Most newly planted young perennials will need winter protection as they will not be mature enough to handle frost.
Bulbs and perennials emerging from the ground do not need to be protected from sub-freezing temperatures. They are growing according to nature's schedule. You don't need to protect plants until after we've had an extended warm period.
Cover plants with cloth; bed sheets and towels work great. If possible, drape the cloth all the way to the ground and secure. Newspaper and straw can also be used.
Frost Tolerant Perennials
Those that are frost tolerant can be planted 2-3 weeks before before the last spring frost to give you some immediate color and interest in your gardens or containers. Be aware though, that they may need to be acclimated to the cold weather.
Seedlings, with their tender new leaves, often give up the ghost when temperatures dip to 32-33°F. Tropical plants have differing low-temperature thresholds. Some keel over when temps fall to 40°F; others crumble at 35°F. Other plants are just hardy by nature and can withstand temperatures as low as 18-20°F.
The newly emerged growth of most perennials can tolerate temperatures in the low 30s and upper 20s Fahrenheit. Freeze damage may occur if temperatures drop into the lower 20s or teens. Most well-established perennials that have been damaged by freezing temperatures should survive.
Know Your Plants' Cold Tolerance
Some frost-tender plants need to be covered as soon as the temperature hits 32°F, while others can handle lower temperatures and/or longer freezing periods. You'll need to look up each of your plants to verify its cold tolerance.
Thoroughly Water plants if it's not going to rain before the freezing temperatures arrive. It may sound illogical. However, a moist ground stays warmer than dry soil. Watering the night before the freeze comes will insulate the root structure of the grass and plants and decreases the potential for cold injury.
When planting: Water plants as soon as you get them in the ground. Allow the water to soak in, then water again until the soil is thoroughly moistened. Week one: Water plants daily or every other day. Recently planted roots will absorb moisture from a small area until they begin to grow.
When Should You Cover Plants? Cover your plants at night and remove them during the day when the temperatures rise above 32 degrees F, so that the soil can warm up again.
A covered porch usually provides protection from light frost, but the garage or sun room is better for freezing temperatures. A couple days in darkness won't hurt the plant. Or move them out during the day and back in at night, if cold temperatures persist.
Some Hardy Perennials Need Winter Protection in the North and Midwest, and Tender Perennials Require Special Protection Methods. Many perennials hardy for your zone make it through winter just fine with no special attention.
How long should I leave my plants covered during cold temperatures? You should cover plants until temperatures are back into the upper 40's to 50's. If it looks like a couple of days of cold temps, leave the coverings on for a couple of days.
Bed sheets or comforters work best for covering large plants and shrubs. Newspaper can be used on low-growing foliage, but it can often be difficult to get it to stay in place. I have used old pillow cases, sheets, towels and even cardboard boxes.
Plants always killed by frost include summer annual flowers and summer vegetables like impatiens, marigolds, coleus, tomatoes and peppers. Subtropical and Tropical plants are the most sensitive to frost (citrus, hibiscus and bougainvillea).
Perennials are slow growers at first: They take the first one to three years to establish deep roots and dense foliage. However, once established, they thrive with little maintenance to give your yard beautiful new growth year after year.
1st Week - Water every day unless there is rainfall that day. 2nd Week - Water every other day, unless there is rainfall that day or the day before. 3rd Week & Beyond - Water 2 to 3 times a week. The top 2 inches of the soil should be dry out in between watering.
If a plant is overwatered, it will likely develop yellow or brown limp, droopy leaves as opposed to dry, crispy leaves (which are a sign of too little water). Wilting leaves combined with wet soil usually mean that root rot has set in and the roots can no longer absorb water.
Morning watering is actually preferable to evening watering as the plant has time to dry before the sun goes down. At night, water tends to rest in the soil, around the roots, and on the foliage, which encourages rot, fungal growth, and insects.
Farmers spray water before an overnight freeze because water produces latent heat when it freezes. The sprayed water on the surface of the plant will be converted to ice, and the heat trapped in it will insulate the plant from the cold environment.
Once frost damage occurs, nothing can be done to reverse the damage. If you find tender plants covered in frost, try spraying them with water to wash the ice crystals away. Letting ice melt in the sun will cause irreversible tissue damage to the plant.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation. Severe or hard freeze - 25° Fahrenheit and colder causes heavy damage to most plants.
Once temperatures drop below 40°F, you may want to start covering your plants with a frost blanket depending on what you're growing, which we'll get into in just a bit. Freeze – A freeze is when the air temperature is 32°F or below.
From that experience, I've found the best frost protection for your outdoor plants is either free or cheap. Cardboard boxes and brown grocery sacks make perfect frost cover and at the end of the season can be recycled. I keep various boxes on the patio and when frost is forecast simply put one over the plant.