We all know people who feel the cold. In fact, as someone who shivers at the mere whiff of an icy draft and squawks at the thought of dipping a toe in the Irish Sea, I count myself among them. Equally, there are those of us who’ll happily wear shorts on all but the frostiest of days and who silently curse the roasty-toasty, thermostatically-controlled interiors of our friends and family’s homes. My teenage son, for example, is immune to the vagaries of an Irish winter. Bemused by requests to wear something warmer than a T-shirt, he’ll leap into an icy river just for the sheer fierce joy of it.
Plants are much the same, their ability to endure very cold temperatures shaped by a complex range of factors that includes their genes and their age as well as their environment. Snowdrops, for example, are helped to endure sub-zero temperatures by a special antifreeze protein the plants produce which protects their cell walls from being ruptured by ice crystals forming and expanding within the plant tissue. The fact that they’re a bulbous species whose leaves and flowers emerge from thick, fleshy structures below ground also helps.
But other kinds of plants – for example, half-hardy dahlias- have evolved in tropical, subtropical or Mediterranean climates where frost is rare or pretty much unknown. Faced with a cold, wet Irish winter like this one, the dahlia’s dormant, fleshy underground tubers can only survive by sitting in a very free-draining soil, ideally protected with a generous mulch of old leaves or straw.
Hardiness can also vary dramatically within a single genus, with certain species and/or varieties much more resilient than others. Similarly, it’s important to bear in mind that not all plant species classified as hardy are equally hardy. So, while the world-respected RHS hardiness ratings systems classifies plants according to one of seven main categories, where the hardiest rating (H7), means that the plant can survive temperatures below minus 20 degrees, and the least hardy rating means that it must be grown under cover of a heated glasshouse (minimum 15 degrees) all year round, inevitably there are subtle but important gradations within those seven categories.
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Plants with a RHS rating of H4, for example, can tolerate temperatures between minus 10 degrees and minus 5 degrees, temperatures reached in many parts of Ireland in the bitingly cold period just before Christmas. But depending on the individual species/variety, the degree of difference between minus 5 degrees and minus 10 degrees might turn out to be the difference between life and death, something that only becomes evident with the arrival of spring.
Likewise, the timing of very cold weather has a huge impact on plants. So, a harsh winter frost won’t do any harm to a magnolia or camellia, for example, but a late spring frost can destroy the flowers of these decorative woody species. That same late harsh frost can sear the emerging young foliage of even ultra hardy species of trees, shrubs and perennials, delaying their growth cycle by many weeks.
Prolonged freezing temperatures can also cause dehydration in a plant by starving it of available water, resulting in symptoms similar to the damage caused by drought. These include wilting of leaves, browning of leaf tips, and the eventual death of some parts (or all parts in the worst case scenario) of the plant. But rather like Lazarus, even plants that might initially appear to have been killed outright can gradually recover over time. So avoid making hasty decisions as regards replacing them.
As all seasoned gardeners know, acclimatisation also plays a key role in helping plants to survive cold temperatures. If, for example, you place young glasshouse-raised plants outdoors in early summer without first gradually hardening them off, then they’ll struggle to immediately adjust to those more challenging growing conditions. Unused to wind and unaccustomed to cooler temperatures, they go into plant shock, sacrificing their flowers, young leaf buds and tender new growth in a determined effort to keep themselves alive until they’ve acclimatised.
But if these same young plants are given the opportunity to slowly adjust to these more challenging growing conditions over a period of several weeks, then they’re very capable of gradually recalibrating their response while suffering no ill effects. Still not fully understood by modern science, this hardening-off process isn’t dissimilar to a seasoned ‘wild’ swimmer whose body gradually becomes used to a daily dip versus an unprepared newbie being forced to suddenly take the icy plunge.
Unsurprisingly, maturity also helps plants to tolerate the kinds of cold temperatures that would kill their younger selves. The baby seedlings of many kinds of hardy species, for example, can be killed by hard frosts that leave their adult equivalents unscathed, the latter’s well-developed root systems and more robust plant tissues helping to sustain them.
