Getting to Know Your Garden’s Soil: Urban Soils

Soil is truly a garden’s foundation.  Plants obtain water and many nutrients from the soil – and interact with soil organisms.  Soil conditions may also limit the types of plants you can grow successfully.  The more you know about the characteristics of your soil, the better you can match plants and watering methods to your unique soil conditions.

A well-developed soil is a complex ecosystem.  When people talk about soils, they’re usually referring to the inorganic part of the soils – the sand, silt and clay particles.  But a good soil contains much more.  It has pores which hold vital air and water.  It contains decomposing organic matter (old roots; decayed leaves; etc.) that provide plant nutrients and help retain soil moisture.  And it’s home to a whole world of soil organisms, ranging in size from gophers and earthworms to microscopic bacteria and fungi.  In fact, a good soil is literally teeming with life!

Soils are characterized by the parent material (rock) from which they were formed and the conditions of their formation. In western Los Angeles County, most native soils were deposited by the L.A. River and its tributaries - or by ancient oceans.  Local soils range from almost pure sand near the beach (remains of ancient sand dunes) to denser, river/lake deposited clays and clay loams further inland.  Soils on the Palos Verdes Peninsula were formed from complex, highly weathered marine deposits.  The most common native soil type on the Peninsula is clay loam.   If you’d like to learn more about the original (native) soils for your local area we suggest the following resources:

 

·     Original L.A. County Soil Survey (1903) - http://soils.usda.gov/survey/online_surveys/california/losangelesCA1903/losangelesCA1903.pdf

 

·    Other California soil surveys (some on-line) see - http://soils.usda.gov/survey/printed_surveys/state.asp?state=California&abbr=CA

 

Urban soils – including those in our gardens - are more complex than native soils. In fact, urban soils are among the most difficult to classify – and work with – of any soils world-wide.  Like all soils, urban soils are composed of a parent rock material that has weathered over time to produce soil.  But it’s the human effects that make urban soils so ‘difficult’.  Soil scientists are just beginning to get a true understanding of the differences between urban and native soils – and their effects on plant growth.   Some of these differences are discussed below.

Depending on where you live, your garden may once have been a farm or ranch – or even had a factory on it!  Human usage leaves an indelible footprint on the soil. And the effects of past land use can influence your gardening success.  For example, farm soils have usually been fertilized and their structure modified by plowing. Farming practices almost always modify the nutrient content and drainage properties of soils.  Recently active farms may also have used herbicidal chemicals to control weeds.  

Plants themselves can affect soil nutrient content and soil organisms. Some plants are ‘heavy feeders’, depleting soils of major nutrients, while others leave plant chemicals in the soil.  For example, it may be difficult to grow plants in soil where a  Walnut tree recently grew. Even weeds, which are often ‘nutrient hogs’,  can deplete soils of important nutrients.  The potential effect of previous vegetation is important whether the plants were agricultural crops, weeds or garden plants planted by a previous home-owner.   We’ll talk more about basic soil nutrient testing next month (October, 2012). 

Your soil may have been compacted by heavy equipment during house or road construction.  Compaction is routinely done to provide a stable base for house foundations, roads, walkways and patios.  In addition, construction equipment itself compresses the soil.   Soil compaction changes soil drainage; compacted soils drain more slowly. Compaction decreases the soil air/water pockets, making it more difficult for roots to grow.  Compaction problems are most acute in the first 20 years after construction.  After that, natural processes, including plant growth and decomposition, gradually return the soil to a more normal composition.  We’ll talk more about testing – and improving - your soil drainage next month (October, 2012).

Your garden soil may be composed of ‘fill dirt’ brought in from somewhere else.  Fill dirt can have a very different composition than the original soil on a site. And fill dirt doesn’t have the complex soil structure that good soils develop over time.  Studies in other areas suggest that fill soils in general have higher levels of clay and less organic matter than native soils, making them drain more slowly.  A recent study suggests that sites with fill/disturbed soils on the Palos Verdes Peninsula also have more clay – and less organic material (humus) – than native soils in nearby areas (see www.csulb.edu/depts/geography/gdep/posters09/zamora.ppt. for more).  The good news is that well-chosen garden plants modify fill soils, converting them from ‘dirt’ to mature, functional soils over time. 

