Site Analysis, or “blimey, Jake, you’re gonna put a garden where?!”

Site analysis kicks off any landscape design / garden design effort with a 5 prong information quest: site history, and an analysis of sun, wind, water, and soil conditions.

Site History

1874 map of washington dc dupont circle georgetown hanes point mount pleasant

(I doubt you'll need to go back this far.)

Knowing a bit about your land let’s you know what possible soil pollutants you may have (urban areas may have lead, etc) and also about the about the likely quality of the land.  One local farm here has been organic for 15 years, but in the not too too distant past, cattle grazed their fields, stomping the land flat and creating peculiar balds and wallows. (This is not a requirement- that cattle muck up a bit of land- but it is common still in outdated industrial-mentality management regimes.)

Anyway, when our farmers went to start an organic orchard for their CSA, the trees took years longer to get any size to them because of the compacted and impoverished soil.  Evrett now says that a year or two of compost and cover crops would have saved time, but he didn’t know about the cattle back then- it had been a conservation easement for years before the family acquired it.

Sun Exposure

I can’t tell you how many gardens’ fall seasons I’ve seen clipped short by bad siting.  The sun at its zenith in late June will light up some patches of land that will never see in an ounce of direct sun come October.  Pay attention.  Run outside at noon today and see how long the shadows are.  And that’s it noon!  By 4:00 pm…

summer versus winter sun sketch up

Left image: June 22, 11 am. Right image: December 25, noon.

This sun study was done quick and dirty and free using Google Sketch Up and Google Earth.  The big cylinders are “trees”. CM lives in the house (A) and put last year’s fall vegetable garden at B.  Next year, it’s being moved to C.       Note: C looks a little dark because of the angle of the fir tree in the aerial photo. There are no shadows on C in either image.

How long the shadows are in winter has everything to do with how far from the equator you live.  Similarly, whether the heat of the sun is enjoyed or endured has to do with your local climate too.  There are different plant zone maps, they disagree with each other, and someday I’ll hold forth on that debate.  For now, I’ll assume you know if your farm or garden freezes in the winter or not.

Wind

Wind is about exposure.  Think about early spring.  There are three different temperatures rising: the air, which is as fickle as can be; the water, which will temper the air if the water body is large enough; and the soil.  At very specific soil temperatures, soil microorganisms return to activity.  At 40° F, the process that converts atmospheric nitrogen to plant soluble nitrogen cranks back up.  That’s why the lawns suddenly green up, seemingly overnight.

The soil temperature of the top few inches can be influenced by the sun, so that a south facing unshaded spot may have the top layers of soil queued up sooner than the surrounding dirt.  If an early spring plant like spinach is there, great.  It’ll get up and go and not be overly put off by the fickle air’s late frosts.  An early spring flower, on the other hand, like star magnolia or Camellia japonica, runs the risk of blooming too early in this location, leading to frost damaged blossoms- unsightly at best unless you have an unusual fondness for little dead brown cabbages dotting your shrubbery.

So wind is actually it wind and walls: where is the exposure to the winter blasts?  Where will the air stall out and bake in the heat of summer?  Is there a wind tunnel on your site, where the wind speeds up to get through a narrow spot?  A windbreak?  Or a frost pocket?

Water

Where the downspouts?  Where does it puddle and where does it flow in a heavy rain?  Can you take advantage of any of this water?  Dig a dry well for under the downspout, install a rain barrel or rain garden?  A number of exceedingly water tolerant plants are quite tasty.  Of course, how easily the water percolates into your soil has a lot to do with your soil…

Soil

Soil quality is determined by looking at multiple factors:  soil structure, soil nutrition, soil acidity (also known as soil pH), and soil moisture, all of which intersect in unexpected and fascinating ways.  I’m focusing on soil moisture right here, which is strongly connected to soil structure.  The gist of it is this: there are two thresholds of concern for every soil: wilt point and field capacity.

soil moisture graph garden soil quality assessment

 

 

 

The Wilt Point is aptly named.  This is the threshold below which there is officially not enough water in the soil, so the plants begin to wilt because all remaining moisture has a chemical bond with the soil.  It’s not that there is not water, it’s that the soil either can’t or won’t share.

Field Capacity is the opposite: the plants begin to wilt because there’s so much water in the soil that all the oxygen spaces are being filled.  Literally, the plants are drowning.  Every single soil out there has these two thresholds, from pure clay to sandy beaches.

The real question, the trick, the holy grail of garden soil, concerns how far apart these two thresholds-  wilt point and field capacity- are from each other.  As it happens, the soil structure that is best for soil moisture is the same structure that is best for nutrition.  Lucky us.

Tomorrow: soil nutrition, or “starved from above, starved from below”.

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4 Responses to Site Analysis, or “blimey, Jake, you’re gonna put a garden where?!”

  1. I’ve got a section in my Weekend Homesteader book about a similar topic, and I’d love to include your Google Sketchup drawings of the sun at different times of the year. Would you be willing to let me use the image?

    I’d need a version that’s as large as possible (300 dpi, about eight inches wide would be perfect) and an email from you with the wording below:

    This email shall constitute an agreement between you and Anna Hess for non-exclusive use of your photo and text in the book tentatively titled Weekend Homesteader. Skyhorse Publishing will publish this book in 2012. You agree that you shall receive no compensation for your contribution.

    I’d be happy to include your website’s URL (and, of course, your name. :-) ) Just let me know if you’re interested. Thanks!

    • Sure! I assume I can save that image at any dpi… I’ll play with the technology. It can very likely do it, I’ve just never asked it to before. ~Molly

  2. Thank you! If they’re difficult about dpi, even the current size of the image might be feasible. I really appreciate you being willing to play a bit more with Google Sketchup and let me use it!

  3. for fun look at the shape of Georgetown on EMBRYO MAP OF DC
    that is the shape of the sundial(gnomon) used to lay off DC. the stone can be used to lay off a garden ie solstice ect for max sun on specific plants .Like the streets of DC.

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