Ash has Southern charm plus the arboreal-nature appeal. Plus your little boy will prize Ash as the name of the hero of the Pokemon cartoons. Ash can also be a dashing short form of Asher, Ashton, or any other "Ash" name.

Alder

Birch is a rarely used nature name that calls to mind the lovely image of the tall, strong but graceful white-barked tree. 
 The best known bearer of the name is former high-ranking Democratic Senator Birch Evans Bayh, who carried the name of his father and was the principal Senate sponsor of the Equal Rights Amendment.
 The birch tree has many notable qualities and uses &mdash; including the making of furniture, paper, canoes, and drums, is considered the national tree of Russia (where it used to be worshiped as a goddess), the state tree of New Hampshire, and has spiritual significance in several cultures.

Birch

Buckthorn

Cypress

Evergreen

Hazel has a pleasantly hazy, brownish-green-eyed, old-fashioned image that more and more parents are choosing to share. Former Old Lady name Hazel reentered the popularity lists in 1998 and now is near the top of the charts.
 
Hazel is a name that comes from the English word hazel, referring to the hazelnut tree. The word was derived from the Old English hæsel of the same meaning. Historically, a wand of hazel symbolized protection and authority.
 
When Julia Roberts named one of her twins Hazel in 2004, there was a great public outcry against "another one of those nutty celebrity names", though we saw it as the deserved revival of a lovely vintage name with roots in nature. Hazel was one of the botanicals that were all the rage at the turn of the twentieth century, when it was in the Top 25, then fell out of favor, off the list completely from 1976 to 1998.
 
Other celebrities with daughters named Hazel include Emily Blunt & John Krasinski, and Taylor Neisen & Liev Schreiber.
 
Hazel Motes is the protagonist of Flannery O'Connor's 1952 novel Wise Blood, and In The Hunger Games, one of the characters is the variant Hazelle. But the most influential Hazel of recent years is the heartrending heroine of Fault in Our Stars.

Hazel

Reddish dye that sounds as if it could be a real name — of the outdated, Old World, Yetta-Hedda variety.

Henna

Holly ranks just in British Top 50, but it's been out of favor here since the 1970s Era of Nickname Names. Still, the name may be on her way back as a rejuvenated nature pick.
 Jolly Holly has always been among the favorite <a href="https://nameberry.com/list/412/christmas-baby-names">names for Christmas babies</a>, evoking the symbolic shrub with dark green leaves and bright red berries. And Holly is the flower for the month of December, so it's also among the perfect <a href="https://nameberry.com/list/577/names-for-december-babies">names for December babies</a>.
 A memorable fictional Holly is the enigmatic heroine of Truman Capote's Breakfast at Tiffany's, Holly Golightly (almost named Connie, and with the full name Holiday), played indelibly by Audrey Hepburn. A real life one is actress Holly Hunter. Holly Short is a fairy character in Eoin Colfer's Artemis Fowl novels. And in Galsworthy's The Forsyte Saga, Holly and Jolly are siblings.

Holly

Hemlock

Hickory

The graceful, natural image of the verdant shade tree transcends any connection with President Johnson.
 A sturdy tree that can live for centuries, Linden was actually quite a popular boys' name from the 1930s to the 1960s--as was the presidential spelling Lyndon--but has now become unisex.

Linden

Magnolia, a sweet-smelling Southern belle of a name made famous via the iconic Edna Ferber novel and musical Showboat, is one of the latest wave of botanical names, along with unexpected blossoms Azalea and Zinnia. It is named for French botanist Pierre Magnol.
 Magnolia has been chosen for his daughter by Adrian Young of No Doubt—which happens to be a fabulous baby-naming consortium as well as a great musical group.
 In Showboat, the Magnolia character goes by nicknames Noa, Nollie, and Maggie.
 Magnolia charted on the popularity list almost every year from 1880 to 1940, topping at Number 420 in 1909—but it's now making a comeback.
 Magnolia Breeland is a character on the TV show Hart of Dixie, who happens to have a sister named Lemon and a father named Brick.
 Maminydjama Magnolia Maymuru is an Indigenous Australian model.

