The Newnham Naturalists – dahlias, bats and box tree moths

From pompom to cactus, waterlily to fimbriated, nothing divides opinion in the shed quite like deciding what’s best in a Dahlia. But for late season colour, flowering well up to the first frosts, they are invaluable in mixed borders and pots in a sunny position. Dahlias are herbaceous perennials that grow from tubers and are members of the Asteraceae family. There are currently 42 recognized species, mostly natives of Mexico, coming from upland and mountain areas of pine and oak woodland.

Three species, D. pinnata, rosea and coccinea, were imported to Europe in the 18th Century and from these plant breeders have developed an incredible 57,000 named varieties. This enormous variation is in part due to the fact that Dahlias are octoploid, that is they have eight sets of homologous chromosomes rather than the usual two.

For classification they are subdivided into 14 groups depending on flower shape (cactus, anemone, single etc) and come in a range of sizes, from the tiny Lilliputs, to the enormous show Dahlias, and in all colours other than the elusive blue.

Dahlias are best grown in rich soil with good drainage, so, preferably 2/3 weeks before planting, prepare the ground by digging over and adding an organic fertilizer such as blood fish and bone. As plants are not frost hardy, they should not be planted out until late May in the south, or June further north, and should be hardened off first.

Once they start growing, nipping out the top shoot will encourage the plant to become bushy and produce more flowers. Once flowering, regular generous  watering, feeding with a high- potash, foliar fertilizer and dead- heading will encourage  non-stop flowering until the first frosts blacken the foliage.

At this point you have to decide whether to risk leaving them in the ground over winter, or to lift them. In a sheltered spot or in a mild winter, the tubers, especially if covered with a thick layer of mulch, should survive. But if you are at all worried about losing your prized ‘Piper’s Pink’, as soon as the foliage is blackened by the first frost, the tubers need carefully lifting, cleaning and the stems trimming to 15cm. There are various methods of storage, but we hang the tubers upside down in a cool greenhouse to dry them out, and then store them, uncovered, in a  cool, dry, frost-free place, regularly checking them for rot, until they shoot the following spring.

If like me you need 50 D. ‘Bishop of Llandaff ’ for your border, propagation is best done by either splitting the tubers, or by taking cuttings in the early spring. When splitting tubers, make sure each new piece has a section of the crown (ie a piece with a stem growing from it) attached.  To take early cuttings, plant tubers in trays of compost and keep warm and watered. Once the shoots appear, select strong healthy shoots about 7.5 cm long, and cut them with a sharp clean knife, preferably taking a little sliver of the parent tuber. Trim the lower leaves, dip in hormone rooting powder, and insert into a pot of free-draining compost (either cutting compost or add grit to multi-purpose).

Finally, although they are one of our most popular garden flowers, most of us in England are pronouncing their name wrongly.  Named in 1791 by Cavanilles, Director of the Royal Botanic Garden Madrid,  after the Swedish botanist  Andreas Dahl, we should be saying  ‘Dahl – ia’ as in Roald, not ‘Day-lia’  as in Doris.

Lottie Collis
Head Gardener

A Freshers’ Week Activity With a difference!

On Friday, October 6 members of Newnham College and CU Nature Society had the opportunity to discover some of Cambridge’s most exciting “night-life”, although not the sort you’d find clubbing! Using heterodyne detectors, we explored Newnham College Gardens and nearby local nature reserves in search of bats.

The bats use high frequency sound waves to navigate in the dark. This is called echolocation. These calls can be converted into audible sounds by bat detectors and used for identification purposes. We were also able to obtain sonograms using a recording bat detector. These confirmed the identifications we had made using the other detectors: for example, the “hockey stick” shape is characteristic of pipistrelle calls.

In the gardens themselves we saw a couple of common pipistrelles. These are the UK’s most common species of bat and can frequently be found in urban areas. They kept flying back and forth amongst the trees near the Old Labs, feeding on insects. A single common pipistrelle can eat up to 3000 midges in a single night! We then set off to explore some local nature reserves, adding two more species to our tally: soprano pipistrelles and a noctule. The noctule is the UK’s largest bat, weighing 18-40g (in comparison, pipistrelles weigh about 5g). The two pipistrelle species are closely related and were for a long time believed to be the same species. Whilst they have similar-sounding echolocation calls, they can be differentiated by the frequency: common pipistrelles call mainly at 45kHz, sopranos at 55kHz.

The event wasn’t just for fun – I wanted to raise awareness of local wildlife, the challenges it faces and dispel some of the misconceptions people have about bats (no, they are not blind, they won’t fly in your hair, and they won’t drink your blood – not in this country, at least!). I’m very grateful to Scudamores Punting Company for lending us their heterodyne bat detectors for the evening – the experience really would not have been the same without them.

It was very pleasing to have so many people come on the walk – 27 in total! I hope everyone found the evening informative and enjoyable!

Bryony Yates, JCR Green Officer 

Moth of the Month – Box tree moth –Cydalima perpectalis

This moth was trapped just down the road in Trumpington but it is one we are hoping NOT to find in the garden at Newnham. A colonist from Asia, it has only been seen in this country for ten years or so but is alarming the garden community as the larvae can defoliate box hedges at a rapid rate. The threat is taken so seriously that the RHS have set up a notification site online. If you do happen to see one locally, try to photograph it, note the date and place, and please inform a member of the garden team.

Chris Thurgood

  • Gardener Chris Thurgood is our ‘moth man’. His keen interest in Lepidoptera (which means “scaly winged”) aka…moth watching, extends to operating a moth trap in Newnham gardens.  This is basically a wooden box with an ultra-violet light on top that attracts and holds the moths until morning, when they are identified, counted and then released.