Results in Roof Growing 2009: Emptying Your Sub-Irrigated Planter

In this link, we show you how to make a two-bucket SIP, or sub-irrigated planter. But what happens when the growing season ends?

You pull the top bucket out and get to view plant behavior underground. In this photo, the lefthand bucket grew greens with shallow roots. On the right, a tomato.

Note the tomato roots have plunged all the way down
to the wicking cup and out its slits into the water reservoir.

This year we tried something new with buckets. While a majority of our SIPs are made from 5-gal in 5-gal recycled food-grade buckets, last spring Art ordered a bunch of 6-gal and 7-gal food-grade buckets to make sets with larger reservoirs.

Click the photo to see the height of the overflow hole
in these 5-gal SIP sets.

Now check the height of the overflow hole
in this 6-gal in 7-gal set.

We haven't measured exactly how much more water the larger bucket set holds, but it's considerable. You don't need a larger bucket set. It just cuts down on the frequency of watering at the height of summer, especially for the heavy drinkers, tomatoes and eggplant among them.

We also wondered if these same veggies might benefit from the extra space for growing medium--obviously more potting mix fits in a 6-gal bucket than in a 5-gal bucket. I'm not sure we saw any difference, but summer weather was very odd this year so it's tough to compare.

I thought you'd be curious to see the root system in the larger buckets. To make the wicking cups for the larger SIPs sets, we used recycled two-liter pop bottles with the top cut off and slits downs the side.

Look at the density of eggplant roots
in the two-liter wicking cup.

These ropey eggplant roots
know where to head for water.

A long view.

The Official Earthbox™ Planting Guide says you can re-use potting mix year after year without emptying the boxes, but I don't do that with my two adopted earthboxes. I believe any type of SIP needs emptying, airing, and drying out to limit the chance for disease.

With bucket SIPs, because the plants so completely occupy the growing space, you need to extract the roots at season's end and toss them away to compost somewhere. Your goal is to pull out as much of the organic matter as possible.

Pulled root mass with potting mix
shaken off for re-use.

We combine used potting mix with fresh mix in a large bin and use it again to plant the following spring. Our blend contains mostly peat and perlite with a little coir. Mea culpa: we're going to try to use less peat next season and boost the coir--coconut fiber that's vastly more renewable than peat.