Thursday 20 September 2018

Nearly myth, you can visually determine your plant’s nutrient status and fertiliser requirement from examining the leaves

He has not got a clue why the chaenomeles on the wall below me is dieing
All this interveinal chlorosis tells him is that it is having a bad time!
My hastily assembled definition off a ‘nearly myth’ is something that is sometimes true but is either grossly inaccurate or only achievable by someone with rare expertise. I scurry to add that such a person is not me.

Perhaps I can illustrate what I mean with an example. Suppose a tomato grower has years of experience of growing this single much researched crop and has had the benefit of the whole paraphernalia of cultural and scientific advice available to growers throughout his lifetime. Such is his experience he can look at his tomatoes and immediately read the tea leaves and give you chapter and verse of his plants nutritional needs. (An unfortunate analogy as tea leaves tell you nothing)
Not only do we not have his experience we have to deal with thousands of different plants that all behave differently

Leaves tell you a lot - but what?
Brenda asked me recently why one of our agapanthus had a few yellow scars at the tip of the leaves. I received a scathing reply when I announced I did not have a clue. I had not noticed or cared!
This set me thinking as to what you can tell from a plant’s leaves. An awful lot really and not just about nutrition. Such things as effects of weather, temperatures, drainage, light levels, pests, diseases such as virus, root disorders, watering or merely senescence. The trouble is these very diverse sources of leaf colourations, markings and distortions often look the same.
You really have to read the runes and consider your plants history. The longer you have gardened the more likely that you will come up with a correct diagnosis of any problem

Just the fungus disease powdery mildew but has it been predisposed by dry weather or lack of water or wrong position  or varietal susceptability or...?

You might take your plants to a so called expert and get a correct diagnosis. Alternatively a sufficiently beguiling snake oiler might confidently tell you a whole lot of gobbledegook and send you away happy even though your plants may be destined to die. Indeed it gives you a kind of satisfaction to ‘know’ what has killed your plant.
Life is too short to worry about every blemish on your plants.

Difficulties of diagnosis

It is tempting to think that certain symptoms will tell you whether a plant lacks a specific nutrient and that this can then be corrected by soil fertilisation, manuring, liquid or even foliar feeding. Dream on

My tomatoes turned out ok
Even where a leaf symptom is correctly interpreted as a shortage of a particular nutrient it is not necessarily deficient in the soil. 
For example Peter Williams was flabbergasted at the phosphate deficiency in my newly planted tomato seedlings last year. An extended period of cold, not enough to kill them had reduced phosphate take up and the leaves where so very purple - classic phosphate deficiency - that you could scarcely visually pick them out from the soil. Warm weather returned and they grew away a healthy green and gave me one of my best crops ever. Cold had inhibited phosphate uptake as Peter well knew. (In fact I did not know until he told me)

Just old age
Robert Pavlis wrote a very fine post on my subject today presenting reliable leaf diagnosis as a myth and gave the illustration of correctly diagnosed phosphate deficiency symptoms due to shortage of nitrogen! One nutrient may be essential to take up adequate amounts of another.

I have already emphasised that there is a myriad of causes of apparent deficiencies not least toxicity caused either by excess of nutrient, or maybe herbicide. 

Temporary low temperature chlorosis on Acanthus spinosa
Iron uptake is frequently inhibited by cold soil early in the season and especially when excessively wet. A perennial plant I know well, Acanthus spinosa invariably goes through a spell of chlorotic young leaves due to this deficiency as it bursts into Spring growth. I remember at horticultural college every year a young lecturer would take the students out to see this ‘herbicide damage’!  I did not have the courage to tell him. The boot is usually on this same foot in gardens where herbicides are used - nutrient deficiency is wrongly blamed on a weedkiller.

A very common cause of nutrient deficiency is ‘induced’ by some factor such as soil pH or excess of a different nutrient. The nutrient is not short in the soil, it is just that the plant cannot absorb it.

I had to pinch myself to really be sure I was looking at rhubarb under the chlorotic vine
Lets face it the real problem of using visual leaf symptoms as indicators of deficiency is that amongst thousands of garden plants there is just too much variability 
And as a final salvo, some plants might carry yield reducing deficiencies for which there are no symptoms at all.

A more positive view

Multi nutrient deficiency (I think not) or is it just senescence or is it merely damaged stem....
What a gardener can recognise is multi nutrient deficiency where his plants are generally starved. A hotch-pot of chloroses and discolourations.
Most often these will be plants in containers that are subject to leaching and not fertilised at all. Many modern amateur composts have abysmal nutrient reserve.

