Exposing herbaceous perennials to cold temperatures, also known as vernalization, can yield a range of effects. For ornamental purposes, the general concern is the effect of vernalization on flowering.
The spectrum of responses to vernalization varies depending on genus, species and even variety: from plants that won’t flower without it (Coreopsis grandiflora ‘Sunray’) to those that have no problem flowering without it (Campanula carpatica ‘Blue Clips’). Some plants are affected not by a yes-or-no response of flowering, but rather because vernalization decreases the time to flower, compared to those plants not vernalized (Rudbeckia ‘Goldsturm’). Some plants may require vernalization to flower only if they’ve gone dormant — if they never go dormant, they’ll flower without a vernalization period (Asclepias tuberosa).
Research shows that some plants need to have a certain number of leaves (maturity vs. juvenility) to perceive the cold for flowering. Additionally, evidence suggests that a correlated mix of cold and day length — not just vernalization alone — affects some plants. It can be very confusing for growers, especially considering the diversity needed for a good perennial product mix. Thankfully, many young-plant suppliers offer great resources to help you determine whether it’s beneficial to buy in vernalized liners or non-vernalized plants, and when to book each for your sales windows.
Over the past 15 to 20 years, perennial breeders have placed much emphasis on developing varieties that are first-year flowering without the need for vernalization. Many of the new varieties of gaillardia and lavandula are perfect examples. But remember, vernalization may only be part of the equation. These plants may still require long-day conditions to flower.
So, what’s a grower to do? First, consider whether you want to sell flowering perennials or plants not in color. If you’re producing plants that don’t need to be in flower for sales, especially in smaller containers, then you may be able to get away with planting in winter/early spring. You may not really need vernalized liners. For flowering perennials in small pots, you might consider plants that the young-plant producer has already vernalized for you. This may induce flowering in the plant, while not having to worry so much about growing in to fill a large container.
Generally, perennials for flowering spring sales fall into two categories: those that you should plant late in the summer or fall and those planted in winter and spring. Depending on where your production is located in the country, those planted in the summer/fall may vernalize naturally as part of your production cycle. Summer/fall planting also has the benefit of giving the plants time to bulk-up and fill out their pots in preparation for spring flowering.
Perennials that typically flower early in the year benefit from a fall planting, such as many of the perennial Iberis. If you’re growing in larger containers (one gallon and up), you may want to plant in the second half of the year just to ensure they’re the right size in spring. Some examples of herbaceous perennials that I recommend for fall planting include most varieties of aquilegia, dianthus and Phlox subulata. Achillea, when grown under the shorter day lengths of fall, will really bulk-up and have the potential to produce more flowers in the spring and summer of the following year.
Vernalized liners, depending on which young-plant supplier you’re ordering from, usually begin to become available in December or January — subject to when the cold weather starts in the supplier’s area. Most young-plant suppliers aren’t vernalizing in coolers, but letting Mother Nature do the vernalizing for them. This means they, too, are at the mercy of the weather, and may or may not guarantee their liners are vernalized. Larger vernalized liners generally allow you the option to plant in late winter/early spring and both fill your pot and have product in color.
For more in-depth information and recommendations on these topics, I highly recommend the book “Perennial Solutions” by Paul Pilon. It’s a wonderful reference guide for perennial production, capturing both Paul’s own research on perennials, as well as the research conducted by teams at Michigan State University and other perennial research programs across the country. Of course, your supplier is another valuable resource for perennial production advice.