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House Beautiful’s gardening report identifies begonias, dahlias and cosmos as vulnerable to the first autumn frost, which can damage tender growth and, in dahlias, threaten tubers. Experts quoted in the report recommend covering plants overnight, moving containers beside a sheltered wall and mulching soil; covers should come off in the morning.
Begonias, dahlias and cosmos are among the popular garden plants that can be damaged by the first autumn frost, according to a House Beautiful report featuring advice from gardening experts. They recommend covering vulnerable plants overnight, sheltering pots and hanging baskets, and insulating soil before freezing temperatures arrive.
The report says the first frost can catch plants while they are still growing actively. Eflorist flower and garden expert David Denyer explains that fresh growth after warm weather can contain substantial water and have soft cell walls. When the water freezes and expands, it can rupture plant cells, leaving affected tissue blackened and unable to recover. Plants generally become more resilient later in winter as they harden off, he says, but the first frost may arrive before that process has taken place.
Begonias are described as half-hardy bedding plants that cannot tolerate freezing temperatures well. Dahlias face a separate risk: their fleshy tubers store water, and frost damage to the tubers can prevent the plant from regrowing the following year. Horticultural expert Monique Kemperman of Plants & Flowers Foundation Holland says cosmos are tender annuals from warmer climates. Frost can blacken and collapse their stems, foliage and flowers, ending their flowering season.
For plants in the ground, Denyer advises placing fleece, cloches or an old sheet over vulnerable growth when a cold night is forecast. He says a cover can hold a thin layer of warmer air around the plant. Remove it the next morning to allow air to circulate; leaving it on through the day can trap moisture and raise the risk of rot or mold.
Frost Can Damage Growth and Tubers
The practical stakes differ by plant. A light frost may blacken or collapse tender cosmos and begonia growth, while damage to a dahlia’s water-storing tuber can affect whether it returns next year. Acting before temperatures fall gives gardeners a chance to limit overnight exposure rather than discovering damage after the frost.
Protection does not require specialized equipment in every case: the report lists fleece, cloches and old sheets as possible covers. The guidance also applies to container plants, which can be moved to a more sheltered position, and to roots, which may benefit from insulation around the soil.
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Why the First Frost Catches Plants
The report’s central point is that the first frost may arrive before plants have hardened off for winter. Denyer says warm conditions and a wet September can be followed by new, water-rich growth that is less prepared for freezing temperatures. Later in the season, many plants naturally become more resilient, but that adjustment may not have happened by the first cold night.
For potted plants and hanging baskets, Denyer recommends moving containers close to a brick or stone wall. Such walls absorb warmth during the day and release it slowly at night, he says, potentially raising the nearby temperature by a few degrees. He also suggests spreading compost, bark or fallen leaves around plant bases to help insulate the soil and reduce temperature and moisture swings around roots.
“The first autumn frost often catches gardeners off-guard because it arrives while your garden still looks full of life.”
— David Denyer, Eflorist flower and garden expert
Local Frost Timing and Plant Damage
The report offers general seasonal advice, not a forecast for a particular location. When the first frost will occur and how cold or long-lasting it will be depend on local conditions; gardeners should check their local forecast. The report does not set out temperature thresholds for each plant or quantify how much protection a cover will provide.
Outcomes also depend on a plant’s condition, its location and the severity of the cold. The guidance describes risks and preventive steps, but it does not guarantee that a covered plant or tuber will avoid damage. No specific plant losses or measured results are reported.
Prepare Before the Forecast Cold Night
Before a frost is forecast, identify vulnerable begonias, dahlias and cosmos, and have suitable covers ready. Cover in the evening, then remove the covering the next morning so air can circulate. Move potted plants and hanging baskets beside a sheltered wall, and add compost, bark or fallen leaves around plant bases to insulate the soil.
After the cold night, check plants for damage and continue following local forecasts as autumn temperatures change. The report does not provide a fixed calendar date for frost protection, so the immediate next step is to respond to local conditions rather than assume the first cold night will arrive at the same time everywhere.
Key Questions
Which three plants does the report say are vulnerable to the first frost?
Begonias, dahlias and cosmos are identified as at risk. Begonias and cosmos have tender growth, while dahlias’ water-storing tubers can be harmed by freezing.
What can I use to cover plants overnight?
The report recommends horticultural fleece, cloches or an old sheet to trap a layer of warmer air around vulnerable plants. Remove the cover in the morning to let air circulate and reduce trapped moisture.
How should I protect potted plants and hanging baskets?
Move vulnerable containers close to a brick or stone wall for shelter. Denyer says walls can release stored daytime warmth at night.
Can a frost stop dahlias from returning next year?
It can, according to the report: frost may damage dahlias’ water-storing tubers, which can prevent regrowth. The report does not say that every frost will kill a dahlia; the outcome depends on conditions and the plant’s exposure.
When should I put covers on and take them off?
Cover vulnerable plants when a cold night is forecast, then remove the covers the next morning. The report advises checking local forecasts rather than giving a single date that applies everywhere.
Source: rss
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