I have discussed fertiliser practice on numerous occasions. It is too big a subject for today but readers will know I take the view that if you use a modern plant fertiliser that is formulated with all the important plant nutrients; major, minor and trace element most (but not all) deficiencies will be automatically corrected.
Plants take up the nutrients they require and there is no need for special formulations tailored to individual plant needs My Yara Mila general fertiliser satisfies virtually all my plant nutrient needs although I always emphasis there are many garden situations which need no fertiliser at all.
It might surprise you that for most gardeners including myself I do not advocate soil analysis. It’s just playing at being a professional grower. 
Do not use amateur ‘kits', they are completely useless perhaps with the exception of pH widgets that provides a very crude guide.


An unusual chlorosis caused by sun scorch on one of Peter's shade loving woodland plants
The description of lack of chlorophyl is much bandied around. The most usual cause is lack of nitrogen although there are a plethora of alternatives. 
Interveinal chlorosis is sometimes seen where the veins stand out green. Occurring in new leaves at the growing point it is generally associated with lack of iron; if in old leaves it might show lack of magnesium although where leaves which are naturally becoming senescent it is of no significance at all.

There are some acid loving plants such as rhododendrons that if grown at too high a pH show interveinal chlorosis around the growing point. It might not just be iron deficiency It might equally be lack of manganese.
The good news here is that fertilisers dubbed sequestrene or chelate are usually cocktails of those chemical molecules that can be absorbed from alkaline soils. (If your rhododendrons are in really alkaline soil they just die).
Against huge odds I hit the jackpot this year when I chucked some old rusty looking chelate which had mouldered unused on my shelf for thirty years over a sick plant and watered it in. It revived in a week.
I looked at this years wonderful crop of tomatoes today, (August) and noticed the new growth was a little chlorotic and sprinkled on a further top dressing of my Yara Mila all nutrient fertiliser

What is wrong with my chaenomeles?

These symptoms are caused by stress but what?
They have performed wonderfully for twelve years and before this season had grown eight foot high. One on a west facing wall, one one facing north. They are now dieing back and are showing apalling symptoms of interveinal chlorosis as well as complete yellowing and partial leaf fall.
Is it magnesium defficiency? Almost certainly not and even if so it is only one symptom of a much more serious cause.
I used to lecture on pest and diseases (and other topics I spout about) and ought to know. In fact I do not have much of a clue.

There is no classic disease of chaenomeles although Swedish research shows that stressed plants - and mine are certainly stressed - succumb and die back from all manner of combinations of very common diseases such as botrytis!

Has my chaenomeles outgrown their position and dehydrated all the soil under the house and garage? On the contrary did last Winter’s repeated deluges cause root death by flooding? 
It has been a very long hot dry Summer.
Have they just exhausted the soil’s supply of essential nutrients - after all the roots are exclusively under buildings or paved (but porous) drive.
Why have two separate varieties both started to die? I think in this case I can absolve this year’s beast from the east!
There is one clue. There are just a few scattered healthy dark green shoots emerging from the chlorosis.

I have taken them in hand. It has been necessary to prune back the dieback. I have generously applied Yaramila  ‘complete nutrient’ fertiliser and heavily watered on a couple occasions. I have scattered a dose of magnesium limestone (My soil is acid, although foundations might be alkaline).
It looked better after I blasted the plants with a heavy deluge from the hosepipe and the dead leaves fell like confetti.
I will report in six months if my false quinces are alive or dead.

Fingers crossed
Robert Pavlis was much more forensic discussing this myth
I wrote about the water scorch myth a long time ago
If you want to know about Yaramila all nutrient fertiliser use my search box

Monday 10 September 2018

Should we worry about poisonous plants?

Irritating plants

Oleander is one nature's most poisonous plants
My opening comments are about this troublesome weed
Yet again I rub my eye in Pavlovian response. My itch becomes more intense and my eyes start to water. I have been weeding again and it’s that damned petty spurge. I prepare myself for a couple of hours discomfort. I am lucky of course and it could be very much worse. 
Last night Cathi thought I had an eye infection and I had to explain

Do not rub your eyes after weeding this plant
This nasty pretty little weed is Euphorbia peplus and has toxic white milky sap. So do the rest of this huge family Euphorbiacea which includes numerous loved garden and indoor plants - not least poinsettia. Handle all of them with care and beware getting their sticky white milk on your skin.

