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Dig a big hole, find a big, strong, undrained container, sink it about 3-4" below ground level, fill it with peat moss, and plant your plant.
Long Version; History and Reasoning:
I invented this system after pondering a system used in Australia and New Zealand for raising plants that demand dry crowns but a constant availability of water at the roots. Plants such as Boronia megastima, B. heterophylla, B. molloyae, and Raoulia australis are all good examples. Think plants that want to grow in granite scree and get watered twice a day. The method used is to dig a planting hole (size unspecified) and line it with plastic, then punch a set of holes half way up and fill below those holes with peat moss. The upper section is backfilled with native soil and the plant is centered within the hole.
I pondered what was going on there when I wanted to plant my second Gunnera manicata, a giant hardy ornamental foliage plant related to Rhubarb, with leaves to over 4' across. The first died because I just didn't water it enough. It was mixed in with plants that needed much less water and I didn't have a sophisticated watering system to account for that difference. I figured if I could give it a water bank I could water it on the same schedule as all my other plants, which is whenever I get around to it. I wondered if I could adapt the method to larger scale plants.
So I decided to think big. I went to Orchard Supply Hardware and bought a heavy duty plastic whiskey barrel (which is just a brilliant idea itself, looks real, no hoops to fall off, doesn't leak). Not a thin whiskey barrel liner mind you, an actual, full-sized, fake barrel. I haven't seen the same model sold there since, unfortunately. The one I bought had very thick plastic walls, presumably PVC, thicker and harder than any gopher was going to get through easily.
Next I pondered the reason for using peat moss in the lower section of the planting hole and wondered whether or not gravel or some other more open material might both hold more water and remain more oxygenated. I decided that oxygenating that water under the plant would not be practical by almost any method and that the original system must work or it wouldn't be used. I also realized unless roots grew down into that water it would be unavailable to the plant. As the water level dropped the root would have to grow down to access it. When the level was high it would probably go sour and kill the root and I would be back to the start. The water had to come to the plant, not the other way around.
Then I realized that the peat moss is the magic ingredient since it has the wonderful and almost unique ability among soil amendments to wick water and evenly distribute moisture. Its acidity inhibits pathogens. It holds an enormous amount of water. It lasts a long time.
When I thought about punching holes in my beautiful container I realized that this would let other plants (like the Japanese Maple) get their greedy, grubby little roots into that water and steal it all. I decided that the Gunnera, which I have seen both at Strybing and at another private garden in Santa Cruz thriving directly adjacent to open water, could take relatively high soil moisture levels and as long as the immediate crown was dry. I decided not to drill holes.
My decisions made, I dug a whiskey barrel sized hole in the yard that the kids had enormous fun jumping in and out of, then sank the barrel about 3" below soil level, filled it with peat moss almost to the brim, filled with soil from there up, and planted my 4" Gunnera on top of the whole mess. It worked, boy did it work.
It grew to over 6' tall within 3 months. It was a monster. It clearly loved being grown that way. I watered every once in a while, when I remembered, which was every couple of weeks or so. It was so easy! And people marveled at the size of the plant and were properly amazed at how quickly it got there.
I have tested a tree fern (Cyathea atrox) using the same method and it has done fine for two years, through two winters, with the water being funneled down the walkway and over its base in order to fill the container during dry periods. It works like a charm. The Elegia capensis (Restionaceae) next to it borrows the water and is the largest example of its species I have ever seen, over 9' tall. The Darmera peltata (a native foliage plant in the Saxifrage Family) and Lysochiton (Skunk Cabbage, another large foliage plant) are planted in the same container and loving it. My blueberries loved it. The Solanum peruvianum (Cape Gooseberry) next to the blueberries loved it. Everything I have tried has loved it.
What's happening is you are in effect is placing a gigantic sponge underneath whatever you plant. The only thing to remember is that when you water you really have to fill that container up. Also, it is important to plant the container a little below grade because this allows the finer textured soil to act as a siphon (via its greater surface area and the miracle of surface tension) to draw excess water off the top of the container. You really don't need holes in the sides, though if you had an invasive plant in the container, or closeby trying to get into the container, you could simply go ahead and drill those holes and lift the container out of the soil a little. I take a shovel around the perimeter once year to take care of any adventitious roots.
This method can almost certainly be adapted for use with other high-water-demand plants such as Ligularia, certain ferns, blueberries, the water loving varieties of giant aroids (Alocasia, Colocasia, Xanthosoma), or Giant Horsetails (Equisetum giganteum, E. myriochaetum). It can also probably be used on those tough to grow plants that like seasonal inundation but dry summers, like the aforementioned Boronias as well as many of the Restio family members, and maybe even South African heathers. You can use containers of different sizes. There is no reason to use a whiskey barrel for everything. Size the container to the plant and anticipated water demand.
Experiment, but remember you are experimenting so if something dies don't bother calling me to complain. I have had nothing but spectacular success so far and think the possibilities are endless.
Frequently Asked Questions
Won't the salts from my water accumulate and damage the plant?
Salts can accumulate in soils. Plants generally incorporate into the tissue whatever is in the water though, so as you remove old leaves and branches you are removing some of the salts that accumulate. I would assume rainwater helps in cutting the salt concentration throughout the year. If there is a salt problem you should find the soil on top growing hard from crystals cementing the dirt together as the water wicks upwards and evaporates. By simply replacing this upper layer of soil you can often remove a large portion of the salts. So far for me, with average city water and 25 inches of rain, I haven't had a problem in several years. Maybe in a dry climate like Southern California with Colorado River water it will be a problem. Maybe not. Your plants may remove salts as fast as they accumulate. You will just have to see.
Won't I kill my plant?
Maybe you will. It is an experiment, remember? Actually this isn't that big a deal, if the plant is struggling just dig it up and move. So far I haven't had any problems, but I am only trying things I think would like to be planted that way. Be selective and use common sense. For example, my Cyathea tree fern is doing well but I would suspect a Dicksonia might not be so forgiving. I would be alert for signs of problems with that one. Also among tree ferns, C. cooperi would probably like it but the deep rooted and very drought tolerant (and also commercially non-existent, at least in the US) C. australis would probably die.
Won't it make my plant rot?
It might. But usually rot starts when roots get stressed, the cell walls get weak, and fungi can enter. If the plant never runs out of water it never gets stressed from drought. It can get stressed by running out of oxygen, but in most cases it will have enough oxygen from roots closer to the surface. But I cannot guarantee you will not lose rot sensitive plants if you plant them over a whiskey barrel full of peat moss. This is for plants that like it wet, remember? And those usually have mechanisms for dealing with overly wet soils anyway.
What about fertilizing?
Go ahead. I haven't had any problems but I have only used half strength soluble types. I would NOT use granular at all, or full strength solubles except on full sized plants and even then carefully. No guarantees here. Be conservative. You can always give more later.
Does it have to be peat moss?
I think so, but I am not sure. Let me know if you find another magic ingredient. Coir might be worth playing with.