, the required flow rate, the type of maintenance required and the price you can pay. The articles Irrigation Water Filters and Irrigation Water Filtration & Filter Recommendations are helpful. In them, they discuss the differences between screen filters, disc filters, centrifugal filters, and the like.
| Water Source | Suggested Filter Types |
| Municipal Water System | Screen Filter, Centrifugal Filter, or Disc Filter |
| Well | Screen Filter, Centrifugal Filter, or Disc Filter |
| River or Creek | Disc Filter, Media Filter and Screen Filter, Centrifugal and Media Filter |
| Pond or Lake | Disc Filter, Media Filter and Screen Filter, Centrifugal and Media Filter. |
| Spring or Artesian Well | Screen Filter, Centrifugal Filter, or Disc Filter |
| Organic material in water | Disc Filter, Media Filter and Screen Filter, Centrifugal and Media Filter. |
| Sand in water | Screen Filter, Centrifugal Filter, or Disc Filter |
| Filter Size Equivalents | ||
| Micron | mm | Mesh |
| 800 | 0.8 | 20 |
| 500 | 0.5 | 30 |
| 300 | 0.3 | 50 |
| 250 | 0.25 | 60 |
| 200 | 0.2 | 75 |
| 180 | 0.18 | 80 |
| 150 | 0.15 | 100 |
| 130 | 0.13 | 120 |
| 100 | 0.1 | 150 |
| 100 | 0.1 | 155 |
| 80 | 0.08 | 200 |
| 50 | 0.05 | 300 |
| 40 | 0.04 | 350 |
| 30 | 0.03 | 500 |
| 25 | 0.025 | 600 |
| 15 | 0.015 | 1000 |
| Irrigationtutorials.com | ||
Pressure Regulator
Header Pipe
Header piping feeds the drip tape. For more permanent installations, PVC piping is often used. If you plan on driving over PVC, it needs to be buried. For ourselves, we planned on using the thin walled polyethylene piping. Not only can it take some abuse by being driven over, but installing connectors that fit the drip tape to the header pipe is childishly simply. You poke a hole in the header pipe and then press in a barbed plastic connector. Shazam!
It’s important to size the header pipe for your flow rate. For that, we looked at the chart above and the article, How Much Water Can Flow Through A Pipe. From these charts, we needed at least a 1-1/4” inner diameter (ID) header pipe. Note: These charts don’t take loses from any elbow, bends, and turbulence.
Unfortunately, inexpensive lightweight poly tubing larger than 1” is not readily available. Rather than going through all the extra work and expense of using PVC piping or special ordering larger poly pipe, we elected to use stock 1” poly tubing. The charts in irrigation sources list the following flow rates:
- 1” tubing at 10 psi has a flow rate of about 18 gpm at best.
- 0-5 gpm for 1/2″ pipe
- 5-8 gpm for 3/4″ pipe
- 8-13 gpm for 1″ pipe
- 13-22 gpm for 1-1/4 pipe
- 22-31 gpm for 1-1/2″ pipe
Hmm, what to do? Given that our header pipe runs are straight and on level ground, we used the upper end of the more conservative flow rates of 13 gpm. As such, our 1” header pipe at 10 psi can feed (13 / 0.34 x 100) = 3,824 feet of drip tape.
At a 4 foot plant spacing, this worked out to (3,824/4) = 956 plants. Given that we’re planting 3,500 high CBD hemp plants, we needed 4 zones using this smaller 1″ diameter header pipe.
number of feet of tape fed = (capacity of header pipe in gpm) / (drip tape flow rate in gpm per 100 feet of tape) x (100)
Timer
Given that both over and under watering limits performance, using a timer makes sense. How easy is it to forget to turn the water on or off while taking care of other chores? Too easy!
