Temperature decides what you can grow and when you can grow it, and it does so twice a year: in spring, when a late frost can kill a tender annual overnight, and in summer, when a heatwave can stop a rose from flowering and scorch a hydrangea by noon. Flowers have a wider range of temperature tolerance than most gardeners realise, but each flower has a limit, and crossing it costs a season. A dahlia left in the ground over a hard winter will rot; a zinnia planted out a week before the last frost will blacken and die; a lavender that survives a cold winter in dry soil will die in the same cold in wet soil.
This guide explains frost, hardiness, and heat stress for garden flowers, and the practical protection that works — mulches, cloches, and the simple habit of waiting a week. Your local extension service can give you the frost dates and the hardiness zone for your area, which are the two numbers this whole guide hangs on.
Frost and the last-frost date
The last-frost date is the average date of the last spring frost in your area, and it is the anchor for planting tender flowers outdoors. Zinnias, marigolds, sunflowers, and dahlias are all frost-tender: a single night below freezing will kill them or damage them badly, so they go outside only after the last frost, and a week later is safer than a week earlier. Hardy annuals such as sweet peas, calendula, and cornflowers tolerate light frost and can go out two or three weeks earlier. The first-frost date in autumn is the other end of the season: it ends the annuals and tells you when to lift tender bulbs. Both dates are averages, not guarantees — a late frost can arrive after the average date, and a sheltered garden near a wall can be several degrees warmer than an open one. Watch the forecast in the weeks after the last-frost date, and keep a few cloches or sheets of horticultural fleece ready to cover tender plants on a cold night. Our flowers seasonal care guide uses these dates as the framework for the whole year.
Hardiness zones and what they mean
Hardiness zones divide the country by the average coldest winter temperature, and they tell you which perennials will survive the winter in the ground. A flower rated hardy to your zone should survive a normal winter; a flower rated for a warmer zone will need lifting, mulching, or growing as an annual. Roses are mostly hardy, though some tender varieties need a mulch of compost mounded over the crown in winter. Lavender is hardy in most areas but dies in cold, wet soil — it is the wet, not the cold, that kills it, which is why our growing lavender guide stresses drainage. Dahlias are not hardy in most cold areas and are lifted in autumn and stored frost-free over winter. Tulips and daffodils, by contrast, need a cold winter to flower well, and they will fail in a climate that never gets cold enough. Check the zone rating on the label, and if you are at the edge of a plant's zone, plant it in the most sheltered, best-drained spot you have.
Heat stress: when summer stops flowers
Heat affects flowers as surely as frost does. Roses stop flowering well when the temperature stays above about 35°C, and their blooms become smaller and paler. Hydrangeas wilt every afternoon in hot sun, even with wet soil, because they lose water through their large leaves faster than their roots can replace it. Zinnias and marigolds, by contrast, love heat and flower best in a hot summer. The signs of heat stress are wilting in the afternoon that recovers by evening, scorched leaf edges, and flowers that open and fade within a day. The remedies are water, mulch, and shade: water deeply in the morning so the soil is moist before the heat arrives, mulch to keep the roots cool, and give afternoon shade to the plants that need it, as our flowers sunlight requirements guide explains. Do not feed a heat-stressed plant — fertilizer on a stressed plant does more harm than good — and do not prune it either. Wait for cooler weather and the plant will usually recover on its own.
Protecting flowers from cold
Cold protection is mostly about timing and insulation. Mulch perennials and roses with a thick layer of compost or bark after the first frosts, when the ground is cold but not yet frozen — mulching too early keeps the soil warm and delays dormancy, while mulching after the ground has frozen traps the cold in. Mound compost over the crown of tender roses. Lift dahlias after the first frost blackens their foliage, dry the tubers, and store them in a frost-free place in slightly damp compost or vermiculite. Cover tender annuals with horticultural fleece or an upturned bucket on a night when frost is forecast, and remove the cover in the morning so the plant does not overheat. A cloche over a young plant can give it several degrees of protection, which is often the difference between life and death on a marginal night. And remember that a plant in a container is far more vulnerable to cold than the same plant in the ground, because its roots have no earth to insulate them — move pots against a wall or into a shed on the coldest nights.
Microclimates: the warm and cold spots in your garden
Every garden has microclimates — spots that are warmer or colder than the garden as a whole. A south-facing wall absorbs heat during the day and releases it at night, so the bed against it can be several degrees warmer than the open garden, and it is the place for the tenderest flowers. A low-lying hollow, by contrast, collects cold air on a still night, and it is where the frost will strike first — never plant tender flowers in a frost pocket. A spot sheltered from the wind is warmer than an exposed one, and a spot under a tree canopy is protected from frost, since the canopy radiates a little heat back down. Use these spots deliberately: put the tender annuals against the warm wall, the hardy ones in the open, and the bulbs that need winter cold in the coldest part of the garden. A gardener who knows the microclimates of their own garden can grow flowers that their neighbours, a street away, cannot.
Temperature and the flowering calendar
Temperature drives the flowering calendar. Tulips and daffodils flower in spring because they need winter cold to trigger their growth, and they will flower later in a cold spring and earlier in a warm one. Roses flower best in the mild temperatures of late spring and early summer, and they pause in the hottest weeks. Dahlias flower from midsummer until the first frost, and the first frost is what ends them. If your flowers are flowering earlier or later than the books say, the temperature is almost always the reason, and it is not a problem — it is the garden responding to the year it has had. Keep a simple note of when each flower first blooms and when the first and last frosts arrive, and after a few years you will have a calendar for your own garden that is more accurate than any book. If you are unsure about a diagnosis, a product, or a local rule, check your local extension service — its advice is tailored to your soil, climate, and the pests and diseases active in your area.
Frequently asked questions
When can I plant zinnias outside?
After the last frost in your area, and a week later is safer than a week earlier. A single night below freezing will kill them.
What temperature kills dahlias?
Dahlias are frost-tender. The first frost blackens their foliage and ends the season, and the tubers will rot if left in frozen ground. Lift them in autumn.
Why do my roses stop flowering in summer?
Heat. Roses flower best in mild temperatures and slow down in sustained heat above about 35°C. They usually resume when the weather cools.
How do I protect flowers from frost?
Cover tender plants with horticultural fleece or a cloche on frosty nights, mulch perennials after the first frosts, and lift tender bulbs before the ground freezes.
Do tulips need cold winters?
Yes. Tulips and daffodils need a period of winter cold to flower well, and they perform poorly in climates that never get cold enough.