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Frost vs. Freeze vs. Ground Freeze

We’ve all heard terms like frost and freeze before, but what do they mean exactly? Is it the end of the planting season? Do we need to cover our plants during a frost or a freeze? Is it too late to plant and transplant? We will explain all those questions and more!

What is frost?

Frost is essentially cold dew. It forms when temperatures drop below 40° F. Some damage is done to tender annuals and perennials if temperatures drop closer to the freezing point.

What is a frost advisory?

A frost advisory is issued when the temperature falls between 36°F and 32°F.

What is a freeze?

This occurs when air temperatures drop below freezing. Frost is not usually visible since wind and below freezing temperatures prevent frost from forming. Noticeable damage is done to certain plants at this point as the freezing temperatures cause the plant’s cells to burst and die. Tender annuals will die off.

What is a freeze warning?

A freeze warning is issued when there is at least an 80% chance the temperature will hit between 32°F and 28°F.

What is a severe or hard freeze?

Hard freezes will cause moderate to heavy damage to most plants. For this to occur, the temperature must fall below 28°F for several hours.

When does the ground actually freeze?

Frost and occasional freezing nightly temperatures will not actually freeze the ground. There are several factors involved for the ground to freeze: sub-freezing temperatures, adequate moisture in the soil, and time. In fact, it takes at least five or more days and nights below freezing for the ground to freeze. This tends to happen in January and February.

How can I protect my plants during a frost?

Many herbaceous and woody plants can go through a frost unscathed. However, houseplants should be brought inside for protection. Annuals, tender perennials, and plants in flower will need to either be brought inside or covered overnight with a frost blanket, burlap, or a plant protector to shelter them. Watering regularly until it freezes not only hydrates our plants, but also keeps the soil a few degrees warmer, acting as insulation.  

Can I protect my plants during a freeze?

During a freeze or a hard freeze tender plants will die, whether protected or not. Both herbaceous and woody plants will show signs of shutting down for the season. Once this occurs, it is a great time to prep certain plants for winter protection!

How should I protect my plants overwinter?

Adding one to two inches of mulch or compost around the base of plants will help with water retention and protect them from heaving this winter. Winter heave occurs when the ground freezes and thaws causing the soil to push upwards, exposing the roots. Many plants will benefit from this extra layer of protection (especially shallow rooted plants like mums and roses). 

Covering woody plants during the winter can help protect them from evaporation loss (also called winter burn in evergreens).  Plants affected by winter burn will show signs in their foliage – it will brown and fall off. In many situations, removing the burnt branches in spring will give space for new growth. In more severe cases replacing the plant may be necessary. 

To avoid this damage, many gardeners will create a temporary barrier with Wilt Stop, Burlap, or Plant Protectors in December. Applying products like Wilt Stop will temporarily coat the pores on leaves, lessening transpiration loss. It is important to fully coat all foliage, otherwise some areas might appear “patchy” next spring. Usually one application is adequate per season

Likewise, burlap can create an adequate shelter to block out most of the drying winter wind. Many gardeners will create a burlap tent or a wrap for their evergreens or other woody plants. Burlap still allows plants to breathe and regulate their temperatures, but will not provide insulation. As a bonus, it can even help to deter deer and rabbits from munching on plants!

Styrofoam Plant Protectors trap air, insulating plants in sub-zero temperatures. Plants should be covered in December or January, and uncovered in the spring when temperatures are consistently above freezing. Like burlap, Plant Protectors also block out drying winds, but are not as breathable. However, they will still provide some protection from deer and rabbits.

fall planting

Is fall a good time to plant?

Yes! The fall months are easier on both established and newly established plants. Autumn brings more rainfall and cooler temperatures which greatly helps with transplant and division shock. Roots will continue to grow until the ground freezes solid, usually some time in January or February.

When should I stop planting during the fall?

The optimal time to plant herbaceous and woody ornamentals is about a month before the last fall frost (typically the first or second week in October). However, if planting closer to or after the last frost, simply bury the root ball slightly deeper than normal and heavily mulch to protect from heaving. Either method will give the plants enough time to establish and take root before the portion above ground goes dormant. In fact, the roots will continue to grow and establish until the ground freezes solid! 

Spring Flowering Bulbs are plantable until the ground freezes! In fact, some gardeners prefer to plant tulips and other bulbs a few weeks before the ground freezes. This ensures squirrels and other animals cannot make a meal out of them! Planting Spring Flowering Bulbs into late fall and early winter do not harm the health or flowering potential. Moreover, they require a period of chilling. Only after their chilling needs are met will warm temperatures wake them from dormancy and bloom.




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