Although our preference would have been to buy a simple, non-electric, timer that a person turns on manually and then it shuts off the water at the set time, we couldn’t find any for our higher flow rate and header pipe size. As such, we elected to use battery operated programmable timers. A one-time-only setting that we can run once on days when watering is required addresses the over-watering concern when it’s rainy.
Barb Connectors
Punch a hole in the poly tubing header pipe and then stuff in a barbed drip tape connector. It’s that simple. We bought an inexpensive “punch” for our connectors to create the right sized hole while eliminating the risk of poking all the way through the pipe. Once the connector is pressed into the poly header pipe, the drip tape then presses onto the other end of the connector and is sealed by tightening a knurled nut.
We purchased connectors with a manual shutoff. This will help if we have to repair a line springs a leak without having to shut off the water. Also, it will allow us to experiment with feeding various nutrients to only some plants – so we can evaluate the benefit. Note: We’ve heard that injecting compost teas into the irrigation system causes drip tape to plug up with bacteria over time.
Drip Tape
Drip tape is thin flat tubing 5/8″ to 1-3/8″ wide (5/8″ is commonly used). Along the tape are tiny holes (emitters) spaced at a fixed interval. The size and spacing of the emitters determines how much water the tape delivers. Generally, tape is sized based upon the soil type, plant type, plant spacing and so on – see Selecting Drip Tape above.
It’s important to lay drip tape with the holes up. This minimizes the risk of dirt, that invariably gets inside the tape, from plugging up the emitter/hole. The dirt will settle on the bottom of the tape away from the hole.
If you’re using planting equipment, your drip tape is offset about 6″ from the center of the row so it’s not damaged by the machinery. Furthermore, the tape itself is best laid a couple of inches below the soil. This prevents the tape from forming into a snake-like line over time due to being heated by the sun.
When designing your system, it’s important to stay within the manufacturer’s pressure and maximum run (lateral) specifications. If you exceed the pressure rating, you’ll split open the tape and if you exceed maximum lateral runs, the water won’t be uniformly distributed along the tape. Each manufacturer has different specs.
In the two chart sections shown, the AquaTraxx 0.34 gpm (0.204 pgh) tape with 12″ emitter spacing uses 10 psi and has a maximum run of 381 feet up a 1% grade. In contrast, the Netafim Streamline 0.35 gpm (0.21 gph/0.8 L/hr) tape with 12″ emitters has a 11.6 psi (0.8 bar) operating pressure with a maximum run of 351 feet up a 1% grade.
Note: AquaTraxx specifies uphill as negative numbers while Netafim uses positive numbers.
Vacuum Break
Putting It All Together
After figuring out that we wanted to use 0.34 gpm (0.204 gph) drip tape with 12″ emitters (based upon our soil type, climate, and hemp’s water requirements), we measured our pump flow rate at 25 gpm. At this rate and for our drip tape, we figured we could water 1,838 plants at a time in 2.45 hours. This meant that for the 3,500 plants we would need 2 zones.
However, in looking at the header pipe, we found that a 1” header pipe at 10 psi can only feed 3,824 feet of our 0.34 gpm (0.204 gph) drip tape and this equates to 956 plants at a 4-foot spacing. Given this, the header pipe is the “bottle neck”. As a result, we increased from 2 zones to 4 zones.
Another important factor to consider was the maximum allowable drip tape length. Since we used Netafim tape, we knew from the specs that the maximum length was 351 feet at 10 psi. At a 4-foot spacing, this worked out to (351/4)= 88 plants per 351 foot length of tape.
However, given that we wanted to use up the entire width of our 350 foot wide field by dividing it up into four 88 foot long zones, the maximum length didn’t come into play. This is because at a 6-foot row spacing, we had (350/6) = 58 rows – we left enough room to be able to mow the cover crop between the rows. At 58 rows, we only need (3,500/58) = 60 plants in each row and this means that at a 4-foot plant spacing, the rows will only need to be (60 x 4) = 240 feet long. This is well under the 351 foot maximum allowable.
