Growing Concerns 93: Non-blooming forsythia, smothering sod

Forsythias won't bloom

My forsythia bushes are 5' tall, about 5 years old, look good and healthy, but never have those nice yellow blooms, except for a very few at the base. They get lots of sun. - D.P. -

They may be genetically incapable of holding onto flower buds over a Michigan winter. If they aren't blooming well now, after such a mild winter, accept them for their greenery alone or replace them.

Most forsythias planted since the 1940's are hybrids, offspring of crosses between greenstem forsythia (Forsythia viridissima) and the weeping forsythia (F. suspensa). This mingling of genes produced Forsythia x intermedia - that x in the name means “hybrid.”

Among these hybrids are ‘Spectabilis’ and ‘Lynwood Gold.’ They can put on a better spring show than their scant-flowering weeping parent and are more graceful of branch than greenstem. Unfortunately for northern gardeners, the hybridizer wasn't breeding for cold hardiness.

Neither parent species nor hybrid offspring are reliably flower bud hardy. Their stems and leaf buds can survive in zone 5 (average lows of -15 to -20°F) or in the milder part of zone 4, called 4b (-20 to -25°F). The flower buds, however, freeze at -10 to -15°. So you've seen flowers only low down, where they were insulated by snow or leaf litter.

Universities and growers in North Dakota, Minnesota, and Ottawa have improved forsythia hardiness by crossing Forsythia europaea and Forsythia ovata to produce super-hardy varieties ‘Meadowlark’ or ‘Northern Sun.’ These both bloom well even into parts of zone 3 where -35°F occurs regularly.

Ray Wiegand's Nursery in Macomb carries ‘Northern Sun.’ In addition to its retail location on Romeo Plank north of 21 Mile Road, Wiegand's supplies many nurseries in Michigan via wholesale sales. If your local garden center doesn't have what you need, ask them to order some hardy stock from Wiegand's.

Our own forsythia is also a blooming failure most years, but we’re letting it stay until the new variety ‘Happy Centennial’ becomes available. It's a 3' tall, 5' wide dwarf with fragrant flowers, hardy to zone 5 lows.

How long before smothered sod dies?

I decided to follow your advice and make a new perennial flower bed by smothering the sod rather than digging. Last September I dug a trench around the bed, which is shady, then stacked about 10" of leaves on it and weighted it all down with some bark chips. Now when I move the mulch aside, it looks ready to plant. Maybe it's this long, cold spring or maybe it's that bed calling to me, but when I go to a nursery now the urge to buy something is overwhelming. My best gardening friend says I should wait until May. C.N., Livonia

If the perennials tempting you are fully leafed out, wait until mid-May. If they're bare root or just breaking the surface in their pots, go ahead. Astilbe and hosta are good examples - hardy here, but if started early in a greenhouse or shipped in from warm North Carolina, it's ahead of the local game and might have all of its foliage killed back by a frost.

That might be a minor set-back or a death blow - it depends on how much stored energy the root has. Last June, frost killed all the foliage on established hostas in our gardens. We had to cut the limp, yellowed leaves off and wait for a new batch to form. If those plants had been very young or newly installed, they might have died.

We use existing gardens as our bell-weather. Right now, astilbe in “the field” are just inch-tall, fuzzy brown sprouts, tiny leaflets curled tight like fern fronds. Hosta leaves are also rolled tight - some are still spear points that have only just broken the surface. The leaf's fuzz, its proximity to warm soil and its curl all combine to preserve heat within the cells in a cold snap. If the nursery stock we’re buying matches field stock development, we buy it. If it's much more advanced than what we see in the garden, we let the garden center hang onto it.

We might buy the plants if they're just a little larger or taller than field-grown stuff. If we do, we harden them off before planting. This means acclimating them to local conditions, treating them as you would treat yourself on a winter vacation to sunny latitudes. Put them out into the weather for a bit longer every day, over a period of three of four days. The fluids in the cells, and the cuticle covering the leaf, will adjust to the change the same way your skin does to an increase in ultraviolet rays. Your skin cells develop a greater concentration of protective pigments; the plants adjust the amount of “anti-freeze” in their cells and reduce their pore openings against dehydration.

Keep them in their pots and move them from a frost-free nighttime position such as an attached garage to a partially shaded, wind-free spot during the day. Leave them exposed to semi-wild air and sun for a few hours on day one, a half day on day two, then all day. After the third night, plant them.

One of the advantages of buying from a local garden center, versus mail order or from department stores, is that knowledgeable people are on hand. We've come to trust garden centers for their advice, both spoken and implied. When our local store advertises a plant as “hardy” or displays it in the “hardy perennial” section, that means a lot to us.

If garden centers offer “hardy plants” for sale that are actually too far advanced for local conditions, we consider it an abuse of customer trust. Perhaps we'll hear from some local garden centers and be able to report for you afterward about the cold-hardiness of their stock and how they're advising their customers to care for tender plants until it's safe to plant them.