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.

Musing on the under-pot zone

Have you noticed how potted plants try hard to get their roots out of the container? Potted pelargoniums, marguerite daisies and cordylines dawdle along until they get their roots into the soil underneath the pot and then they put on a spectacular flourish.

What causes this? It can't simply be that their blueprints tell them to grow their roots to a certain defined extent, because roots try to escape from very large pots as well as small ones. They must be looking for greener pastures. Perhaps the soil under the pot is cooler and moister than that inside the pot, or perhaps there are better combinations of water and air in the under-pot zone.

I am thinking about this in designing the strawberry towers mentioned in an earlier post. I would like the plants to be as happy as possible in their hanging containers while at the same time I would like them to be low maintenance and only require watering and turning every two or three days.

So I am thinking about ways to simulate the under-pot conditions that plants seem to like and ways to create these conditions in the hanging containers. My earlier designs for the hanging containers included a water reservoir (like the pop bottle planters or a self-watering pot) but now I am inclined to try a base that is filled with coarse sand (like a miniature capillary watering tray) with a top unit containing potting mix set on top of the sand. Plant roots should be encouraged to spread from the top unit into the sand-filled base.

Experiments are needed to establish whether plant growth would be better in this type of system than in the simple water well pots, and to establish whether plants would prefer the top unit to be set into the sand or suspended slightly above it, and whether sub-irrigation or top watering would be best, and whether fertiliser pellets would be better placed in the top unit or in the sand... There are lots of possibilities to try. I looked on the internet to see if anyone has done any of these experiments already. Pot-in-pot growing looked similar but I didn't find enough published information to answer my questions. I will have to find out the slow way.

Summer will officially be finished in two weeks so I had better get busy.

Thinking about the strawberry tower

I say "strawberry tower" although what I have in mind is actually a hanging garden, a column of plant pots suspended one above another.

Several years ago, Geoff & I grew strawberries hydroponically in hanging tubes filled with perlite medium. No electricity was available on site so I watered them manually. They cropped satisfactorily, but only if they were watered with nutrient solution at least twice a day, and three or four waterings per day was better.

Growing strawberries in hanging containers is great - you don't have to bend down to pick them and they are out of reach of snails and other ground-based pests. The need to water them so frequently was a big minus, though. I don't want to set up an elaborate automatic watering system and I don't intend to hand-water 3 or 4 times a day, so I want to see whether putting a water reservoir in each individual hanging pot will solve the watering problem.

Simply hanging PET bottle planters in a vertical chain would meet the minimum requirements of what I have in mind, although if each planter had to be individually watered it would be very tedious. Ideally, I want to be able to water all the planter units in a column at once. Plastic tubing is going to be involved.

In spite of having to use a bit of plastic tube, the aim is to build planter units that re-use materials that would otherwise become landfill or go to be recycled. Construction must be simple, without the need to use special tools. If possible, I would like to find a planter unit that can be used for conventional growing using potting mix as well as for hydroculture. (Increased weight of potting mix will put greater demands on the suspension system. Potting soil could have possible drainage problems, as well.)

I have three different types of planter unit on the drawing board, and will try to have prototypes ready for planting by Monday so growing trials can get underway. More anon.