You can leave daffodil bulbs in ground over winter without any worry at all. In fact your daffodils need that cold time to bloom. They don't just survive winter. They depend on it to make flowers.
In my experience gardening in Zone 5, winters get rough. Temps drop well below 0°F (-17.8°C) on the coldest nights here. My daffodil bulbs have stayed in the ground through every one of those harsh winters for eight years now. Each spring they push up on time and bloom like nothing happened. I haven't lost a single bulb to cold damage.
Your daffodils need that cold for a reason. The bulb needs 12 to 16 weeks of soil below 48°F (8.9°C) to form flower buds inside. Without enough cold, you get leaves but zero flowers in spring. This is why warm-zone gardeners chill bulbs in the fridge before they plant them.
The daffodil winter hardiness range covers most of the country. UF IFAS Extension shows they grow well in USDA Zones 3B through 10. That means you can grow them from northern Minnesota to central Florida. The only difference is how you handle planting and chilling for your zone.
Your bulbs show strong daffodil cold tolerance even above ground. Iowa State Extension says open blooms handle frost down to the upper 20s°F (-2°C to -3°C) without big damage. I've seen my flowers coated in frost on cold April mornings. They bounce right back once the sun warms them. A hard freeze below 25°F (-3.9°C) can hurt open petals but the bulb stays safe underground.
Deep planting protects your bulbs from the worst cold. Set them at three times their height below the surface. That puts them 6 to 8 inches deep. At that depth your soil stays much more stable than the air above. Even when the surface freezes solid, your bulbs sit in insulated ground that keeps them safe.
If you garden in Zones 3 or 4 where winters hit hardest, add a layer of mulch for extra safety. Spread 2 to 3 inches (5 to 7.5 cm) of straw or shredded leaves over your beds after the ground freezes. This mulch helps smooth out the freeze-thaw cycles that can push bulbs up toward the cold surface. You don't need mulch in Zone 5 or warmer at all.
Leave your bulbs in the ground, let winter do its job, and wait for those first green shoots to break through the soil. Your daffodils have been handling cold winters on their own for thousands of years. They don't need you to dig them up and bring them inside. Trust them and you'll get strong blooms every spring with almost zero effort from you.
Read the full article: Daffodil Bulbs: Planting and Care Guide