Petty spurge is of delicate nature and pervades almost un-noticed amongst and within your plant clumps. You are more likely to have a problem if you are rather good at weed control where it has less competition and you are at first blind to its presence. It is an extremely common weed and few gardens avoid it.
It comes from seed and is easily controlled if you remember to do so. One just forgets.

Just pull it out at the roots with a gentle pull. Take care not to snap the stem and get toxic sap on your hand. Leave it to desiccate and die or if you are just too tidy take it to  compost. You ought to wear gloves but I constantly lose them somewhere round the garden. None of mine match

Petty spurge has had a field day in this Summer's drought
Very shallow hoeing is ideal and is the safest method. Unfortunately most modern Dutch hoes are next to useless and difficult to skim with. My own onion hoe would be ideal but I have lost it.
I do not find spraying works very well. The weed  starts so very small and the glyphosate tends to run off and I suspect it does not translocate very well.

Even the nice euphorbias have toxic sap
Hot chillis
I am told if you have been slicing hot chilli the sap on your hand can cause very similar stinging sensations in sensitive places.

A different kind of irritant

Rue, Ruta graveolons
Another plant to which I have fallen victim is rue, Ruta graveolons. This so called herb is a nondescript  dwarf evergreen glaucous shrub I have encountered in past client’s gardens when I have pruned. This has a photosensitive sap that causes phytophotodermatitis. You might get away with exposure to its sap on intact skin but any cut or graze - common when pruning, especially if you catch yourself on a cut ruta stump, is real nasty. You will recover but it’s not nice at the time

The affect of sap of the herb rue is not very nice
Many plant sensitivities are specific to just a few people. I once had a student who after pruning common ivy came out in a dreadful rash. I myself hate handling Christmas trees which give my hands a sore feeling. The house plant Primula obconica is rather notorious for causing a rash (and breeders have bred a safe variety called ‘Touch me’). 
Less well known is that chrysanthemum sap can irritate, so can the small tree rhus

Rhus sucker
Really poisonous plants

Deadly wolfsbane can be extracted from Monkshood, aconitum
It’s not my intention to prepare a compendium of poisonous plants but merely to make a few observations. 
There are hundreds of garden plants that are poisonous. Some fatally so. Should we be worried that many much loved plants have the capacity to kill you?
I do not think so but be very careful. Never eat any plant part unless you know it to be safe. 
I have at least a dozen really toxic plants in my garden including an oleander that I struggle to grow! My only concession to their nature is to thoroughly wash my hands after any close contact such as cutting or pulling. 
Last month in a French garden I very carefully pulled out a very strong growing deadly nightshade with my bare hands and I am still here. (I don’t mean in France).

Weeds such as ragwort are deadly poisonous to grazing animals

If you read a list of potentially poisonous plants you might give up and grow nothing at all. I am firmly against any suggestion that poisonous plants be compulsory labelled. Such is human nature that most of the plants in the garden centre would be so marked. Just think of a few examples

Lily of the valley. Now that’s really poisonous

Arums.  Although there are several cases each year of hospitalisation when the red berries have been eaten it is a long time since there were any recorded deaths.

Asparagus. Its berries are poisonous albeit mildly. 

Tomato. The only edible and none poisonous part of the plant is the tomato fruit. (Fortunately there are no dangers growing them)

Rhubarb. Fears of oxalic acid in the leaves are grossly overrated although the leaves contain another chemical which is also toxic

My poisonous pokeweed is regularly asked about on my open days

Parsley. My favourite herb that I safely devour in some quantity in sauces or in sandwiches contains five times the concentration of oxalic acid as rhubarb leaves

Aconite. I am not about to prepare wolfbane from this plant's deadly content

Potato.  Young tubers exposed to sunlight produce deadly solanin  indicated by the presence of harmless green chlorophyll 

Even my beloved dicentra are toxic

Narcissus. There have been cases of thinking they were onions with nearly fatal consequences

Laburnum. A client gave her pet rabbits a treat placing them in a bottomless cage on the lawn under a laburnum that was casting its seeds. Oh dear.

Digitalin is extracted from poisonous foxgloves to treat heart disorders
This is an excellent reference list of poisonous plants in wikipedia which will either reassure or persuade you gardening is not for you.

There is a garden of poisonous plants in North Yorkshire. I understand you can only go round with a chaperone. That sounds a gimmick to me.

And a final thought, most vegetables contain tiny levels of toxins and there is very strong evidence that this is the reason why they are good for you

I write about the 'nearly myth' of poisonous oxalic acid in rhubarb leaves here

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