Sea influence
The sea can soften winter extremes because water warms and cools more slowly than land, but coastal gardens may still face salt, wind and winter wet.
Garden notes
Is this plant tender?
The short answer: In a coastal garden such as Earlscliffe, the most useful response to the question “Is this plant tender?” is “Tender where?”
A tender plant is not simply a plant from somewhere warm. It is a plant whose survival, appearance or performance outdoors is limited by cold, frost, winter wet, wind or insufficient summer heat, or some combination of these.
In practical terms, a tender plant is one whose long-term success outdoors depends strongly on local microclimate and cultivation.
In ordinary gardening language, a tender plant is one that needs winter protection, glasshouse conditions, or a particularly sheltered outdoor position. That is a helpful starting point, but it is too simple for real gardens. Tenderness depends on the plant, the site, the weather, the soil, the season and the gardener’s tolerance for risk.
At Earlscliffe in Howth, this question is especially interesting. The garden is coastal and maritime, so the sea can soften winter extremes. Yet the same setting can also bring wind, salt, humidity, storms and winter wet. A plant may be protected from deep frost but still be damaged by exposure or saturated soil.
A practical definition: a tender plant is one whose long-term success outdoors depends strongly on local microclimate and cultivation.
Six kinds of tenderness
It is often more useful to ask what kind of tenderness a plant has than to call it tender as a single category.
| Type of tenderness | What it means | Practical response |
|---|---|---|
| Frost-tender | Damaged or killed by temperatures near or below freezing. | Grow frost-free, protect in winter, or plant out only after frost risk has passed. |
| Cold-tender | Damaged by prolonged low temperatures, even without obvious frost. | Use the warmest microclimate and avoid cold, drying winds. |
| Wet-winter tender | Can tolerate cold better when dry, but suffers in saturated soil. | Use raised beds, slopes, gravel, sharp drainage, or pot culture under cover. |
| Wind-tender | Hardy enough in temperature terms but damaged by exposure, salt or desiccating winds. | Provide shelter and avoid exposed coastal positions. |
| Heat-demanding | Survives winter but needs more summer warmth to flower, fruit or ripen growth. | Use a warm wall, reflective surfaces, containers, or the sunniest part of the garden. |
| Marginally hardy | Survives many mild winters but may be damaged or lost in exceptional cold. | Keep a backup plant, take cuttings, mulch crowns and record what happens. |
How to interpret hardiness zones, climate classification, and plant hardiness ratings
Before deciding whether a plant is tender, it helps to separate three related but different ideas: the climate of a place, the cold tolerance of a plant, and the local conditions of a particular garden.
- USDA plant hardiness zones describe average winter minimum temperatures for locations
- Köppen climate classification describes broader climate patterns such as oceanic, Mediterranean or continental climates
- RHS plant hardiness ratings describe how much cold individual plants are expected to tolerate.
Used together, they are useful guides, but none of them can replace local observation of soil, shelter, wind, winter wet, summer warmth and microclimate.
1. USDA Plant Hardiness Zones
The widely used USDA Plant Hardiness Zone Map was developed for the United States, but is often used as a broad international reference. It divides places according to their average annual extreme minimum winter temperature. Zone numbers run from 1, the coldest, to 13, the warmest. Each zone is further subdivided into half zones, designated as "a" and "b".6
Important Limitation of USDA zones
USDA zones consider only winter minimum temperature, not other climate factors. They do not account for things like winter wetness, summer heat, or length of growing season. Thus two places in the same USDA zone can have very different climates – for instance, Seattle and Dallas are both roughly Zone 8, but Seattle has cool summers and wet winters, while Dallas has scorching summers and dry spells, which matter to plant growth. 7
USDA Plant Hardiness Zones for Ireland and Britain
Approximate USDA-style mapping places much of Ireland and Britain between Zones 8 and 10, with colder inland areas generally lower and some very mild coastal or island locations higher.19 20
The highest zones in Ireland are very localised and typically confined to small coastal or offshore situations.
