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Garden Drainage Help Needed

11 years ago

Hello everyone. This is my first post, so please be kind if it's the wrong forum, or something that has been discussed at length previously.
I have a garden on top of a timber retaining wall. It contains a row of Miscanthus grass and many dollars of dead phlox. The plants are dieing because the area stays wet. It does get a good amount of sun. I'm looking for ways to address this. The wall is 29 in tall by 35 feet long and the garden is 3 feet deep.
Digging a hole approx 2ft deep, I discovered the soil is quite dense, much like loam.
Because of the 'dead men' that run perpendicular to the wall, under the garden, I cannot add a perforated drainage pipe the length of the wall.

Do I dig everything out, add stone and sand? Then top off with 12in of garden soil? If so, how many inches deep of each? Would this have to be done for the entire footprint of the garden (my back already hurts :)) ?

Can I just add peat moss or sand to the top 12in of soil? This would be a lot easier, but not sure it will be effective.

I don't mind the effort, but if I do it, want to make sure it will work. Thanks for any advise or suggestions.

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Comments (3)

jean001a
11 years ago
Consider hiring a soil engineer for an onsite visit & suggestions.
Kimmsr
11 years ago
Loam is not a soil type that would be dense mostly because it is about 45 percent Sand, 25 percent Silt, 25 percent Clay, and maybe 5 percent organic matter. So you need to determine what type of soil you do have.
Clay soils tend to hold too much moisture because the very small soil particles do not allow the moisture to flow easily. Adding sand or stone for drainage may be too costly.
Perhaps these simple soil tests mightbe of some help.
1) Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drainsÃ¢ÂÂ too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.
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You should at least tape the pipe joints or find a coupler. I have an article on drainage at my website, since I installed many of them while working at university campuses and golf courses. At my site link below, click Advice in the menu, then afterward, choose Advice page 2 from the flash buttons. When you get to Advice Page 2, find the drainage topic in the list. You may find the soil article relevant too. There should be a topic about making dry creekbeds functional by running drainage to them. It turns the dry creekbed into a storage or drainage cistern. Here is a link that might be useful: mdvaden.com > go to advice, then advice page 2
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If the lowest place in the yard is at the left corner, but you have a low spot right of that, it sounds like it needs to be filled, maintaining the overall uniform slope of the yard, so that water doesn't collect there. Looks like you have plenty of slope toward the back lot line but that it's fairly level along the back line. One would need to know grade information outside of your lot lines in order to know what is possible to do. A french drain is not going to help. Is there a drainage swale/gully beyond the back fence? It would not be abnormal to see flowing water near an overflowing swale during a heavy storm. As long as the water keeps moving, no problem. It's when it stops and ponds that you don't want. During a heavy rain, check to see if water is flowing beyond your left neighbor's property, or if it stops there and backs up into your yard.
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If the surface remains nicely moist because you have a lake underneath, you've got a problem. As Dave said, root rot is then pretty much assured. I'm not sure how you're judging that the bed is drying out. Sure, the surface is going to get dry, if you don't have that lake underneath. That would be completely normal. If the surface stays moist, things are probably too wet. I can't imagine that several inches down it's drying out, however. That's where you want the moisture.
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I would also add that some herbs will require very different growing conditions, making them not very suitable housemates in the same container. For example, rosemary and sage like it very dry and very fast draining; oregano and thyme can take bit more moisture but not a lot and something like basil is a very thirsty plant requiring regular water. If you water enough to keep the basil happy, you will drown the others!
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ODog
Original Author
11 years ago
Wow. Thank you!

I'll work on this over the coming days. I'm sure I'll have follow up questions, of which I'll post, if that's OK with you.

Thanks again.
O

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