Your garden soil may even include ‘buried treasure’.  For example, the soil in Mother Nature’s Backyard has old bricks, asphalt, glass, chunks of cement and other debris dumped since the 1940’s (see top picture for some of our more interesting ‘finds’).  In Mother Nature’s Backyard, we chose to leave the majority of the debris in place. The choice was both a practical and conscious one.  We plan to study the effects of native plants on the garden's soil over time. We also want to see whether our ‘junk filled’ soil has adverse effects on the plants. We hypothesize that native plants are far hardier – and more effective soil-changing agents – than anyone suspects.  We’ll keep you posted of our results over the next few years.  

Old industrial sites are of particular concern for gardeners.  They may contain chemical and other debris harmful to humans.  If you live on a former industrial site – and particularly if want to grow edibles – we suggest the following:

 

·    Trace Element and Urban Gardens -http://celosangeles.ucdavis.edu/Environmental_Horticulture/Trace_Elements_and_Urban_Gardens_568/

·    Urban Gardens and Soil Contaminants - http://www.misa.umn.edu/prod/groups/cfans/@pub/@cfans/@misa/documents/asset/cfans_asset_287228.pdf

 

Urban soils are challenging and interesting.  There’s still much we don’t understand about them.  But it’s becoming clear that certain aspects of urban soils predict gardening success.  Next month (October, 2012) we’ll discuss some simple soil tests that you can do to learn more about your soil’s characteristics. In the following months we’ll discuss things you can do to promote healthy, productive garden soils.   In the meantime, if you’d like to learn more about urban soils we suggest the following:

 

·    Urban Soil Primer (USDA Natural Resources Conservation Service publication)  - http://soils.usda.gov/use/urban/downloads/primer(screen).pdf

 

·    Urban Soils - by Phillip J. Craul, Professor of Soil Science, SUNY-College of Environmental Science & Forestry  - http://www.ces.ncsu.edu/fletcher/programs/nursery/metria/metria05/m57.pdf

 

 

As always, you can e-mail your Garden Soil questions to: mothernaturesbackyard10@gmail.com .


 

Metallic Green Bees – Genus Agapostemon

Have you noticed any interesting insects recently? It seems like we discover new insect visitors every time we look in Mother Nature’s Backyard. Right now our Plant of the Month (Sweet-scent – Pluchea odorata) is attracting a number of interesting insects (see above).

Late summer and early fall can be a good time to observe insects, depending upon the weather and the types of plants that are blooming. Some insects hatch in the summer and are active now; others are completing their life cycle in early fall. And all can provide hours of interesting viewing. One of the more interesting insect groups – and among the more showy – are the Metallic Green Bees in the genus Agapostemon. These are true native bees in the family Halictidae (the Sweat Bees). But they are quite different from the familiar – but non-native - European Honey Bees.

Metallic Green Bees are found in North and South America. There are about 40 species, with the greatest abundance in temperate regions and Southwestern United States. The Agapostemon are floral generalists, which means that they visit a number of different flower species. Despite their lack of specificity, generalists like Metallic Green Bees can be important pollinators in gardens and in the wild.

Metallic Green Bees are short-tongued, so they favor flowers with a relatively open architecture and easily accessible nectar, which serves as food for adults. Like all foraging insects, they appreciate the convenience of plants with many small flowers clustered together. This in part explains why you’ll notice Metallic Green Bees visiting plants in the Sunflower family (Asteraceae; including Sweet-scent) and the Buckwheats (genus Eriogonum). Both of these plant groups feature many small simple flowers that produce high-quality nectar.

You’ll recognize Metallic Green Bees by their unique coloration. Our local females are often metallic green all over while the males have a yellow-and-black striped abdomen (the rear segment of the body) and metallic green head and thorax (see photo above). They are medium sized bees (about 0.3 to 0.6 inches long) somewhat smaller and slenderer than European Honey Bees. You’re likely to see the females busily visiting flowers, collecting pollen on the brushy hairs of their hind legs. The males will be seen nectaring (sipping nectar) or slowly cruising flowers in search of females.

Most species of Agapostemon are solitary ground nesting bees, although individual bees may nest in close proximity. They build deep vertical nests in the ground or in earthen banks. Metallic Green Bees have two generations per year: the summer generation which is almost all female and the fall-spring generation which includes both males and females. Fertilized females from the fall-spring generation over-winter from late fall until early spring. These females accumulate a layer of fall fat that allows them to survive the winter (note: cold weather kills the fall-spring males, which do not have the fat layer).