Examples on the island of Ireland include:
- Zone 8a (-12.2°C to -9.4°C) in places such as Carlow
- Zone 8b (-9.4°C to -6.7°C) in places such as Longford, Kilkenny, Portlaoise, Enniskillen, Edenderry, and Tullamore
- Zone 9a (-6.7°C to -3.9°C) in places such as Dublin, Belfast, Arklow, Castlebar, Limerick, Roscommon, and Wicklow
- Zone 9b (-3.9°C to -1.1°C) in places such as Clonakilty, Midleton, Bangor, Rosslare, and Tralee
- Zone 10a* (-1.1°C to 1.7°C) in places such as Portstewart, and off the west coast of Cork and Kerry
- Zone 10b* (1.7°C to 4.4°C) in places such as off the west coast of Cork and Kerry 19 20
* The upper zones (10a/10b) are very marginal and mostly offshore/microclimatic.
Britain shows a similarly maritime pattern, with milder western and coastal districts, especially Cornwall, the Isles of Scilly and parts of western Scotland, contrasting with colder inland and upland areas.
However, USDA zones need careful interpretation in Ireland and Britain. They are based on average annual extreme minimum winter temperatures, which makes them useful for comparing winter cold, but less useful for describing the full growing conditions of a cool maritime climate. Ireland and Britain have relatively mild winters for their latitude due to the North Atlantic. However, summers are cooler and growing seasons shorter than in many places with similar USDA ratings.
This matters because winter survival is not determined by minimum temperature alone. In cooler summers, new growth may not ripen fully before winter, leaving stems, buds and young shoots more vulnerable to frost damage. So while many parts of Ireland and Britain may appear relatively mild on a USDA map, those zones can give a misleading impression unless summer warmth, winter wet, wind exposure, soil drainage and local microclimate are also considered.
USDA Plant Hardiness Zone Map 2023 updates
The USDA Plant Hardiness Zone Map was updated in 2023 because a newer 30-year climate period was available, more weather station data could be incorporated, and improved GIS and PRISM mapping methods allowed finer resolution. The 2023 map uses 1991–2020 data, replacing the 1976–2005 period used for the 2012 map.
Although many areas shifted warmer, USDA cautions that changes in hardiness zones should not be read as a simple measure of climate change, because the map is based on average annual extreme minimum temperatures and is also affected by improved data and mapping methods. 18
However, the map still measures only one thing: the average annual extreme minimum winter temperature of a place. It is therefore useful for comparing winter cold, but it does not account for winter wet, wind, soil drainage, summer warmth or garden-scale microclimates.
2. Köppen climate classification
The Köppen climate classification is a global system for grouping climates according to long-term patterns of temperature and precipitation. It uses letter codes: the first letter identifies the broad climate group, while later letters describe rainfall seasonality and summer or winter temperature patterns.
For example, much of Ireland and Britain is classified as Cfb. In plain English, this means a temperate oceanic climate: mild to cool winters, mild or warm summers, and rainfall spread through the year.
| Region | Common Köppen types | Plain-English description |
|---|---|---|
| Western Europe, including Ireland and much of Britain | Cfb | Temperate oceanic: mild winters, mild or warm summers, and rainfall in all seasons. |
| Southern Europe and Mediterranean coasts | Csa / Csb | Mediterranean: dry summers and mild, wetter winters. |
| Central and Eastern Europe | Dfb / Dfa | More continental: colder winters and warmer summers than western maritime areas. |
| Northern Scandinavia and high mountains | Dfc / ET | Subarctic, tundra or alpine climates, with colder conditions and shorter growing seasons. |
| Parts of south-east Spain and other dry areas | BSh / BSk | Semi-arid climates, with limited rainfall. |
For gardening, the important point is that Cfb does not mean frost-free. It describes the broad regional climate, not the detailed conditions of a particular garden. A coastal garden in Howth may share Ireland’s temperate oceanic climate, while still having warmer walls, exposed windy slopes, sheltered corners, frost pockets, wet winter soil and other local microclimates. See the The science behind tender plants and cold below for more details.
3. RHS plant hardiness ratings
In 2012, the Royal Horticultural Society introduced a revised hardiness rating system for plants. Unlike USDA zones, which describe the climate of places, RHS ratings describe the cold tolerance of individual plants. 1
The scale runs from H1a, for plants that need temperatures above 15°C, to H7, for plants able to tolerate temperatures below −20°C.