It’s important to add that winter cold isn’t always the gardener’s enemy. It helps to control many common garden pests and diseases, for example, and prevents weeds from germinating as well as helping to create a crumbly soil structure. For some plant species, exposure to freezing or near-freezing temperatures is also the crucial trigger for successful germination and/or flower formation.
If you’ve ever tried unsuccessfully to grow delphiniums from seed, for example, or wondered why you’ve had no luck getting the seed of bupleurum to germinate, or pondered the secret to breaking dormancy in penstemon seed, then the answer in each case is a period of cold, or what’s properly known as cold stratification. Pop those seeds in a bag in the fridge for a few weeks or leave them in a pot outdoors to overwinter and you can provide the chilly temperatures required.
Similarly, some species- for example tulips and narcissi- need a period of winter cold- or vernalisation- to flower properly in spring. Without this, their flower stems will be unnaturally short and their flowers small and malformed. The same is true of many kinds of fruit trees n which can need between 1000 and 1200 ‘chilling hours’ to fruit well. Likewise overwintering garlic also needs cold temperatures for the bulbs to develop properly.
Cold is also the friend of hardy annuals such as sweet pea that dislike being mollycoddled indoors. Capable of tolerating temperatures as low as minus 5 degrees (sweet pea have an RHS hardiness rating of H3), the young, containerised plants should be ‘grown hard’ in either a sheltered spot outdoors or under cover of an unheated glasshouse or polytunnel before being transplanted into their final growing spots in the garden or allotment by late March/ early April.
But of course this isn’t the case for all annuals. So over the coming months, make sure to keep any tender heat-loving species such as tomatoes, peppers, cucumbers, aubergines and most bedding plants as well as most succulents (these all typically have a rating of H1c/ 5 degrees -10 degrees or H2/1 degrees -5 degrees) in a bright, sheltered spot, under cover, where you can guarantee their protection from Jack Frost.
This week in the garden
This is a great time of the year to clean polytunnels/ glasshouses in preparation for the start of another growing season. Start by clearing away weeds along with any dead or diseased plants before thoroughly washing down the structure including any doors, vents and general fittings such as gutters, shelving, staging and water butts. Then use a long-handled soft brush, a sponge and a bucket of warm, soapy water (ideally an environmentally safe washing-up liquid) to remove algae and accumulated dirt.
To clean the hard-to-reach outer roof of your tunnel, use a few old sheets knotted together and then pulled back and forth over it, or a soft brush fixed to a telescopic handle or to a length of garden hose. Then hose it down with clean water before thoroughly washing down the inside, taking extra care to clean out any creases/ folds in the polythene cover where pests/ diseases could be lurking.
Wash down seed trays and ensure that you have enough seed compost, vermiculite, some horticultural grit, plant labels, a waterproof fine pen (for writing plant labels) in stock in readiness for the beginning of this year’s busy sowing season. Also check in advance that electric equipment such as heated propagators and glasshouse heaters are working.
Dates for your Diary
Annual Snowdrop Gala & Other Treasures, Ballykealey House, Ballon, Co Carlow, Saturday February 4th, tickets via firstname.lastname@example.org or from Robert Miller (087 9822135)
Ballyrobert Gardens, 154 Ballyrobert Road, Ballyclare Co. Antrim BT39 9RT), Snowdrop Open Days on Saturday February 11th & Saturday, February 18th (2pm-3.30pm), ballyrobertgardens.com
Hunting Brook, Lamb Hill, Tinode, Blessington, County Wicklow, guided tours of its snowdrop collection by owner Jimi Blake on February Wednesday 15th, Friday 17th, Saturday 18th and Sunday 19th at 11am & 2pm, huntingbrookgardens.com
Snowdrop month at Altamont Gardens, Ballon, County Carlow, throughout February, carlowtourism.com; Primrose Hill, Lucan, Co Kildare K78 C1W9, snowdrop garden open to the public 2pm-5pm throughout February