Sub-irrigated Planter (SIP)
What is a sub-irrigated planter (SIP)? Click here for a Wikipedia article on it. It is this marvelous design that has totally opened up the possibilities of apartment gardening. Without it, the challenge of keeping a plant properly watered is a daunting affair. When I discovered the SIP idea on YouTube, it made complete sense to me and encouraged me to try it for myself. It's also called an SWC, which is "self watering container."
How Does a SIP work?
The main principle is this: you have a pot that houses a water reservoir beneath the soil at the base of the pot, and a side opening is provided for adding water as well as allowing excess water to drain out. What this essentially does is provide a potted plant with a continuous source of water without super saturating the root system. A typical pot without drainage holes sitting outside will become flooded in heavy rains. It's also harder to gauge how often to water the plant so that the lower half of the pot isn't water logged (best is just moist). A pot that has drainage holes will allow water to flow out, but no reservoir exists for the plant to have continuous access to water (only a small amount in the pot saucer that the lower most roots can sip at while water is still present).
How Expensive is a SIP Container?
That all depends upon how much you want to spend. If you want, you can buy a completely pre-made kit. The two most popular ones you'll see floating around the internet are the "Earth Box" and the "Grow Box". These two retail products are essentially doing the same thing, but with slightly different designs, quality of parts, and prices. The Earth Box is roughly $60 and the Grow Box averages around $40, depending upon what options you buy (they each offer different sizes and accessories). You can also buy generic planter pots that feature a built-in water reservoir and essentially achieve a similar principle, for quite a bit less (I bought mine at Home Depot for just $8 each). Lastly, you can make your own SIP for less than $10, or even cheaper if you can scrounge up the main materials for free.
The advantage of making your own is that you can customize it exactly as you wish. The cheaper you go, the more unsightly it'll probably end up (like utilizing a discarded 5 gallon painter's bucket). But when first starting out, it is probably best to buy something already made and get into the experimentation in the next season. Either way, the great thing about a SIP is that anyone can afford one by choosing the appropriate materials.
How is a SIP Container Designed?
Here is an image of a self-made SIP, courtesy of Wikipedia:
In this variation, two buckets are used and the watering pipe extends up through the top. While this design is fairly convenient to make, the main problem is that an entire bucket that could be it's own SIP is being used as a reservoir. Frankly, it's not hard to create your own reservoir in an existing bucket. Also, you'll frequently see designs using PVC tubing for the watering pipe, but not everyone has easy access to PVC and of a sensible diameter for the job. If you're doing a large array of SIP containers and set up automated watering, the top mounted watering pipe is a sensible approach. But for the apartment gardener that may have only a handful or fewer pots, a side mounted flexible tube with funnel is an easier way to go. If you go searching around on YouTube, you will find a number of SIP design variations and you can pick the one that makes the most sense for you, both in terms of effort involved and cost of parts.
Some SIP Considerations
The Top Covering
One primary consideration is the top of the pot. Some people utilize the 5 gallon bucket solid plastic cover with a central cut-out for the plant. The top has a radius slit at one point where the top can be flexed to slide around the plant. Basically the top is pretty well shielded from water coming in from above, making the plant mostly dependent upon a manual refill of the reservoir. Some people prefer this method because the top acts as an excellent barrier to foil potential weeds and also eliminates soil splash from hitting the lower branches of the plant (something known for spreading disease). The cover also helps with moisture retention so that the soil does not easily dry out at the top. As other options, you can cover the top with a landscaping cloth (synthetic material that is permeable for air and water, but does not rot), plastic sheeting, or mulch. Much of this will depend upon your growing environment.
- Hard plastic lid with cut-out for plant and a slit to accommodate easy installation around the plant.
- Plastic sheeting, with a center cut-out for the plant (e.g. "X" slits), secured around the planter with string/elastic.
- Landscaping cloth can be used as a barrier that allows air and water to pass while keeping pests out. The material is petroleum based so it's not exactly eco-friendly.
- Mulch is an excellent material for covering, which is not only organic material but also does a great job of retaining soil moisture while allowing rainfall to pass into the bucket. It is probably not a good idea to use it in environments that experience frequent heavy rains, as you'd probably want to protect the soil from having too much water pass through (which can accelerate the leaching of nutrients).
NOTE: If you have issues with fungus gnats, mulch will become a haven for them. Not recommended unless you are able to dispense with those annoying insects.
There are several ways that you can build the reservoir at the bottom, depending upon what materials you have access to. The most popular method I've seen is to have a thick plastic circular shelf with a cut-out for a slotted basket. The main idea is that you have a soil barrier where the water reservoir exists beneath and a central section that penetrates into the reservoir to serve as a "wick" that delivers moisture up through the soil. The wick can be tightly compacted soil, perlite, moss, or a mix of different things. The main idea is to keep the soil below consistently moist by being in controlled contact with water, without becoming too saturated. Some methods involve using other materials to create a raised surface with a central plastic bottle punched through with holes.
My Choice
Before I settled on a SIP design and began to acquire the materials, I stumbled upon a low priced nice looking sub-irrigated pot at Home Depot, by Southern Patio Dynamic Design. The pot space is equivalent to about 5 gallons, including the water reservoir. There is an overflow opening on the side that you could add water to, but with the open top I sometimes add water through the top if there hasn't been any rain for more than 5 days. Instead of using plastic, I covered the soil with a layer of mulch that helps retain moisture, warmth, and discourages weeds. I figured I'd give these a try and see how they work out. Maybe next season I'll try my hand at a self-made SIP. |
| Southern Patio -- Dynamic Design |
"Self-watering reservoir lets you feed your plant from the roots up.
Plant absorbs the precise amount of water it needs every time.
Easy access reservoir allows you to monitor water levels and water less frequently."
