Rough plan for small pot-in-pot planter made from plastic milk bottle

Use post-consumer plastic bottles for this project! We get milk in 3 litre bottles with handles as in the attached sketch and I think 3 litres will be a suitable size for a basic stacking unit of the "strawberry tower".

Cut an access hole near the top of the milk bottle. This hole must be big enough for a small plant pot to be put inside the milk bottle. Cut a drainage hole near the bottom of the bottle, about 2 cm above the bottom.

The water reservoir will hold about 500 ml. A tube made of synthetic fabric (spunbonded polypropelene or polyester - think geotextile, agricultural fleece, or nonfusible nonwoven interfacing) will connect the water reservoir to the plant pot. Right now I am thinking I will make a tube of synthetic fabric about 5 to 7 cm high and 2 cm in diameter and fill it with sand to act as a capillary tube to wick water up to the plant but I have not tested this yet.

A "floor" is needed to separate the water reservoir from the upper chamber of the bottle. Cut a margarine container down to about 2.5 cm in height, cut several drainage holes (about 1 to 2 cm in diameter) in its bottom and invert it --bottoms up!-- in the base of the milk bottle. The margarine container will not fit perfectly but it should be large enough to hold the actual "floor". Cut the "floor" to fit the milk bottle cross section. Use a disposable plastic plate or similar material to make the "floor". Remember drainage holes. We are trying to make a "floor" sufficiently strong and stable to support the sand/coir and the plant in its pot. Do not insert the "floor" in its final position just yet.

The capillary tube will have to pass through the "floor". Cut a 2 cm hole near the centre of the "floor" and thread the tube through the hole so the "floor" is at the approximate mid-point of the tube's length. Now the "floor" can be put in position on the margarine tub. The capillary tube should reach to the bottom of the water reservoir.

Now put about a 2 or 2.5 cm layer of the sand / coir mix on top of the "floor". The capillary tube should not be buried. Make sure that the end of the tube extends out of the sand mix distinctly. The plant pot will have to be centred on the tube, and good contact will have to be made between the tube and the potting soil in the plant pot.

Now take the plant pot that will be put inside the milk bottle chamber. Cut several 1 cm drainage holes in its base, and in the centre of the base cut one 2 cm diameter hole (or whatever diameter the capillary tube ended up as). Insert the plant pot into the milk bottle and make sure that the capillary tube reaches through into the plant pot. Fill the pot with potting mix and plant the seedling and water in.


Use the overflow hole to fill the water reservoir. Occasionally it may be necessary to add water from the top - because constant bottom-watering may cause salts to build up on the surface of the potting mix - but generally watering will be done via the water reservoir. Evaporative loss should be fairly small. It would be interesting to see whether the relative humidity inside the shell formed by the milk bottle is very different from the humidity outside, but I don't have a good way to quantify it. The test of this growing system will simply be whether the plants thrive, and I hope they will.

Now for something completely different. I expect my new flush cutters to arrive tomorrow so I am likely to get obsessed with wire work for a while. This is probably the last post on Gardening for at least two or three weeks.