The Pros and Cons of Flow-Control and Free-Flow Irrigation Systems

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Free-flow irrigation is the simplest form of irrigation. Examples of free-flow irrigation are deep water culture, flood-and-drain/ebb-and-flow, nutrient film, and some styles of drip irrigation. Hand-watering falls in a grey area. Methods that involve weighing, watering, and re-weighing are closer to flow-controlled, while “just spray the hose at the plant for a while” methods are closer to free-flow. As a general rule, free-flow methods rely on the plant to take what it needs from a large supply of water and nutrients, rather than the supply of water and nutrients being dictated by the grower.

gravity drip irrigation
gravity drip irrigation

Pros of Free-Flow Irrigation

One of the biggest draws to free-flow irrigation methods is simplicity. Deep water culture, or DWC, is about as simple as it gets: just let the roots grow in a pool of aerated water. This style of irrigation has very few moving parts  if any at all), is easy to set up, and requires relatively little maintenance. Ebb-and-flow systems are a little more involved, but still far less so than a standard drip system.

When it comes to nutrients, free-flow systems offer one big advantage over flow-controlled systems: they can run just about any nutrient. It’s pretty much impossible to clog something like a DWC or flood-and-drain system. Free-flow drip systems lack the small channels used to control flow rate that standard drip systems do, making them much harder to clog. If you’re looking to run something like molasses, kelp, or another nutrient mix with high viscosity or large amounts of particulate matter, a free-flow system is likely right for you.

The other big benefit of free-flow systems is that they tend to be less expensive than flow-controlled systems. Free-flow systems are usually the ones that pop up when you search for “how to grow weed cheaply” or “grow weed in grandma’s house without her knowing,” as it doesn’t get much cheaper than putting a potted plant above a tub of water with an aquarium pump in it. Many free-flow systems are made using jury-rigged or reclaimed items, or with standard (and inexpensive!) schedule 40 PVC. Free-flow drip systems cost less than their more precise cousins because all of the flow control devices – your spray heads and drip emitters – aren’t necessary, and the decreased danger of clogging means filters can safely be skipped in many scenarios.

Cons of Free-Flow Irrigation

Free-flow systems are highly inaccurate. Since there is nothing regulating how much water a given plant gets, it can be near impossible to know how much of any given nutrient is being delivered to said plant. You may have 15ml/gallon of nutrient A, but there’s no way of knowing what portion of that gallon went to plant A.

Free flow systems are also very imprecise. Yes, “precision” and “accuracy” mean different things! Without any flow-control devices, there is no way to compensate for pressure within your irrigation system. This means that a free-flow drip system will give far more water and nutrients to the plants at the beginning of the system than the ones at the end. When you combine this with the inaccuracy of a free-flow system, you are not only unable to measure how much of nutrient A your plants get, but you are also unable to ensure that plant B gets the same mystery amount of nutrients as plant A does.

Both of these cons can be mitigated somewhat with some extra attention on the part of the grower, but that can require a lot of time that the grower may not have or be willing to spend weighing plants and measuring runoff.

Flow-Controlled Systems

Flow-controlled irrigation systems are inherently more complex than free-flow systems, as they ensure a measured amount of water is delivered to each plant. Drip irrigation accounts for most flow-controlled systems, although outdoor sprinkler-based systems are also technically flow-controlled. If free-flow is a shotgun, flow-controlled is a sniper rifle. The water and nutrients each plant gets are dictated by the grower, and various technologies can be used to tailor watering to the needs of each plant.

Pros of Flow-Control Irrigation

The biggest pros of flow-controlled systems are the biggest cons of free-flow: flow controlled systems are highly accurate and very precise. Between the physical emitters themselves and the various automation options, plants can be fed the exact amount of water and nutrients they need, exactly when they need them, with minimal waste generated and low variation between plants. Pressure compensation allows growers to ensure that each plant is getting fed the same amount as all the others on a given system. This precision and accuracy takes the guesswork out of budgeting for nutrients and greatly reduces the logistical problems that runoff can cause.

Flow-controlled systems are also famously flexible. By their nature, free-flow systems force a grower into a one-size-fits-all irrigation scheme. Flow-controlled systems let the grower tailor their irrigation to plants of different strains, maturity levels, and growing media. A flow-controlled drip system doesn’t restrict how you need to grow the plant, either. A flow-controlled system can be used to water a multi-tiered rack of vegetating plants, a closet full of clones in dixie cups or a greenhouse of flowering plants in 200-gallon pots. With the right design, the same system could even do all of them at the same time!

Cons of Flow-Control Irrigation

Flow-controlled systems are complex, and can be intimidating to design and set up. There are a lot of moving pieces, a lot of connections, and a lot of math involved in making sure everything works correctly. A small error in the available pressure or having a few too many drippers on a line can cripple an entire system, and troubleshooting issues can be time consuming. Not to say that you can’t learn how to do this, but it takes time. Consulting with an irrigation professional is highly recommended when it comes to using any drip system, especially a flow-controlled one. Another downside of flow-controlled systems is that the potential for user error is much larger than with their free-flow counterparts. Sure, your dripper may emit exactly one gallon every hour, but a sufficiently inattentive grower can still mess it up. Without a controller it can be easy to turn the system on and forget it, leading to an unintentional flood. Things can get out of hand even with a controller, as a small mistake during programming could mean that your plants are getting watered for one minute three days a week instead of one hour three days a week.

Systems with flow-control also tend to require a larger initial capital outlay than those without flow-control. This is especially true of automated systems, as automation options can range from a $30 hose end timer to integrated systems costing upwards of $200,000. Individual drippers and rolls of tubing are fairly inexpensive, but the total initial cost for a grow with thousands of plants can be substantial.

There is one major point to be made that is an exception to the things I’ve said about cost. Namely, automated, flow-controlled irrigation systems end up saving most cultivators money in the long run. This is especially true when compared to hand watering. The initial cost can be high, but over the life of a system the time savings add up considerably.