Ask a local master gardener about  tender perennials

  • Verbena bonariensis Karelj/wikimedia commons

For the Gazette
Published: 1/11/2019 10:46:29 AM


Q: In looking through my seed catalogs, I see plants such as Tall Verbena (Verbena bonariensis) listed as a “tender perennial”, zones 7-10.  I know what annuals and perennials are, but what is a “tender” perennial? And how do I confirm which temperature zone I am in here in Southampton? —W.B., Southampton

 

A: Your questions made me smile, W.B., as you illuminate a good example of quirky gardening vocabulary that often leaves people scratching their heads. Not everything is black and white for us northern gardeners. 

As you know, annuals are plants that complete their entire lifecycle — growth, reproduction, and decline — all in one year. Examples of common northeastern annuals include petunias, amaranth and marigolds. Perennials are herbaceous (green stemmed, not woody) plants you plant once and they come back year after year, typically taking 3-5 years to mature. Daylilies, coneflowers and peonies are examples of northeastern perennials.

Tender perennials are plants that, if planted far enough south, grow as perennials. They cannot tolerate the cold winters of the north, however, so up here they will grow, but they die once frost hits. Think of them and treat them as annuals. 

Now, how do we know if we are too cold for a tender perennial to grow as a perennial? Let us refer to the USDA’s handy Plant Hardiness Zone map. You can find it on-line at https://planthardiness. ars.usda. gov/ PHZMWeb. In it, they divide our entire country into 11 temperature zones based on the coldest average temperature each area gets during the winter.

Here in western Mass we range from zone 6a (-10 to -5 degrees F) to zone 5a (-20 to -15 degrees F), mostly 5b (-15 to – 10 degrees F). Enter your zip code or select the Massachusetts-specific map to find your local temperature zone.

Armed with that information, tak another look at the catalog.  Compare your USDA zone with those listed in your seed catalog. If you are Zone 6a, for example, which I believe you are in Southampton, any plant listed as Zone 6 or lower (Zone 5, 4, 3, etc.) will grow as a perennial in your area. Any plant that is listed as Zone 7 or higher (Zone 8, 9, 10, etc.) will grow as an annual. 

Thus, a plant listed as a tender perennial in zone 7-10, such as the verbena you found, will act as an annual where you are. Given how close Southampton is to the Zone 5b line, I would suggest using Zone 5 as your guideline for plants so you are not disappointed by plants not surviving a random, crazy cold winter. Weather can be wacky. 

Hardy, tender, annual, perennial — the gardening world certainly does have its own vocabulary. Good questions, W.B.. I hope this answer helps and thanks for asking a local master gardener!

Have a gardening dilemma? Please send questions, along with your name/initials and community, to the Western Massachusetts Master Gardener Association at AskAMasterGardener@wmmga.org. One question will be selected and answered per week.

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