Is there enough time in the season for the new branches to harden when they emerge?

Where in the world are you?? Are these species maple trees or Japanese maples? Potted?

Chances are, once new growth emerges, it can take several weeks before it can 'harden off'. In my neck of the woods, this is not all that big of a deal. I've had red maples (Acer rubrum) come out of dormancy in the spring and have several inches of last years growth froze dead from the previous winter. The trees just send shoots out from below the dead stuff and keep growing just fine ( I snip the dead stuff).

Depending on your location, pouring ice water on small trees will help them decide to stay dormant during a warm spell or even cause them to go dormant if they haven't already.

If your season is long enough, new growth should harden off by about a month after it emerges but depending on a lot of variables, may still 'winter kill'. Again, usually NBD.

Edit: Not over watering late into the season helps trees to slow down and keeps them from sending out late new growth.

Thanks. I am in NYC. Tempartures today in the 80s!!!

Leafing out is not the same as producing new woody growth or branches. Maples will typically have viable buds on standby almost year round - if weather conditions are right, they will produce foliage. It is nothing to be concerned about.

Thanks Gardengal - it's backbudded new branch growth. Hopefully there will be enough time to harden off as I appreciate the fuller multibranch look for my peve multicolor