- At the tender end, H1 plants generally need heated glasshouse or warm indoor conditions.
- H2 plants tolerate cool conditions but not freezing.
- H3 plants are half-hardy and may overwinter in mild, coastal or well-sheltered places, but remain vulnerable to early or sudden frosts. 14
- H4 to H7 describe increasing levels of outdoor hardiness.
Ratings are useful, but not absolute: The important point is that a rating is a guide, not a guarantee. A plant rated H3 may survive for years in a sheltered coastal garden in Howth, Cornwall, the Isles of Scilly or the west of Scotland, yet fail in a frost hollow inland. A plant rated H4 may tolerate the stated temperature but still suffer in heavy wet soil or harsh wind.
RHS plant hardiness ratings and broad USDA equivalents
The RHS and USDA systems are not directly interchangeable. RHS ratings describe the cold tolerance of individual plants, while USDA zones describe the typical winter minimum temperatures of locations.
Chart 1 below gives a broad comparison of the two systems, rather than a direct conversion. RHS plant hardiness ratings indicate the level of cold a plant is expected to tolerate, while USDA hardiness zones describe the average annual extreme minimum winter temperature of a location. Both are useful guides, but neither fully accounts for wind, winter wet, soil drainage, salt exposure, summer warmth, plant age, provenance, or the microclimate of a particular garden.1 6
Location, Location, Location: Earlscliffe, Howth and microclimate in a maritime setting
Whether a plant is considered tender depends greatly on where it is grown. The same plant may be hardy enough in a sheltered coastal garden, marginal in an exposed suburb, and impossible outdoors in a colder inland frost pocket. This is why the question “Is it tender?” often needs the follow-up: “Tender where?”
As mentioned above, Ireland’s climate is strongly maritime (Köppen Cfb) compared with more continental climates at similar latitudes. The Atlantic Ocean, including the warm North Atlantic Drift associated with the wider Gulf Stream system, helps moderate temperatures around Ireland and Britain. This contributes to milder winters, smaller seasonal temperature ranges and fewer severe frosts in many coastal areas than would otherwise be expected at this latitude.
A key feature of the Irish climate, however, is variability. Ireland lies close to the polar front, where warm and cold air masses meet, so weather can change quickly. Mild, humid conditions may be followed by colder, clearer nights, especially in winter. For gardeners, these sudden shifts can matter as much as the average climate, because plants are often damaged by abrupt cold after a mild spell rather than by steady, predictable cold.22
In maritime climates, the sea warms and cools more slowly than land. Coastal areas therefore often avoid the lowest night-time temperatures experienced inland.3 This helps explain why some marginal plants can grow outdoors in coastal Irish gardens. Tree ferns, palms, South African perennials and other warm-temperate plants may survive in places such as Howth, West Cork, Cornwall or the west of Scotland, while needing winter protection in colder inland gardens at similar latitudes.
This does not mean that coastal gardens are automatically easy places for tender plants. Maritime conditions can also bring salt-laden winds, high humidity, winter wet, storms and slow spring warming. At Earlscliffe, the sea may reduce the severity of frost, but exposure, drainage, shelter and local topography still decide which plants thrive, merely survive, or fail.
What shapes a garden microclimate?
A microclimate is the small-scale climate of a particular place within a wider climate. In gardening, it can explain why the same plant succeeds in one corner of a garden but fails only a few metres away.
Shelter and exposure
Walls, hedges, trees and buildings can reduce wind damage, while exposed slopes may suffer from salt-laden or drying winds.
Elevation and slope
Cold air can drain downhill and collect in low spots, while slopes and raised areas may avoid the worst frost pockets.
Walls and built surfaces
Stone, brick, paving and walls can absorb heat during the day and release it slowly at night, creating warmer planting positions.
Aspect and sunlight
South- and west-facing positions are often warmer and brighter, while shaded north-facing areas may stay colder and wetter for longer.
Soil and drainage
Free-draining soil can help marginal plants survive cold, while saturated winter soil can damage roots and crowns even in relatively mild weather. 2
These nuances mean that gardeners in milder microclimates can sometimes grow plants that would normally be considered tender for the wider region. A Howth gardener, for example, may be able to use maritime influence, urban warmth, walls and shelter to grow borderline hardy plants outdoors, while the same plants may need winter protection in frostier inland gardens.
