Summer Sudden-Death Syndrome

 Allan Robinson, UK

  I was interested to see the article on Page 15 of the Spring 2007 issue of the Group News. This is a devastating problem on certain occasions and something I have tried to cure by observation and experiment over many years. Here is a summary of some thoughts and efforts.

   It was several decades ago when John Watson, one of England’s present day plant hunters, made a very interesting comment (John has spent a great deal of time in Turkey and S.America and described himself as “The un-employable in pursuit of the un-growable”). He said “you can take two plants growing side by side on a mountain and bring them back to England, one will take to cultivation immediately and the other will find all manner of problems, even though they enjoyed the same plot back home”. Why? I just assume that plants have different tolerances when conditions differ from their natural habitat. We can help make them happier by changing a few cultural conditions in their favour.

    A few years ago I took some Geranium cinereum group plants that were collapsing throughout somebody’s garden to Plant Pathology at R.H.S. Wisley. Even though the stems and roots had developed a distinct reddish hue and were accompanied by a foul smell, Plant Pathology could not culture anything from them to enable us to point the finger at the fungal culprit. As no solution was found here, I delved deeper.

    Looking at the problem generally, various Genera have a tendency to collapse in summer as if dying of drought. Many gardeners will then water the victim, their kindness just ensures the plant rots even quicker.

    Listing out some of the problem Genera we get Erodium, Geraniums in the Cinereum group, European Daphne plus Lewisia and Penstemon from Western U.S.A. The first point that comes to mind is that all these come from a Mediterranean climate, so water would be a scarce commodity for them during the height of the summer. Plants from this category should NOT be mixed with others who are happy being watered whenever dry. Our old friend the Aubrieta and even the Hellebore have a distinct summer dormant period but can take summer water without too much ill effect.

    In addition to this, many species of plants will develop a long tap root system from germination in order to find enough moisture to enable them to tick over during dry periods. Young Erodium chrysanthum (true species) grown in small clay pots, plunged into a metre deep sand bed, grew roots over a metre long without increasing the size of the plant itself. Seeds sown directly into a deep, well drained bed will germinate and form their taproot. Plants removed from a seed tray, potted etc. etc. will form a root system which is shorter and more branched thus increasing their need for irrigation during the summer.

         Altitude is another factor that is often overlooked. Although the plant is thought to emanate from a hot climate in the Mediterranean region, they do in fact, come from higher altitudes which means a reduced summer temperature.

        Another killer comes in the form of the modern Nursery practice of potting into a peat based compost. Nice and easy for the nursery but placing this lump of compost into the ground with its’ plant firmly embedded, will either result in it being soaking wet or drying out to a moisture repelling nightmare. Fungal problems really enjoy these peaty playgrounds we provide for them. In Garden Centres, Phytophthora, a very nasty fungal problem, spreads from one pot to another during watering, brown Rhododendrons being an obvious indicator. Far better is a loam based compost with plenty of sharp sand and crushed limestone for Erodiums and cinereum group Geraniums, as well as our European Daphnes. Stone helps cool a compost in a pot or raised bed, moisture condenses on its’ surface and can then be used by the plant. I also use either a little sphagnum moss peat or some potting grade composted bark, which has fairly small particles, to open up the compost. Coir can cause fungal problems as it seems to dry out at the edges but remains moist towards the centre of the pot. A dry surface will no doubt prompt watering and the centre gets wetter, disaster again.

    Most of the European Genera we are discussing are more often than not found on limestone. Probably one of the most obvious selectors of a limestone environment is the Cyclamen, one learns not to bother to go and look for these if the rocks aren’t limestone. The Cyclamen Society has recorded pH levels well into double figures on their Field Studies, these findings no doubt apply to our Geraniaceae victims. A few years ago in the Northern Peloponnese we had been finding Geranium subcaulescens on some mountains, but upon reaching the summit of Mt.Parnonas we realised none had been encountered. We then noticed the limestone was not quite the same and noted curved strata covering lava plugs, so the rock had been heated. Somehow the chemistry had been affected and the Geranium was not happy. Apparently an alkaline soil keeps certain fungal problems under control and allows the plant to thrive.

    So here we have a few guidelines to keep the plants happier.

a)   Watering. Too much summer moisture is a killer, July and August seem the worst months. Many of our mountain dwellers will be covered in snow in winter, commencing growth as the snow melts around them, flowering and seed production goes on as the ground gets drier during the summer. This problem of summer-death was completely eradicated by growing plants in a deep sharp sand plunge under a glass roof. (Either a modified greenhouse as glass sides are a nuisance, causing overheating or clear corrugated plastic roofing on a wooden framework. The plastic should be high above the plants to allow good air movement, probably around a metre would be best or higher if practicable). Water well when in full growth but reduce water down to just a drop in summer when the plant shows sign of wilt. Just keeping the plunge slightly moist when using terracotta pots seems best, plastic pots need direct watering – but not much in summer! The plunge helps keep the roots cool. Practice makes perfect.

b)   Troughs. It is probably worth mentioning at this point that sinks and troughs can be killing grounds for our “victims”. Stone troughs in particular heat up in prolonged sunny weather and when watered well seem to develop a “fungal soup” which is simmered nicely. The deeper the sink / trough the better it is.

c)   Alkalinity. Use crushed Cotswold or Purbeck stone in the compost, these are readily available. I also add crushed chalk to the compost to help raise the pH, this can be purchased from Horticultural Trade Sundriesmen, normally labelled “Carbonate of Lime”. It is not expensive, just a few pounds for a large bag. (Mine is Needham’s Garden Lime which is ground chalk).

d)   Tap Roots. Try sowing seeds directly into raised beds etc., to promote tap roots. Good drainage is essential and I have found that a minimum depth of 45cms (18 inches) is a starting point. Drainage doesn’t seem to work well when only shallow.

e)   Peat. Refrain from multipurpose composts, this quantity of peat is a killer.

f)      Good ventilation. Many plants grow thousands of feet up on a mountain, where they encounter cool conditions but plenty of sun.

g)   Pots. When growing specimen plants I have found “Rose” or “Hellebore” pots are the best. 4 litre or sometimes the 3 litre size with a layer of crushed limestone at the bottom for drainage (up to 2”) before the compost is added. There are several makes of these deep pots, available from Horticultural Sundriesmen such as Monro and LBS Horticulture.

      Adhering to the above points will help reduce losses and when overhead water is controlled, “Sudden Summer-Death Syndrome” becomes a thing of the past.

                               Allan Robinson, Sutton Bridge, 5^th August 2007.