When I started Good Scents I really wanted to be able to grow astilbes and trollius for cut flowers so decided to try to artificially create the conditions they need. The solution I came up with has worked surprisingly well and so I wanted to pass it on. I created large 4'x25' beds but the same system could probably be used on a smaller scale. The basic idea is to create a growing area that drains slowly but is also higher than the surrounding ground. This way the crown of the plants will not flood, but the roots will always have access to plenty of moisture. This is what I did:
- Construct a raised bed frame around the new bed using 2x8 lumber. If you are doing a smaller area you can probably skip creating the frame as long as the resulting bed is higher than the surrounding ground.
- Excavate about 18 inches of soil from the bed. This will make a big mess and yes, it is really hard work. When you are done the distance from the top of the frame to the bottom of the excavation is about 2 feet. If you aren't using a frame it will be about 18 inches.
- Line the bottom and sides of the excavated area with a single sheet of heavy duty (6-8mil) plastic. Plastic like this comes in 10 foot widths so there should be plenty if the bed isn't too wide. If you are using a lumber frame, the plastic should extend to the top of it.
- Use a pitchfork or other tool to punch holes in the plastic. I made maybe 5 pitch fork punches along the 25 feet. The holes prevent too much water from accumulating in the bed.
- Put the soil back into the bed, adding plenty of compost, peat moss and fertilizer as you go.
- When you're done the soil will be well over the top of the lumber frame. Much of this will settle so don't be alarmed.
- Plant your thirsty plants in the new bed and mulch.
Over a couple years I constructed 4 of these beds and have never before grown such great astilbes and trollius, not to mention other damp loving plants like primroses. I do water them occasionally when the weather is really dry, but for the most part the soil in the beds stays constantly moist but not wet.
One caveat - if your soil is dry because you have surface rooted trees like maples, pines, or cherries nearby, this method will probably not work because 1) excavating the bed will be impossible and 2) if you do manage to clear the roots from the bed, they will quickly return.