A well-known extreme of Irish microclimate is Gulf Stream-warmed West Cork, where gardens like Ilnacullin (Garinish Island) near the shore of the Beara Peninsula host subtropical species outdoors year-round – essentially subtropical or warm-temperate plants that elsewhere in Ireland are strictly “tender greenhouse” subjects. 21
Local Earlscliffe records
David Robinson’s notes, articles and scientific papers show how plants actually behaved in the microclimate at Earlscliffe in Howth.
He used Earlscliffe as a living trial ground for plants close to the edge of their climatic range in Ireland. Many were associated with USDA Climatic Zone 9 (the RHS levels were not defined in 2001 when this study was completed) and the plants came from regions such as the Canary Islands, South Africa, South America, Australasia and New Zealand.13
David Robinson recorded regular air frosts between early December and mid April, but freezing days were rare except in exceptional winters. Summer temperatures were moderated by the sea and seldom exceeded 23°C, while annual rainfall was relatively low for Ireland, around 650 mm and spread through the year.12 13 See the case study below for more details.
In more recent Earlscliffe observations, with the exception of the severe winter of 2010/11, this broad pattern appears to have continued.
Earlscliffe also differs from many gardens in County Dublin because its soil is derived from Cambrian shale and quartzite and is mostly acid to neutral. That matters because hardiness is rarely about temperature alone: drainage, pH, soil structure, wind exposure, shelter, rainfall pattern and plant health all affect whether a plant survives or merely struggles.
Other maritime gardens show the same principle. Kilmacurragh in Co. Wicklow has long been used for trialling tender introductions, especially from the Southern Hemisphere.8 Logan Botanic Garden in south-west Scotland is well-known for growing plants outdoors that would be difficult in colder inland districts.9 Tresco Abbey Garden in the Isles of Scilly is an even more extreme maritime example, where shelter and mildness allow many warm-temperate plants to flourish outdoors.10
Earlscliffe case study from David Robinson
David Robinson’s 2001 papers are used here as local observational evidence. They record actual temperatures, actual plants and actual outcomes at Earlscliffe, and are best read alongside current climate data, modern plant physiology and present-day hardiness guidance.
One winter that tested the Earlscliffe garden: 2000/01
David Robinson’s notes on the winter of 2000/01 are valuable because they record actual garden conditions and actual plant responses.
Autumn 2000 was mild and wet. Then, on 26/27 December, the air temperature at Earlscliffe fell to −6°C. Over the next four nights it fell to −4°C, −7°C, −5.5°C and −4.5°C. For 3 days between 28 and 30 December, the temperature did not rise above −3°C or −4°C. At the end of the cold spell, the temperature rose by about 7.5°C overnight.11
A second cold spell in late February 2001 brought further frost, snow and a severe northerly gale. For a garden where snow had been rare over the previous 30 years, these conditions were exceptional.
−7°C
Lowest recorded temperature in the December 2000 cold spell
3 days
Temperatures did not rise above −3°C or −4°C
7.5°C
Approximate overnight rise at the end of the cold spell
30 years
Snow had been rare in David Robinson’s previous records
The plant responses were varied. Young Musa basjoo were killed to ground level, although they had previously survived winters at Earlscliffe with stems intact since 1973. Erica canaliculata, usually one of the hardier South African heaths in the garden, had more than 50 cm of previous year’s growth killed back on some plants. Many succulents, especially Aeonium species, collapsed rapidly. Other plants, including Metrosideros excelsa, Psoralea pinnata and Corymbia ficifolia, showed damage more slowly, with leaves wilting and browning over several weeks.11
| Plant or group | Observed response at Earlscliffe | What it illustrates |
|---|---|---|
| Aeonium species | Many collapsed rapidly in severe frost, especially during the December 2000 cold spell. | Water-rich succulent tissues can be damaged quickly. |
| Brugmansia sanguinea | Top growth could be killed, but plants were capable of resprouting from the base. | A plant may be top-tender but root-hardy in a favourable site. |
| Echium pininana | Seedlings were killed in some earlier severe winters, but later populations showed better survival.13 | Self-seeding populations may become locally adapted through selection. |
| Corymbia ficifolia / Eucalyptus ficifolia | Flowered spectacularly in 1976 but was later killed; seedlings raised from collected seed were badly damaged in 2000.13 | A plant may grow and flower for years before an exceptional winter exposes its limit. |
| Palms including Juania australis | Several palms came through the 2000/01 winter well, including Juania australis. | “Exotic-looking” does not automatically mean highly frost-tender. |
| Metrosideros excelsa and M. robusta | David Robinson recorded marked differences between species in the same genus. | Hardiness cannot safely be assumed at genus level. |
Taken together, these observations show that in a mild coastal garden, the duration of cold and exposure to wind can be as important as the absolute minimum temperature.
