Freezing temperatures have some maple farmers concerned that they will not have enough sap to boil during the 18th annual Maple Weekend in New Hampshire, just a few days away. ''It has been in the teens at night,'' said Armand Bolduc of Bolduc Farm in Gilford. ''That's too cold. The trees are frozen.'' He said there is no sap flowing right now, preventing his sugarhouse from gathering up gallons of sap to boil this weekend. The farm is one of over 110 sugarhouses in the state scheduled to hold activities.