If a tree has chill hours 200 and the tree gets 200 hrs by the end of January; if there is a warm spell in February will the tree will bloom? It might, but it depends in part on what happened during the period the chill units were accumulating and how long and warm the warm spell is. Chill units don't necessarily accumulate in a linear or even a hockey stick shaped graph line. Chill units accumulate when temperatures are between freezing and about 42*F. Temps below freezing cannot be counted as chill units, and temperatures much above 42-44* "undo" some of the effect of chill units already under the plant's belt. IOW and as an example, if a plant requiring 200 chill units has undergone chilling such that 100 chill units have accrued, then experienced a warm spell, when the warm spell is over it might need another120 more chill units.
If the tree blooms, will the tree start to produce leaves? When a plant's chilling obligations have been met, the plant passes unnoticed into a state of quiescence, during which only temperatures below about 42* keep the tree from blooming or producing leaves. At any point after the tree has passed into quiescence, a warming trend can stimulate either/both bloom and leaf growth in whichever order the plant is programmed to produce those organs.
If the tree produces leaves and there is more nights of 28 degrees average, will the leaves freeze? It can, but all resistance to chill/freezing isn't immediately lost at the onset of growth. Partially open leaves will still have some protection due to the increases concentration of dissolved solids in the cell solution, which serve as antifreeze (just like salt or sugar) to help the plant tolerate a greater degree of chill. A cold spell w/o temperatures dropping to killing lows can somewhat restore the resistance to chill lost during the preceding warm spell.
Will the tree be damaged or die? Speaking only about temperate deciduous trees ...... they could, but most likely not. Whether the tree lives or not depends in large degree on it's level of vitality and stored food/energy when it entered dormancy in the fall. Trees most at risk would be those which, when entering dormancy in fall, were highly stressed and had low levels of stored energy. Leaves of most trees do not become net producers of food/energy until they are somewhere near 75% mature, so the tree must have sufficient energy reserves to at least carry some of the leaves to near maturity so they can provide food/energy to developing foliage.
I know that an early bloom is usually killed by late frost but I haven't seen what happens to a tree that blooms way too early and whether or not it sets leaves while there is still a possibility of killing frosts. When blooms are killed, it's not usually as serious as foliage suffering from chill injury. For plants that bloom on old wood, there is usually a deposit of starch at the base of each bud that needs only an adequate supply of water to metabolize the starch as an energy source for the blooms.
I want to plant a tree with low chill hours, 250 when my area receives 500 -800 and does have warm spells in midwinter followed by normal cold. Trees with greater chill requirements tend to be more forgiving of unusually mild mid-winter warm spells as a dormant tree is more likely to keep snoozing during a warm spell than a tree that has passed from dormancy into its quiet state.
Al
