Polishing Your Perennial Production

Susan Martin
You’ve faced your fears and made note of common pitfalls when it comes to growing perennials so it’s time to get growing. Successful perennial production starts with mastering the fundamentals and the learning never really ends, so whether you’re new to growing perennials or have spent most of your career in a greenhouse, there’s always something new to discover.

Growing conditions: One size does not fit all   
If you ask Jeremy Windemuller, a well-respected grower and consultant from Walters Gardens in Zeeland, Michigan, he’ll tell you that the biggest mistake he sees growers make is trying to grow their annual and perennial crops in the same soil and same growing environment. It just doesn’t work.

Pictured: Perennial crops finish more efficiently when separated into warm and cool growing environments in sun and shade. This fall-planted crop of heuchera and leucanthemum cultivars is rooting in well in a cool, open-air greenhouse.

Annual growing media holds far too much moisture than perennials can handle; they need bark in their mix to facilitate drainage. Compounding the issue, annual growers hang rows of hanging baskets above their perennials, blocking out the light and wetting the foliage with excess water from the drip lines. That’s a sure recipe for

At minimum, Jeremy advises that growers set up two separate growing environments for perennials—one warm and one cool. Better yet, set up warm and cool greenhouses for sun and shade and an additional growing area outdoors. Within those environments, further group plants by watering needs. Setting up proper growing environments will go a long way toward staving off future growing issues.

Commonly grown perennials that prefer warm growing conditions include: agastache, coreopsis, echinacea, heuchera and monarda. Those that prefer cool growing conditions include: astilbe, baptisia, clematis, dianthus, helleborus and hosta. Perennials that do best in outdoor growing environments include: hibiscus, sedum, nepeta, lavender, ornamental grasses and others that thrive under high light conditions.

Maximizing your production space
Timing is everything when it comes to growing perennials. As you gain more growing experience, you’ll learn which crops to bring in at what time to maximize greenhouse efficiency. Crop scheduling is highly dependent on your production model. If you’re a small grower filling your own retail needs, you may need to schedule most of your product to be ready at once in spring. Larger wholesale growers will need to learn how to supply seasonal products over a span of many months.

At Garden Crossings, a small but mighty retail and mail-order business based in Michigan, Rod Grasman schedules all of his annuals, perennials and shrubs to be ready at the same time in late March. After the annuals move out of the greenhouses, they’re backfilled in early summer with newly potted 1-gal. shrubs for sale the following year. Perennials such as echinacea that need more time to bulk up are planted midsummer, followed by fall-planted perennials in mid-September. The remainder are potted in January or February and grown with minimal heat until they’re finished by late March.   

Large growers producing big numbers of perennials tend to start their spring crops with small plugs in late summer or fall prior to sale and move them out by mid-spring. New crops of perennials for summer sales take their place. Ornamental grasses and other fall favorites are started in early summer for fall sales. Higher light levels, longer days and warmer temperatures help these crops finish faster so the production space can be turned more quickly. Many large growers stagger their planting dates of popular crops within these seasonal windows in order to have a consistent supply of retail-ready product available over many weeks.

Choosing liners to suit your growing needs
When you look through your suppliers’ catalogs, you’ll find a wide variety of plugs and bare root sizes available for perennials. Some varieties are almost always offered in plug form, while others are typically offered as a bare root. Take a close look at your production costs to determine which starting materials will finish most efficiently in your climate. Often, the smallest plugs yield the biggest margins, but some plants, like bare root perennial hibiscus, consistently yield top dollar.

Pay close attention to the vernalization requirements of all of the perennials you grow, as this detail can mean the difference between an entire crop that either flowers or doesn’t. If planting your perennials in late summer or fall, be sure they receive eight to 12 weeks of vernalization on average (study individual genera for specific requirements) or purchase vernalized plugs in spring.

Proper planting technique   

Years ago, after advising a man on how to plant 500 tulip bulbs as a surprise for his wife, he returned with a cheeky grin and dirty knees and said, “You said to plant them with the pointy tip down, right?” He wasn’t joking.

So it is with perennials, too—they’re quite particular about the depth at which they’re planted. Most perennials need to be potted up with their crown even with the soil line. Too low and you’ll risk rot, too high and you’ll risk drying out. Iris and liatris are exceptions in that their bare root should be planted high. Dicentra, hosta, baptisia and clematis prefer to be planted with their crown just below the soil line. If you’re planting a bare root, you can often see a mark where the soil line was when it was growing in the field. Use this as your guide.

Preventing pests and diseases   
As with any crop, perennials have their fair share of pests and diseases. Most commonly, aphids, mites and whiteflies tend to enjoy perennials’ soft new growth. If you walk through a perennial greenhouse, you’ll likely see yellow sticky cards to catch the nasty buggers.

Diseases can be largely prevented through good growing practices, like maintaining good air circulation, watering in the morning and spacing plants well. Applying a preventative fungicide at time of transplant can also help to prevent diseases. Keeping floors clean and cleared eliminates hiding places and breeding grounds for greenhouse pests. As a general practice, be proactive rather than reactive and scout regularly for problems.

Production tips       
Before you begin growing a perennial crop that’s new to you, it’s very important to develop a grower recipe so you’ll know exactly when to plant, pinch and feed it. Your supplier should be able to guide you with its production requirements.

Perennial growing media usually contains a mix of peat, perlite and bark with a typical pH of 5.8 to 6.2. It’s important to consistently test and know the pH of your water supply and make any necessary adjustments to avoid a pH that’s much higher or lower than the average range.

Many perennials like agastache, hibiscus and monarda benefit greatly from a soft pinch after transplant to promote branching. Finished crops like nepeta, heliopsis, salvia and leucanthemum benefit from shearing or deadheading to promote new growth and stimulate rebloom. Though each extra touch can mean extra cost, higher quality finished product can garner higher prices.

Growers with automatic watering systems typically incorporate water-soluble fertilizer with every watering at a rate specific to each crop. Learning which perennials are hungrier than others and grouping them in your production space by these needs will make it much easier to apply the proper amounts of fertilizer. Be sure to test the EC rates often to check for high salt levels, which can decimate perennial crops over time.

Overwintering advice   
One of the most challenging parts of perennial production can be overwintering, particularly for growers in northern regions and places where wide temperature fluctuations in winter are common. More often than not, it’s the swings in temperature that kill plants, not the cold.

When overwintering perennials in containers outdoors, a heavy frost blanket is essential for cold nights, but it must be removed on warm days. Many growers who overwinter their perennials under cover will regulate the temperature to maintain an average of 35 to 38F (1.6 to 3C) to keep plants from freezing. Ventilation fans are essential for warm, sunny days.

Talk to other local growers in your area to see what’s worked for them and what hasn’t. Some years, you’ll call it good if you can just manage to keep the mice out. Every winter brings a new challenge and even experienced growers lose some plants. You’ll be better for it and there’s always next year, right? GT

Susan Martin specializes in horticultural marketing, content generation and management, working with green industry clients in trade and consumer sectors. She can be reached at gardenersuesnews@gmail.com.