Clean water and plenty of it is the only essential ingredient for maintaining tree freshness.

As a general rule, your tree stand should hold at least 1 quart of water per inch of diameter of the tree trunk.
Keeping your real Christmas tree fresh throughout the
holiday season involves giving it proper care from the time you purchase it
until it is disposed of. Before you set up your tree, make a fresh, straight
cut across the base of the tree and place the tree in a tree stand that holds 1
gallon of water or more. If the tree has been cut within the last six to eight
hours, it will not need to be re-cut; however, any longer than that then the
end should be re-cut. Make a straight cut across the trunk, removing 0.5 inches
or more from the bottom.
Cut Christmas trees
will absorb a surprising amount of water, particularly during the first week.
As an example, a tree with a 2-inch diameter trunk may initially use 2 quarts
of water per day; a tree with a 4-inch diameter trunk may use more than 1
gallon per day. The water capacity listed on a stand's label or box can
be misleading. Usually they list the capacity of the reservoir when the stand
is empty, but you also need to allow for the amount of water that will be
displaced when the tree trunk is put in the stand. So when choosing a stand for
an average tree, you will want to choose a stand that holds at least 1 to 1.5
gallons of water. In addition, make sure you are checking your tree stand at
least daily to make sure the container holds enough water and refill it often
to make sure the water does not fall below the level of the trunk bottom.
One of the most common
questions educators
receive concerning Christmas trees relates to the use of additives in the
Christmas tree stand. Some people have seen TV or newspaper advertisements for
products that you add to the water in your tree stand. Others have concocted
their own "home remedies' with ingredients such as sugar, aspirin, bleach and
7UP. Research in Washington and North Carolina has shown that your best bet is
plain tap water. Some of the home remedies such as bleach and aspirin caused
heavy needle loss and should be avoided.