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updated: 11/7/2013 11:40 AM

How late can I plant perennials in my garden?

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By Mary Boldan

Q. How late can I plant perennials in my garden? What should I do if I have perennials that I wasn't able to get planted in time?

A. In order to have success in planting your perennials in the fall, it is important to have an idea of your frost times to know when to plant your perennials accordingly. According to the National Weather Service (NWS), frost is when the temperature of the Earth's surface falls below 32 degrees. This forms thin ice crystals that can cover the ground. Since a frost is temporary (usually over one night), you can protect your plants by covering them or bringing them indoors the night the frost is predicted.

On the other hand, a freeze is when the temperature is expected to remain under 32 degrees for an extended period of time. A freeze generally spells the end of the growing season for tender plants

With this in mind, planting too close to the first fall frost can hinder the success of plants by not allowing enough time to establish and take root. The right timing is often instrumental in the vitality of overwintering plants.

Although each plant's needs may vary and some are more winter-hardy than others, planting a month before the last fall frost is generally sufficient in giving the plant enough time to establish itself and take root. If you have to plant closer to the last frost, be sure it is not in a low spot where water accumulates. This and a layer of mulch will help to prevent frost heaves from dislodging the plant over winter.

• Provided by Master Gardener Mary Boldan. Master Gardener Answer Desk, located at Friendship Park Conservatory, 395 Algonquin, Des Plaines, is open 9 a.m. to noon on Wednesdays. Call (847) 298-3502 or email

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