Q. I need to water a 2.5′ wide by 21′ long grass strip in the middle of my driveway. What is a good method for this narrow an area? My home is located in Southern California.
A. Irrigating lawn in areas less than 4′ wide is very hard and results in a lot of wasted water. It is illegal to install a grass area less than 4 to 6 feet wide in many cities, especially in California and other western States, including ALL of Arizona and most of Nevada (the minimum width varies from town to town.) Enforcement is typically limited to new development, but if you get a permit from the city for the work you may get nailed on this issue.
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If you do use sprinklers there is going to be a lot of water waste from over-spray onto the concrete. It will likely run down your driveway and when (not if) the next big drought cycle hits and they start with the “water police” thing you will likely have to stop watering your strip or risk a “fix it” ticket.
If you do use sprinklers you will reduce the radius of each sprinkler to your 30″ width and then you reduce the distance between sprinklers by a similar %. I recommend using side-strip sprays rather than the center-strip type as you will have a lot less over-spray on to the concrete with them. The side strip side are installed down both sides of the strip. Center spray types are installed down the center of the strip. Using center strips type will require half as many sprinklers, but the cost of this initial savings is lousy performance, poor efficiency, and lots of wasted water (it is common when using center strips that 50% or more of the water applied will be wasted.) So let’s say you decide to use 4′ x 12′ pattern side-strip spray nozzles in 4″ pop-up bodies. Since your area is only 30″ wide you would need to reduce the spray width from 48″ to 30″. That would be 62% of 48″ ( 30″ / 48″ = 0.625). So you would also need to reduce the 12′ distance down to 62% also, which is 7.5′ (12′ x 0.62 = 7.44′). So in your 30″ x 21′ area you would space the heads 7′ apart on both sides. After installing the system you would reduce the radius of each head as best you can using the radius reduction screw. It is unlikely you will be able to avoid some over-spray onto the walk as noted earlier. If you decide to use center strip nozzles the procedure and spacing would be the same, there would only be one row of heads, however, installed right down the center of the strip. With center strips you will have to allow more water to overspray onto the cement driveway, if you don’t you will get dry yellow edges. If you want a better explanation of why see this page on sprinkler spacing.
Using Subsurface Drip
Your other option is to use subsurface drip. This is what I would do. In this case I would use three drip tubes running the length of the grass strip. Place one down the middle and the other two should be 4″ in from the edge of the driveway concrete on each side. Use dripperline with 1 gph emitters spaced 12″ apart. Netafim, Rainbird, and Toro all make subsurface dripperline. Make sure the dripperline is a model that the manufacturer claims in their literature is for subsurface installation. Subsurface dripperline uses a different type of emitter designed to keep out dirt and roots. Read my drip guidelines for info on filters and pressure regulators you will need. The salesperson at the irrigation store may tell you that you only need two tubes, which normally would be correct, they typically are spaced 18″ apart, not 12″. There are a couple of reasons I am suggesting 3 tubes rather than 2. First is to get the total flow up because the area is so small and most automatic solenoid valves don’t work very good at really low flows. Another reason is that the concrete on both sides absorbs and radiates a lot of heat, and this is going to make your little lawn strip dry out fast. That’s also why I suggest the dripperlines at the edges of the area be 4″ from the concrete, otherwise the lawn edges right up against the concrete tend to dry up and turn yellow. You are going to need to be careful in selecting your valve, the dripperlines in your 21′ long area are so short that the total flow using 1gph emitters is only going to be 1 GPM; (3 tubes, 20′ long with 1gph emitters every foot. So 20′ x 3 tubes = 60′ of tube. 60′ of tube x 1gph/ft = 60 gph. 1 gpm = 60 gph.) A lot of automatic valves will not work at flows that low. Make sure the rated flow range in the literature for the valve goes that low.
To install your drip system remove the top 5″ of soil from your planter. Now till the soil another 4″ deep. Tamp down the soil to lightly compact it and get rid of air voids. A 8″x8″ hand tamper tool is good for this, you can buy one at any decent garden shop or home improvement store. Now place your dripperline tubes down on top of the soil and use steel erosion control staples to hold the tubes in place. Put a stake every 36″. You can buy the stakes at the irrigation store, they all carry them. The metal stakes work much better than the plastic ones made for drip tube. The stakes are very important, they will rust into the soil and hold the tubes in place. Without them the empty tubes will float to the surface during the winter when the soil becomes saturated during rain storms.
Now put down the final 3″ of soil over the top of the tubes, tamp it down and install your sod (which should be about 1″ thick and should bring the sod surface up even with the top of the concrete.) You will need to lightly hand water the sod for a week or two to keep it cool and moist. It needs time to grow roots down to where the subsurface water from the dripperline is. Slowly back off the hand watering after a couple of weeks. Watch the sod’s color to see how well it is rooting in. If the sod is still in need of top-watering by hand it will turn a dull “flat green” color. When you first install it, the sod will be that dull flat green color because it is stressed from the cutting,shipping and installation. It’s easier to see the color if you stand back and look at it from a distance. Right after you install the sod take a minute to look carefully at it and notice the stressed dull color. Then also note the brighter green color it changes to after you water it the first time it. Now you know what stressed sod looks like and what to look for over the next few weeks. If it is hot or the warm winds are blowing when you install the sod you may need to hand water it more often. Usually watering a couple of times a day is sufficient until the sod is established.