Local adaptation and the self-seeding test
One of David Robinson’s most interesting observations concerns self-seeding plants. When an exotic plant produces self-sown seedlings year after year, it suggests that at least part of its life cycle is well matched to local conditions. At Earlscliffe, plants such as Luma apiculata, Solanum laciniatum, Cordyline australis and Echium pininana were recorded as self-seeding freely.13
Echium pininana is especially revealing. David Robinson noted that earlier cold winters, including 1978/79 and 1986/87, killed all seedlings, but that after later severe winters, including 1995/96 and 2000/01, many seedlings survived. His interpretation was cautious but suggestive: repeated cold stress may have selected for a hardier local population.13
The severe winter of 2010–11 repeated this widespread loss of Echium seedlings, and it is now thought that the remaining Echium seed stock may be more cold-hardy than earlier generations.
Conclusion
The process observed by David Robinson on the winter of 2000/01 is a reminder that hardiness is not always static. It can vary between species, provenances, seedlings and generations. For gardeners, local seed-grown plants from surviving parents may sometimes be more useful than a generic plant label from a very different climate.
The science behind tender plants and cold
The visible signs of cold damage are familiar: blackened leaves, collapsed stems, scorched young growth, split bark, rotted crowns, or plants that look alive in winter but fail to restart in spring. Behind those symptoms are several physiological processes.16
Why cold can kill plants
When temperatures fall below freezing, ice often forms first outside plant cells rather than inside them. This draws water out of the cells, causing dehydration stress. If freezing is severe, prolonged or rapid, cell membranes can be disrupted. If ice forms inside cells, damage is often fatal.15
Plants from colder climates have evolved ways to manage this stress. They may change membrane composition, accumulate sugars and protective proteins, reduce free water in tissues, and enter a hardened state after exposure to cool but non-freezing conditions. This process is known as cold acclimation or frost hardening.5
Timing matters as much as the absolute minimum. A plant that has gradually hardened in autumn may tolerate a temperature that would damage it badly after a mild spell in late winter. This is why a warm February followed by a sharp March frost can be so damaging.4
David Robinson’s observations make the same point in garden terms: some plants collapsed immediately, some died slowly with drought-like symptoms, and others lost their top growth but later resprouted from the base. The morning after a frost does not always tell the full story.12
Evolutionary perspective
Freeze tolerance is a specialised evolutionary trait. Many ornamental plants grown in Irish and British gardens originate from regions with little or no severe freezing, so their tissues and seasonal rhythms are not well adapted to cold, wet winters.17
As a result, these plants are often being grown outside the climate in which they evolved. This helps explain why so many ornamentals need protection in cool maritime gardens: they are essentially living beyond their natural comfort zone. 17
By contrast, traits such as dormancy, cold acclimation and protective compounds evolved over long periods in temperate climates. Even so, cold tolerance is not fixed. A plant that is normally hardy to around −5 °C may be damaged at −2 °C if it has not acclimated properly or is under stress.15
What this means in practice
In practice, most failures can be traced back to one or more of the six kinds of tenderness outlined earlier, whether frost, cold, wet soil or exposure.
In a maritime Irish garden, success with tender plants usually depends on matching the plant to the right risk. If the weakness is frost, choose the warmest and most protected part of the garden. If the weakness is winter wet, focus on drainage rather than wrapping. If the weakness is wind, shelter may matter more than temperature.
