A Frosty Farewell

By Martin Kral

The end is near, but . . .

Because of its long growing and blooming season in several regions of this continent, we can enjoy the dahlia’s bounty until a killer frost puts an end to the splendor. If your area dithers on inclement weather, high winds, and frigid temperatures, consider yourself lucky. In Europe the dahlia is known as the Queen of Fall, and the flower’s spell there ends all too early. Variations in these conditions makes it difficult to recommend to American growers when the dahlias succumb, but a night of frost often is a hint that they waited just too late.

It has been my ambition to present the last dahlias of the season to my Thanksgiving dinner hosts. The prevailing climatic conditions of our Puget Sound basin sometimes allow such extraordinary goals, but a couple of years ago a frigid Halloween put the kibosh on such illusions. Providently, I had already cut down the majority of bushes that had shown no interest in developing new growth. Indeed, dahlias will let you know: Bright red blooms turn brick red, stems become thin and weak, flower production is reduced, and immature buds don’t grow. Nature does know best when to call it quits.

The slimy experience avoided, a recommendation is to determine at this time whether it is advisable to hold out for some late-blooming bouquet dahlias or simply call it quits. Certainly here in Western Washington many passionate growers start cutting stalks at this time, undeterred by changing weather and periods of rain. Particularly those facing hundreds of bushes take tuber harvest seriously, since the arduous digging takes a toll on a gardener’s physical health and mental pluck.

Depending on your soil type and weather conditions, digging after lopping off the stalks (leaving a 6-in. ‘handle’) can take place over days, if possible. My experience tells me to wait at least three rainless days to allow moisture to percolate downward - such a rarity in our ‘Northwet’ autumns!

Entyloma dahliae

Some early preventive measures are advisable in any case: Avoid crowding of foliage and aerate space between plants. Deadhead spent blooms and tidy up foliage to prevent diseases such as botrytis. Lower leaves should be removed to discourage powdery mildew and the appearance of other fungus diseases common in fall gardens. These harm the critical photosynthesis and transmission of fluids so necessary for good plant health. Often moving up from ground level, such infections eventually spread throughout the bush and may cause the plant’s collapse. For example, Entyloma dahliae* is a type of smut fungus that thrives during cool, wet conditions. Pale spots on lower leaves indicate a beginning infection. As these spots mature, they darken and develop spores. Eventually, these spots become necrotic and fall out, giving the leaf a buckshot appearance. Better air circulation, sunshine, and reduced foliage wetness help control the spread of the disease.

Verticillium and Fusarium wilt growth within the plant’s waterways (xylem), and so inhibit water transport. Most often it is revealed by yellowing foliage and dark stem tissue. Verticillium wilt is exacerbated by wild swings in temperature, while Fusarium wilt thrives in warm areas. All affected foliage must be destroyed. In warm climates, Southern blight (Sclerotium rolfsii) thrives in milder regions and in gardens that are untidy. It’s a soil-borne fungus, and it can persist there for years. Remove all fallen foliage and blooms, trim off spent ones, and grow dahlias in raised beds, if necessary. To prevent the garden from becoming a fungus infirmary, all dahlia foliage needs to be treated as trash. It is not compostable.

This may also be your final opportunity to properly label the bushes. Once the blooms have wizened or no longer can flower, every tuber clump will look much like another. Avoid the NOID, I say - although I confess that have my share of those too every year.

*Actual name – Entyloma calendulae f. sp. Dahlia