Fall-spring females from the previous year emerge in early spring and lay eggs in cells in the underground nests. They provide each cell with a pollen ball to feed the developing larva. The early spring-laid eggs give rise to the summer generation, which is mostly female. The summer generation, which emerges and nests in summer, lays eggs that produce the fall-spring generation. Surprisingly, the fertilized eggs laid by the summer generation give rise to fall-spring females while the unfertilized eggs develop into fall-spring males. The fall-spring generation emerges in fall and includes both males and females. And so the cycle continues, from one year to the next.

The warm days of fall provide a wonderful time to get out and enjoy the out-of- doors. So get out in your garden – or in the wilds - and look for these interesting native bees.

Plant of the Month (September) : Sweetscent (Salt-marsh Fleabane) - Pluchea odorata

Mother Nature’s Backyard features pink and purple flowers. While there are a sprinkling of other flower colors, this theme helps tie together the many plant species featured in our garden. One of our favorite pink-purple species – Sweetscent or Salt-marsh Fleabane – is in peak bloom around our rain garden this month.

Pluchea odorata 

(pronounced PLOO-shuh oh-dor-AY-tuh) is a plant of moist places including brackish marshes, freshwater springs/marshes and other seasonally moist places. It can be found from the west to the east coast of North America and south to the Caribbean Islands and northern South America. Locally, it grew along streams and marshes in western Los Angeles county and on Catalina Island. There likely were patches of it along the Dominguez Slough and Dominguez Creek.

Sweetscent is a member of the Sunflower family (Asteraceae) and its flowers look similar (except in color) to those of Mule Fat (Baccharis salicifolia) another local Sunflower. Pluchea can grow as an annual or perennial, depending on winter temperatures and the amount of summer water. It has rather stout, 2-4 ft upright stems that may become woody at the base but are mostly herbaceous. The leaves are simple, oval to lance-shaped and alternate on the stem. The leaves and stems are dotted with glands that release a mildly camphor-like scent when rubbed or brushed against. To some, the foliage scent is down-right weird. To others, it is a traditional scent associated with our local wetlands. We find it an unusual – but refreshing – addition to our rain garden.

One of the best features of Sweetscent is its colors. If you like pastels – and your tastes run to pink-purple – you should consider growing this plant. The foliage is a pale green pastel – one of the prettiest greens around. Contrast this with the purple flowers and you have an artist’s dream. Monet would have loved the clouds of pink-violet-purple flowers that seem to float like clouds above the pastel foliage.

Like sunflowers in general, Sweetscent has many small flowers that grow in dense clusters (think of the ‘heads’ of sunflowers). As seen above, each flower head includes a number of flat ray flowers surrounding a small number of disc flowers. The heads are grouped in rather loose, flat clusters – located primarily above the foliage. Pluchea blooms from late summer into fall, making it an important color spot in the fall garden. The sweet scent of the flowers – and the abundant nectar – attract a wide range of insects.

The insect visitors are mostly of the smaller variety: the smaller butterflies like Skippers, Blues and Hairstreaks and native bees and small flies. You can easily spend a delightful hour watching the variety of insects attracted to this plant - so be sure to plan seating close by. Below are pictures of just some of the insects we saw on a recent morning. We’ll introduce you the Metallic Green Bee in a posting later this month (September, 2012).

Pluchea will grow in just about any garden soil including those that are alkali and salty. If you have clay soil you’re in luck – Pluchea does very well in local clay soils. Pluchea likes full sun to part-shade. It prefers regular water (Water Zone 2-3 or 3 – see April 26, 2012 posting for more on Water Zone Gardening) although it can take drier conditions once established (occasional water – Water Zone 2). We planted it around our rain garden where it can take advantage of some summer water (we currently water every three days in this part of the garden). This plant can also take the seasonal flooding it will get in the rain garden.

Pluchea spreads via rhizomes. In our experience, the spread is not particularly vigorous – but we manage our garden on the dry side which will decrease the rate of spread. If spread concerns you, Sweetscent does fine in a large container. It will thrive around a lawn, pond or any water feature that provides some summer water. It looks right at home when combined with native warm weather grasses, rushes and sedges. It really is spectacular when massed, giving a needed burst of color in late summer and fall. Sweetscent is easy to start from fresh seed.

Sweetscent is widely used medicinally in Caribbean countries. A tea made from the leaves stimulates perspiration and urination. It is used to combat stomach cramps and as an eyewash for itchy eyes due to hay fever. The tea is a stimulant that works as a vasodilator (increases the size of small blood vessels). It should not be used in excess or in persons prone to migraines or by pregnant women

. The fragrant dried leaves have been used as an insect repellant. They would make a good addition to old-fashioned sachets. Dried flowers are also attractive in floral arrangements.