For marginal plants, it is often wise to keep a duplicate in a pot, take cuttings, or protect the crown rather than the whole plant. Some plants can be treated almost as die-back perennials: the top growth may be damaged, but the crown or roots survive and regrow. Others must keep their growing point intact, which makes them much riskier outdoors.
The most useful evidence is local evidence. Record minimum temperatures, frost dates, soil conditions, exposure and actual plant performance. Over time, a garden develops its own hardiness map. For Earlscliffe, that local record may be more useful than any generic hardiness chart.
Conclusion
Tenderness is not a fixed quality that belongs to a plant in isolation. It is the result of a relationship between the plant and the place where it is grown. Cold tolerance, winter wet, wind exposure, soil drainage, summer warmth, plant age, provenance and microclimate all help decide whether a plant merely survives, grows well, flowers reliably, or eventually fails.
Hardiness ratings and climate zones are useful guides, but they should not be treated as guarantees. The RHS system helps describe the cold tolerance of individual plants, while USDA zones describe the winter minimum temperatures of places. Neither system can fully describe the conditions in a real garden, especially a maritime garden where mild winters, cool summers, salt-laden winds, wet soil and occasional severe cold may all interact.
At Earlscliffe, David Robinson’s records show why local observation matters. Some plants that look tender can prove surprisingly resilient in a sheltered coastal position, while others may grow well for many years before an exceptional winter exposes their limits. Some fail suddenly, some decline slowly, and some recover from the base or persist through self-sown seedlings.
The most useful question is therefore not simply whether a plant is tender, but how and where it is tender. In a garden such as Earlscliffe, success comes from matching the plant to the right place, understanding the particular risk, and learning from what actually happens over time.
In the end, the most honest answer to “Is this plant tender?” remains: “Tender where?”
References & Further Reading
- Royal Horticultural Society. RHS hardiness rating . ↩
- Royal Horticultural Society. Microclimates: assessing your garden . ↩
- Met Éireann. 1991–2020 climate averages . ↩
- Food and Agriculture Organization of the United Nations. Frost protection: fundamentals, practice and economics . ↩
- Thomashow, M. F. “Plant cold acclimation: freezing tolerance genes and regulatory mechanisms.” Annual Review of Plant Physiology and Plant Molecular Biology . ↩
- USDA Agricultural Research Service. USDA Plant Hardiness Zone Map . ↩
- McKenney, D. W. et al. “Beyond Traditional Hardiness Zones: Using Climate Envelopes to Map Plant Range Limits.” BioScience . ↩
- National Botanic Gardens of Ireland. Kilmacurragh . ↩
- Royal Botanic Garden Edinburgh. Logan Botanic Garden . ↩
- Tresco Abbey Garden. Abbey Garden . ↩
- Robinson, David. Cold hardiness and Climatic Zone 9 plants . Paper prepared for the International Magnolia Society 2001 Annual Meeting, Dublin. ↩
- Robinson, David. Plant Hardiness . Further notes for the 2001 International Magnolia Society Annual Meeting. ↩
- Robinson, David. Climatic Zone Plants at Earlscliffe, Baily, Co. Dublin . Paper presented at the Federation of Zoological Gardens of Great Britain and Ireland, Fifth Annual Plant Group Conference, Zoological Society of Ireland, 5–7 September 2001. ↩
- Royal Horticultural Society. When to plant out tender plants – avoiding late frosts . ↩
- University of California - Agriculture and Natural Resources. Protecting Plants from Extreme Cold . ↩
- University of California - Agriculture and Natural Resources. Chilling Injury . ↩
- Frontiers in Plant Science. Adaptation to seasonality and the winter freeze . ↩
- U.S. Department of Agriculture - Agricultural Research Service. USDA Unveils Updated Plant Hardiness Zone Map . ↩
- Plantmaps interactive maps displaying the USDA Hardiness Zone Map. Ireland Plant Hardiness Zone Map . ↩
- Plantmaps interactive maps displaying the USDA Hardiness Zone Map. United Kingdom Plant Hardiness Zone Map . ↩
- Office of Public Works (OPW). Heritage Ireland Ilnacullin – Garinish Island. An island garden of rare beauty . ↩
- Met Éireann - The Irish Meteorological Service Climate of Ireland . ↩
This page is periodically reviewed and expanded. Last substantial update: 9 July 2026.