Midwinter warmth no sweat for plants

Maple bloom in very early spring. Hardy plants are more likely to be hurt by spring frosts than midwinter freeze. This maple flower was tough as nails while in bud during the winter's depths. Then, its new tissue was covered by resin filled scales. Now the cells are filling up with water and are likely to freeze and burst the weather turns even half as cold as it was at midwinter.

Season finale has most impact

Plants may beat their species' chill requirement.

Weird weather's effect on other creatures.

Hardinesss zone change? Plants don't read.

Although I've rather liked this warmer winter for a change, what pluses and minuses can this mean for my perennial flower garden and newly planted trees? - T.K. -

Ongoing discussion at the Forum

No problem for hardy plants to slip a zone or two

A mild winter's no problem for plants so long as it doesn't entirely deprive rest-dependent species of their time out. Many plants geared to zone 5 winters can be fine in a zone 6 or 7 year, where the coldest day is 10- or even 20 degrees warmer than it could be.

What counts is the ticking clock

Plants need time at or below 40°F to acquire full hardiness and remain dormant. Once top growth stops in late fall those with chill requirements -- such as flower formation dependent on a certain number of cold hours -- begin to tick off the time. They remain "dormant" until they reach their species' target, which might be 500, 1,000, or even 1,500 hours at 40°F or less. There is no bonus for extra cold. Every hour below 40°F counts, no matter how far below that mark.

Racking up that time may take 3 -to 6 weeks in a real winter, below 40°F 'round the clock. If days are warm so that only sunless hours count, it may take 6- or 10 weeks. Either way, the need is met and result's the same. What's different is that in the warmer scenario a plant may come through with every branch tip, flower bud and root intact, rather than losing bits to extreme cold.

Light sleepers most likely to run into trouble

Most hardy plants won't 'wake up' until they've had an extended period at 50°F or warmer. If that warmth comes early, at the end of winter when time's up, it can dissipate the blocks plants developed during hardening in fall. Then plants' tips begin to grow, pulling water up (the 'sap rises'). If it's too early, and frigid air touches new growth that has no secondary freeze defense, it can be a killer.

Yet that situation -- late warm-up followed by hard freeze -- can happen in any winter. Warm year or cold, unsettled springs happen.

People say, "Old fashioned bleeding heart is so pretty, I wish it would rebloom if we deadheaded it!" ButDicentra sepctabilis can't help itself. After it blooms, it will rest, and bloom-inhibiting chemicals within its cells will not wear off until a winter's worth of cold hours pass.

Chill before opening

Some species that require cold to bloom fully:

Common lilac (Syringa vulgaris and French hybrids)*
Peony (Paeonia lactiflora, the common Memorial Day peony)
Old fashioned bleeding heart (Dicentra spectabilis)
Apple/Crabapple (Malus species and hybrids)*
Tulip*
Daffodil

*If a species is marginal in your area because your winters are too warm, check into varieties with lesser chilling requirements.