Author: Linda Chalker-Scott

Some good and creative guesses about why the Clematis leaves had interveinal necrosis.  While iron and manganese deficiencies both cause interveinal chlorosis (veins are green, areas between are yellow), the necrosis indicates tissue death between the still-living and green veins.  Very simply, this has been caused by water loss.

During transplanting of the vine, I had to remove them from the fence and lay them out on the ground.  They remained this way for a couple of days.  For much of the foliage, this meant that the lower leaf surfaces were now exposed to the sun.  As with many broadleaved plants, the upper and lower leaf surfaces are morphologically distinct:  the upper surfaces have a thicker waxy cuticle and epidermis, with few stomata, while the lower surface lacks much of the cuticle and is loaded with stomata.  When the leaves are turned upside down, the shade-adapted lower surfaces now receive intense sun exposure: water evaporates quickly from these unprotected leaves and the tissue dies.  The only parts of the leaf that don’t die are the veins, which remain full of water as long as the roots are functional.

So both LisaB and Benjamin identified sun exposure as the culprit behind the damage.  But as with many environmentally-induced plant problems, the ultimate cause is water stress.

If you’ve been following the saga of our clematis, you’ll know that first they suffered iron toxicity (from the waterlogged soil they were in) and then were dug up and replanted in containers.  Last week I showed you what happens when you vigorously work wet soil – yet more waterlogging!  During the transplanting process, I took more pictures:

This damage is NOT from the iron toxicity problem.  It appeared during the transplant process.  What caused it?

Answer on Monday!

Friday’s puzzle was tricky!  I will preface the answer by saying I don’t do a lot of container plants except for annuals.  When I plant up annuals into soilless potting mix (which is dehydrated), I work in the water thoroughly into this fluffy medium.  Friday’s situation was a little different.  The Clematis are more or less permanent residents in these planters, so I use real soil rather than potting mix.  (This will reduce or eliminate the shrinkage you’ll get if you use soilless media, as these highly organic materials steadily decompose.)

But what I thoughtlessly did was to mix water into my nice screened clay loam like it was a potting mix.  The result of vigorous mixing is to break down the soil structure and drive water into all of the pore spaces, resulting in a totally waterlogged soil:

After my "D’oh" moment when I realized my error, I potted the second Clematis into loose soil and let gravity work the water into the larger pore spaces. This passive approach protected the soil structure and left some of the pore spaces filled with air rather than water, so that the water drained through rather than sitting on top of the compacted soil:

This demonstrates one of the rules of working with landscape soils:  you should avoid any potential compaction when the soil is wet.  (It’s the same reason you shouldn’t stomp on wet backfill as you’re transplanting trees and shrubs.)  As Ginny pointed out, you’ll create a cement-like structure – not conducive for water movement, oxygen availability, or root growth.

A few weeks ago I showed you photos of iron toxicity in a Clematis planted in a soggy soil (perched water table).  Because this area is just not conducive to plants, we’re putting in a small deck.  This necessitated the excavation of two Clematis, which were both suffering from wet feet.  (Needless to say the root mass was very small and shallow on both plants.)  We decided to put them into large planters with conical trellises and use them as deck plants.

During our pond excavation we retained the topsoil and sieved it for uses just like this (for woody planting, not annuals).  The soil is a clay loam and has been stored in a garbage can to keep it dry.  Anyway, the first pot I filled with this good soil, added water, and worked the soil with my hands to ensure it was thoroughly hydrated.  As holes developed, I added more soil and continued to work it in by hand.  I then installed the first plant and watered it thoroughly.

After observing what happened during the next several hours with this plant, I installed the second plant differently – I watered the soil but did not work it by hand to hydrate it.  I added the plant as before and then watered everything thoroughly; I added soil where holes developed.  I then redid the first container in the same manner as the first.

What happened to the first container that caused me to change my installation technique?  And why didn’t it happen with the second container?  Answers and photos Monday!

Well, Dr. Rohwer was right – he thought you’d get this more easily than I did!  Ray and Jon were spot on – this is classic juglone toxicity from those walnut trees (Juglans spp.) you see in the background.  Many of these leaves ended up on the roof, leaching into the rain barrels (good sleuthing Ray and KennyG!), the water in which was used in irrigating the vegetable garden.  In fact, Dr. Rohwer divulged that the rain barrel water was quite brown from the walnut leaves.  Thus, the tomatoes met an unhappy end, because tomatoes definitely do not love walnuts.

Juglone toxicity, an example of allelopathy, is an interesting phenomenon – it does not affect all plants equally.  In fact, this clue was given in the puzzle, when Dr. Rohwer mentioned the "garlic and carrots, amongst young beets, onions, kohlrabi, and bolting radishes and spinach" also in the garden.  Many of these vegetables are known to tolerate juglone, as are various landscape plants.  (There is an excellent publication from Purdue on juglone toxicity with lists of tolerant and sensitive species.)

Juglone is most prevalent in the nuts, roots and leaves of walnut trees.  This makes sense:  germinating walnut seeds and expanding root systems are both able to kill their competitors, retaining more resources for themselves.

Juglone tends to be less of a problem for established trees and shrubs, as their root systems are more extensive; seedlings would be the most sensitive stage.  And walnut wood used as part of a wood chip mulch has not, as far as I know, been shown to cause juglone toxicity in landscape plants. 

Friday’s quiz was a tough one. Bernadette and Lisa B. gave it a good shot, guessing that this might be a phosphorus deficiency. While they’re on the right track (it is a nutritional disorder), the mineral of interest is iron, and it’s a toxicity problem, not a deficiency.

Under wet conditions (the affected Clematis is in the part of our landscape with a perched water table – see the March 15 posting), iron is predominantly in the Fe⁺² form (ferrous) rather than the Fe⁺³ form (ferric). The ferrous form is easily taken up by roots, and when leaves accumulate too much iron they turn reddish-brown.  This diagnostic characteristic is called bronzing, and it’s different than the reddening caused by anthocyanin accumulation.  (As an aside, nonagricultural landscapes rarely have phosphorus deficiencies and are more likely to contain excessive levels of phosphorus.)

Finally, all this talk of iron has reminded me of one of my favorite chemistry puzzles – see if you can figure out what this is:

Give up?  It’s a ferrous wheel!