Philo and I were invited to a pig roast on Saturday afternoon. As I gathered ingredients for corn bread and some Meyer's Lemon/Cranberry relish that morning, Tom Spencer's garden radio show played in the background. Tom's answer to one caller sent me running down the hall to the office - Philo had to hear about this! A woman wondered whether she should put the shells from her pecans into the compost or use them as mulch - and she was told it might not be such a good idea... while pecans are not as allelopathic [not alleopathic*] as Genie's infamous Black Walnut, they do contain juglone, a substance that can prevent the healthy growth of certain other plants. 
With that large canopy of pecan trees hanging over our garden and the ground covered with husks and shells discarded by the squirrels, how concerned should we be? Was this the reason our tomato plants grew well the first summer, but seemed progressively more spindly the last two seasons?
We'd heard for decades that some plants, including tomatoes, wouldn't grow under Black Walnut trees- friends in Illinois had that experience - but we hadn't realized Pecans also had some juglone. The two of us started searching and reading, finding references to Black Walnuts, Pecans, Allelopathy and juglone in many articles, advice columns, forums, sites from universities and extension sites. As always, clip-and-paste ran rampant, with many sites using exactly the same wording and the authors disagreeing completely on the toxicity.
The conflicting articles stated that the leaves had juglone but the shells did not; that the juglone was concentrated in shells and bark; that it could persist for a long time or that a few weeks of composting eliminated it; that there was enough juglone in pecans to stop the growth of tomatoes or that the amount was so minimal it didn't count. Whether the soil was sandy or clay seemed to make a difference, as did the amount of moisture. Some sources recommended that adding lots of compost and improving drainage could remedy an area with high amounts of juglone. We read that juglone from black walnuts is used in traditional medicine to heal ringworm in humans and also read that allelopathy* swings both ways - the Department of Horticulture at Oklahoma State in Stillwater found that bermuda grass may inhibit the growth of pecan trees.
When I browsed a Project Gutenberg book called Growing Nuts in the North by Carl Weschke, one chapter told of the author's experiments in the early 1920's. It was pretty funny to hear him call the pesky squirrels "bushy-tailed rats" - that was definitely a bonding moment with someone from the past.
Our tentative conclusion after a couple of hours research was that for most areas of the garden, any juglone found in the pecan tree did not seem to have done any harm. On the other hand, it had probably been a mistake to incorporate such large amounts of pecan leaves into the tomato & pepper garden over the last 3 years and the waterlogged conditions of early summer may have intensified the effect. 
Although our conclusions are tentative, we've noticed that several other plants in the nightshade family declined when in the root zone of the pecans - the Brugmansia got smaller instead of larger and only grew when moved to the far side of the garden. So to be on the safe side, we'll keep the pecan leaves, twigs and hulls out of the vegetable garden and not use them anywhere unless they've composted for a year. We'll add different compost to the tomato bed in spring, perhaps treating it with liquid compost now. A few weeks ago I transplanted that Solanum in the photo above to the triangle bed. It's one of the ornamental potato flowers but it wasn't growing or flowering. Now I realize this member of the nightshade family was also under the drip line of the pecan. Was that a factor in its failure to thrive?
This is more than you wanted to know about juglone, isn't it! If a pecan appears here again, I promise it will be in a pie! Let's move on to flower photos. 
There were a few openings since the blooms were posted on the 15th - here's the peach iris unfolded. It sure looks odd backed by blue Plumbago, lemons, a 9-foot Brugmansia and the pink cuphea in bloom!
The Sasanqua camellia 'Shishi Gashira' opened a few flowers yesterday, but a fast moving gust of rain and wind shattered the flowers. More buds await their turn. 
The next flower is difficult to photograph - a polaroid filter held over the lens helped a little. 
That white line is the odd flower of a plant called Hoja santa, or Piper auritum, with large aromatic leaves that are used in Central Mexican cuisine. We once tried them parboiled and cut into squares, then filled with a chicken, pepper and rice mixture before folding into packets and baking. 
Austin garden photographer Valerie was the person who first introduced me to Hoja santa - she has more information on the plant. 
A cold front with rain is expected tomorrow evening followed by a weekend of cold, wet weather with highs in the fifties and lows in the upper-thirties. [from 10 degrees down to 2 degrees for you who use Celsius].
But today I can still admire a gulf fritillary on the pink cuphea. 
Today I can enjoy the red berries on the Yaupon holly as it arches across from the left side of the path to the gate, mingling with the sharp and pointed leaves of the holly tree growing next to the garage.
This post, "The Old Dogs Learn Something New", was written for a blogspot blog called The Transplantable Rose by Annie in Austin.
Happy Thanksgiving to all of you from Philo and Annie in our garden in Austin, Texas.
*Edited Nov 26 - my son pointed out that the words are allelopathic not alleopathic, and allelopathy not alleopathy. Thanks, kiddo - still learning .... Arf, arf.
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