The tree, not the charred bit of soot in the fireplace, usually used for boys, but, as a short form of Ashley, works for girls too. 

One of the most usable of the pleasant, newly adopted nature/water names (like Lake and Ocean), especially in middle position. 
 There was a character named Bay Kennish on the show Switched at Birth; conservative political commentator Bay Buchanan was born Angela Marie.
 Other associations with bay are the bay laurel tree, prized since ancient times for its medicinal and culinary (bay leaves!) properties, and the reddish-brown color usually applied to horses.

One of the rarer tree names, Birch evokes imagery of the beautiful paper-white bark tree. Although a unisex name, the most notable people named Birch are men, such as Indiana senator Birch Evans Bayh Jr. Birch trees are very symbolic in many cultures. In Russia, birch trees are emblematic of grace, beauty, and strength. In Celtic culture, birch trees represent growth, stability, and adaptability.

Birch

Blue suddenly came into the spotlight, as the unusual color name chosen by Beyonce and Jay-Z for their baby girl Blue Ivy. Blue is also a starbaby middle name du jour, used for both sexes in different spellings and forms, from John Travolta and Kelly Preston's Ella Bleu to Alicia Silverstone's Bear Blu. Dave 'The Edge' Evans named his daughter Blue Angel back in 1989.
 Blue is the heroine of the trendy Marisha Pessl novel Special Topics in Calamity Physics.

Blue

Blythe originated as a nickname for an upbeat person, coming from the Old English word bliðe, meaning "merry" or "cheerful." Today the homophone blithe shares the same meaning. Blythe was eventually adapted to a surname before it became a feminine given name.
 
Blythe embodies a cheerful, carefree spirit and could be the next Blair/Blaine/Blake/Brett/Britt, with the advantage that it's clearly pink. It joins other other light-hearted names for girls: Felicity, Hilary.
 Blythe is the middle name of Drew Barrymore—in her case a surname from her rich theatrical history.
 The title of the memorable Noel Coward play Blithe Spirit is taken from Shelley's poetic line, "Hail to thee, blithe Spirit"—a nice tribute to a baby Blythe.

Blythe

Chase is starting to be seen on a handful of girls each year, and has been heard as a nickname to Chelsea. 

Chase

One of the new bird names, like Lark and Wren, this one's associated with the billing and cooing sounds of love. Soft and gentle, Dove also has the admirable association with peace.
 Dove was actually an early bird, used as a first name in the seventeenth century and in the U.S. Top 1000 through the 1890's.
 One admirable surname association is with Rita Dove, the Pulitzer Prize winning poet and former United States Poet Laureate.

Dove

In English, a fen is a wetland fed by surface and/or groundwater. In Dutch and Frisian, it's a traditional diminutive of names containing the element frid, meaning "peace".

Glass

Just like Flora is for botanicals, Jewel is the generic gemstone name, not used much since the early twentieth century, when it was seen as a symbol of how precious a daughter could be. The French version, Bijou, feels more modern, as do Pearl and Ruby. 

Jewel

June, a sweetly old-fashioned month name derived from the goddess Juno, was long locked in a time capsule with June Allyson (born Ella) and June Cleaver, but is rising again especially as a middle name. 
 June was a Top 100 name from 1915 to 1941, peaking in 1925 at Number 39. It fell off the U.S. Top 1000 in 1987 and returned in 2008 after a two-decade hiatus. Now, we'd call it one of the vintage girls' names starting with J due for a comeback. Balthazar Getty has a daughter named June and Amanda Peet used it as her daughter Molly's middle name.
 Juni is the Scandinavian version; other related choices are Junia, Juno, Juna and Djuna.

June

We're hearing Kat more and more being used by Katherines who are tired of Kathy, Kate and Katie. Katti or Kattie is another possibility.

Lune

March has never been popular as a month name, possibly because of its slightly abrupt sound. But with all kinds of month (and season, day, and holiday) names, from January to December, Sunday to Easter, Winter to Midnight, coming to the fore, March is beginning to seem eminently baby-ready.

 An added bonus of lovers of classic literature: March is the surname of Jo, Meg, Beth and Amy in Louisa May Alcott's Little Women.

March

The original oddball celebrity baby name, via Frank Zappa's daughter Moon Unit, who claims she's always liked it. 

Moon

In Greek mythology, Nyx was a powerful goddess and the embodiment of the night, but when spoken, its negative meaning can't be ignored. 

Prim

Quinn is one of the first popular Irish unisex surnames, a strong and attractive choice on the rise for girls but still popular for boys. Quinn was used for about 3000 baby girls and 700 boys in the US last year.
 
Quinn is the Anglicized version of the Irish patronymic surname Ó Cuinn, meaning "descendent of Conn." Conn has two possible derivations—the Old Irish cond, meaning "intellect," or cenn, meaning "chief." One of the most notable Quinn clans was from County Tyrone in Northern Ireland. 
 Quinn started taking off for girls in the mid-nineties, possibly influenced by the popular show Dr. Quinn, Medicine Woman. Since then, there have been female Quinns on the animated MTV show Daria, on Zoey 101, and—most prominently—on Glee, featuring cute blond cheerleader character Quinn Fabray.
 Some other, more <a href="https://nameberry.com/lists/category/unusual-names">unusual names</a> with the Q-factor: Quincy, Quintina, Quintana, Quinta, Quilla--and, on a different note, Queenie.

Quinn

Fiery but slight middle name choice; much more apt to be a redhead's nickname. Instead, check out other names that mean red: Rory, Rowan, Ruby.

Rue has gone from Golden Girls actress to Hunger Games heroine. This botanical name is also a coincidental double word name, meaning "regret" in English and "street in" French. Despite these unfortunate secondary meanings, Rue has real potential to be one of the most popular new <a href="https://nameberry.com/list/479/middle-names-for-girls">middle names for girls</a>. 
 Actress Rue McClanahan, who played Blanche in The Golden Girls, was born Eddi-Rue, named for her parents' middle names Edwin and Rheua-Nell.
 Actress Sara Rue made her surname the middle name of daughter Talulah Rue.
 The herbal plant Rue was introduced by the Romans and has been prized for its medicinal properties – the ancients believed it guarded against epilepsy and vertigo and improved the sharpness of vision.

Ross

Sloane is a sleek, sophisticated surname name that has gradually morphed over to the girls' side. Sloane is definitely a name that's going to continue to rise. Spelled without the final "e," Sloan joined Sloane in the Top 1000 for the first time in 2011.
 Sloane has been the name of girlfriend characters in Ferris Bueller's Day Off and Entourage, and it is currently the character played by Olivia Munn on The Newsroom. Sloane Crosley is a bestselling author, and comedian Rob Corddry named his daughter Sloane Sullivan.
 The association with London's Sloane Square and the upscale clique of Sloane Rangers, who have included Princesses Di and Kate, gives it some extra panache.

Sloane

Miss Mayhem's list "Unique One-Syllable Girl Middle Names!" of 23 great name ideas: Ash - Sloane!

Unique One-Syllable Girl Middle Names!

The list's title is pretty self explanatory, but the description must exceed fifteen words, which is why I'm bothering to write all of this.