My wife and I recently returned from a vacation that took us to Pennsylvania, Maryland, and Washington DC. While there, I noticed that many trees and shrubs are planted on top of small mounds (about 2-3 ft in diameter and 1-2 ft high for a shrub, larger for a tree).
No, these were not planted "at grade" and then soil mounded around them. I could tell by looking at the base of a tree or shrub that a mound was formed first and then the plant was placed at the top of the mound.
Why is this done? We don't plant things that way where I live. Here, some trees and shrubs are actually planted in a slight depression (an "anti-mound") to catch water and prevent run-off during irrigation.
In some areas there're the opposite conditions. Mounding is a way to circumvent drainage problems; either standing water or the ground is too porous. The most common reason mounds are used for planting in those areas is that there's bedrock or a shale bank just below the surface... in many coastal areas there's only sand, the ground perks so well that were not for mounding even small plants wouldn't survive... could be clay, or a high water table too. When properly constructed a large saucer shaped depression is scooped out and the mound is built up over a large enough area that it's not very noticable... obviously the mound should be appropriately sized for the planting when it reaches mature growth... what you noticed are probably mounds that were skimpily constructed without taking into account that the plantings would grow, or were purposely meant to be berms to simulate outcroppings but not properly planted.
It could depend on the soil types and local rainfall patterns and amounts. If the areas had heavy clay soils and reasonable rainfall, they might have a problem of soggy soils with bad drainage causing root rot. (Such as where I grew up in western NY state.) Planting in a raised mound helps alleviate these problems.
On the other hand if the soil drains well and/or there is not good rainfall year-round, planting in a depression can help maximize the amount of water captured for the plant from what rain there is. I think this is the more common case in states like California.
You say you are in the CA Mediterranean climate. I assume that means most of your rain is in the cold half of the year. (The same rainfall pattern we have here in the Willamette Valley of Oregon, although it is not warm enough to be a "Med" climate here.) This is a very rare rainfall pattern in North America. Most of the rest of the lower 48 get most of their precipitation in the warm half of the year (when the plants really need it!). Thus different regions have different sorts of challenges. :-)
-- If you dug a hole one foot deep in the back yard of my parents' house and filled it with water, two weeks later it would still be half full. (They eventually had a contractor put in a drainage system so the yard would not be a swamp for three days after every heavy rain.)
-- If you do the same in my current back yard, five minutes later the water will be gone. (In some areas it will drain away as fast as you can pour it in.)
This helps give an idea of how much soil drainage can vary, depending on the soil type.
Utopia in Decay
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Cherkauer
To From: "Kevin Cherkauer" Newsgroups: rec.gardens Sent: Monday, June 09, 2008 7:31 PM Subject: Re: Planting in a Mound
It could depend on the soil types and local rainfall patterns and amounts. If the areas had heavy clay soils and reasonable rainfall, they might have a problem of soggy soils with bad drainage causing root rot. (Such as where I grew up in western NY state.) Planting in a raised mound helps alleviate these problems.
On the other hand if the soil drains well and/or there is not good rainfall year-round, planting in a depression can help maximize the amount of water captured for the plant from what rain there is. I think this is the more common case in states like California.
You say you are in the CA Mediterranean climate. I assume that means most of your rain is in the cold half of the year. (The same rainfall pattern we have here in the Willamette Valley of Oregon, although it is not warm enough to be a "Med" climate here.) This is a very rare rainfall pattern in North America. Most of the rest of the lower 48 get most of their precipitation in the warm half of the year (when the plants really need it!). Thus different regions have different sorts of challenges. :-)
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