Generally sub-zero is below freezing point, so its 0 degree Celsius (which is 32 degree F and 273.15 Kelvin).

You can plant these hardy annuals when there are 40 degree temperatures as long as your plants are not brand-new seedlings. Planted in the fall, the plants bloom in late winter to early spring and die back in hot summer temperatures.

Use appropriate covers, irrigate the ground thoroughly, apply an insulating layer of mulch, move container plants indoors and take other steps to protect sensitive plants when cold threatens. Anti-transpirant sprays, supplemental heating and cold frames can also prove helpful to protect plants.

Bed sheets, drop cloths, blankets and plastic sheets make suitable covers for vulnerable plants. Use stakes to keep material, especially plastic, from touching foliage. Remove the coverings when temperatures rise the next day. For a short cold period, low plantings can be covered with mulch, such as straw or leaf mold.

Keep the plants moist and apply a light fertilizer after all danger of frost has passed. More tender plants will end up like annuals and will not withstand the freeze damage. Occasionally, freeze-damaged perennials will have just some damage to the root and you can divide the plant and install the pieces in the ground.

Here are few common symptoms and what they may mean:

• Wilting leaves.
• Plant/soil not holding water.
• Yellowing leaves.
• Roots at the surface or coming through drainage holes.
• Tips/edges of leaves turning brown.
• Flower buds form then drop off before opening, or shrivel soon after opening.

20 Hacks That Will Bring Your Dead (or Dying) Plant Back to Life

1. Find Out if the Plant is Actually Dead First. 1/20.
2. Trim Back the Dead Parts. 2/20.
3. Leave Bits of Stem Intact. 3/20.
4. Diagnose the Problem.
5. Water a Thirsty Plant.
6. Move a Thirsty Plant to a Humid Spot.
7. Use Filtered Water on Your Plants.
8. Replant an Overwatered Plant.

If the leaves are brown, cut them off at the trunk. Most years, new growth will come out in the spring and the plant will recover nicely. Confederate Star Jasmine: This vine is also rated Zone 8, but a sustained freeze will often kill it back, sometimes to the ground.

Check the water needs of plants after a freeze. Water that is still in the soil may be frozen and unavailable to the roots and plants can dry out. It is best to water in the afternoon or evening the day after a freeze so plants have had a chance to slowly raise their temperature.

Watering Plants At Night Isn’t Needed: Although the idea has been around for years, most plants don’t need extra care by watering them at night. Night time watering promotes disease — and no one wants to deal with that now do they? Try early morning or early evening waterings for the most absorption.

How much cold will kill a plant? Not much, although this is usually dependent on the hardiness of the plant as well as the climate. Typically, temperatures falling below freezing will quickly damage or even kill many types of plants. However, with prompt care, many of these cold damaged plants can be rescued.

How to Protect Your Plants from Frost

1. Bring Potted Plants Inside.
2. Water Plants in the Afternoon.
3. Add a Thick Layer of Mulch.
4. Cover Up Individual Plants with a Cloche.
5. Give them a Blanket.
6. Wrap Your Trees.
7. Keep the Air Moving.