Magnolia

If Apple and Juniper, Oak and Pine can be baby names, why not Maple? Why not indeed. We've heard Maple starting to be used quietly, but with its lush sound and attractive image, we predict its use as a first name will grow &mdash; and its choice by the Jason Batemans &mdash; who combined it with the sweet middle name Sylvie &mdash; will only accelerate that growth.

Maple

Oak, a symbol of solidity, strength, and longevity, is joining Cedar and Pine as a viable name, one that would work especially well in the middle.
 If you'd prefer to take the longer route to the name, you can consider the surnames Oakes or Oakley.

Oleander

Worthy sibling for Oak, Elm, Juniper, and Spruce.

Pine

Pyrus

With its gentle sounds and earthy vibes, the name Rowan feels like a fusion of different styles. A word name, a surname, and a gender-neutral name, Rowan is rustic but trendy, blending the vibes of both Owen and Oakley. 
Just as it’s hard to fit Rowan into one category, it’s equally difficult to identify one definitive root. It could be considered an Anglicized form of the Irish name Ruadhán, meaning “little redhead” and it was used as an English surname before it became a given name. 
Alternatively, it could be linked to the rowan tree, whose name derives from Old Norse and means “to redden.” In folklore and mythology, rowan trees are seen as a symbol of protection and of life, adding extra meaning to Rowan. 
 To complicate matters further, some scholars identify Rowan as a girls’ name, deriving from Rowena and Rhonwen, meaning “famous friend” or “white spear.” This gives it solid credentials as a unisex name and lends it a slightly more rugged vibe. 
What is certain, however, is that Rowan has quickly become a stylish choice, chosen for over 3000 boys every year. Once an obscure name in the US, it rapidly climbed the charts during the 2010s and has now entered the Top 100 Boy Names for the first time. 
Rowan is popular in the UK, Canada, and the Netherlands too, but even more so among Nameberry users, where it ranks in the <a href=“https://nameberry.com/popular-names/nameberry/boys/allNameberry Top 1000”> Nameberry Top 30.</a> 
The most prominent bearer of the name is probably Rowan Atkinson, the British comic actor of Blackadder and Mr Bean fame. Other namesakes include the former Archbishop of Canterbury, Rowan Douglas Williams, and the Australian swimmer, Rowan Crothers. Pokémon fans and Sarah J. Maas readers might recognize it too as the name of a character in both the game franchise and in the Throne of Glass series. 
Gentle but powerful, stylish but not oversaturated, Rowan may well continue to climb the charts, so if you love Rowan but want to stray from the trends, Ruan, Sylvan, Lorcan, Lennox, Wren and Sloan might inspire you.

Rowan

This name of a giant tree, itself named for a nineteenth-century Cherokee who invented a way to write his tribe's language, makes a strong, stately statement. 

Sequoia

Sassafras

An ancient tree that figures in literature from Shakespeare to Harry Potter and is believed to possess magical powers, Willow is a lovely name, as graceful as its inspiration.
 Willow's use as a given name derives directly from the willow tree, whose name was derived from the Old English word welig, meaning "willow." Willow trees are associated with grace and elegance, which gave way to the adjective "willowy." Other names with meanings related to willows include Arava, Willoughby, Dozier, Wellesley, Selby, and Salton.
 
This elegant and charming nature name has been chosen for their daughters by Will Smith and Jada Pinkett Smith, Michelle Monaghan and Pink. It's been a fast riser in recent years, climbing along with other Will names Willa, William and Will, having taken off here a year after the character of Willow was introduced on Buffy the Vampire Slayer in 1997.
 
Willow is also popular in other English-speaking countries: Australia and the UK, Canada, Ireland, and Scotland.
 
The name has transcended its one-time hippie aura to move into the realm of reappraised and appreciated <a href="https://nameberry.com/list/329/nature-baby-names">nature names</a>. The willow is believed by the Romani to possess the ability to heal the sick and rejuvenate the aged.
 
Pink was quoted as saying her choice was influenced by a willow tree she grew up near, and that it is the most flexible tree that nothing can break—no wind, no elements, it can bend and withstand anything.

Willow

Frankie's list "Nature Names - Trees" of 22 great name ideas: Ash - Willow!