Help identifying a story where kids discover some kind of wormhole in a garden

I'm not at all sure about this...but could the book in question posssibly be one of Madelaine L'Engle's? If I remember correctly, the first book in that series "A Wrinkle in Time" involved children moving through other worlds, and was fairly dark in nature. It's been some time since I've read it but I believe the method used to travel was called a 'Tesseract'.

The story sounds incredibly like "The Secret Garden", 1910, by Frances Hodgson Burnett. It was a novel, originally published in serial format, and in 1993, a film with Maggie Smith. I remember the film, and the lovely story.

I checked on IMDb to find the availability and to see a clip. It is available for viewing on Amazon.

The story is called A Wrinkle In Time... I've read it a million times as a kid. The movie sucked but I loved the book.

It sounds like it could be The Silver Nutmeg: The Story of Anna Lavinia and Toby (1956) by Palmer Brown. This is from NY Times Book Review:

Anna Lavinia’s father wanted her to have another point of view, so what did he do? He made a peephole in the garden wall. But he couldn’t have known that this new view would lead Anna Lavinia all the way to the upside-down mirror land that lies on the other side of the pond. Here Anna Lavinia meets Toby, who explains that on the other side, instead of gravity, there’s something called “the tingle,” which feels like “the tickle that comes before a sneeze, or the thrill that comes when the knot in a ribbon just begins to loosen,” and allows for floating and spectacular feats of tree-climbing (but mind your furniture doesn’t drift away!). Toby introduces Anna Lavinia to a variety of wonders and oddballs, including an uncanny fortune-teller, a turtle with a jungle on its back, and Aunt Cornelia, who’s never quite recovered from the disappearance of a certain young man into Anna Lavinia’s world a very long time ago.

The Silver Nutmeg continues the adventures begun in Beyond the Pawpaw Trees, and features loads of sense, a little nonsense, and more charming verses from Anna Lavinia’s favorite book of rhymes. Best of all, fans of Palmer Brown’s intricate drawings will find every page a delight for the eyes.

Dunno about a wormhole in the garden, but The Lion, the Witch and the Wardrobe is definitely set in Britain (on this side of the wormhole, at least), features a bunch of kids / teenagers (two brothers and two sisters) and is definitely quite dark (Edmund selling out his older brother and his sisters is pretty dark, as well as the evil witch blanketing Narnia with winter and turning various characters into stone). The wormhole is found by entering an old wardrobe in the home where they're staying.

And author C. S. Lewis had quite a series of books about Narnia, of which this was the first.

This may be "The Secret Country" by Pamela Dean, the first book of a trilogy. The summary on Goodreads is:

Each vacation for the past nine years, cousins Patrick, Ruth, Ellen, Ted, and Laura have played a game they call the “Secret”—and invented, scripted world full of witches, unicorns, a magic ring, court intrigue, and the Dragon King. In the Secret, they can imagine anything into reality, and shape destiny. Then the unbelievable happens: by trick or by chance, they actually find themselves in the Secret Country, their made-up identities now real. The five have arrived at the start of their games, with the Country on the edge of war. What was once exciting and wonderful now looms threateningly before them, and no one is sure how to stop it… or if they will ever get back home.

My memory is a little fuzzy on the details, but I believe the kids fall through some kind of portal under a hedge in a garden? The children are believed to be the characters they pretended to be- one is a Prince. Colours are highly symbolic in Secret, each has a meaning, and they use their knowledge of the 'characters' to figure out what is really happening, and what the truth is, while trying to protect the country they find themselves in from the dangers they find.

Hope that helps!

edit: unfortunately the first one was published in 2003 which puts it at the tail end of your estimated timeline.

Possibly An Acceptable An Acceptable Time by Madeleine L'Engle.

Polly O'Keefe is visiting her maternal grandparents, the Murrys. While in the garden/fields near the house, she meets characters from 3000 years ago; some sort of portal has opened. There is travel both ways, and at some point Polly, her friend Zachary, and Bishop Colubra all travel to the earlier time.

Directly relating to the details you mention:

What I can remember is that the stories/books had a dark atmosphere,

This is somewhat subjective, but when dealing with a selfish young man with a serious heart condition (Zachary), a young modern teenager who travels back in time, is kidnapped, and almost becomes a human sacrifice, etc., you could certainly term it somewhat 'dark'.

that the main characters were young, that they found some kind of wormhole in a garden that would take them to another place.

In this book, it was another time, not another place, actually, but the time was distant enough that the place was quite different (standing stones where in Polly's time there is a house and pool, a lake where in her time there is a valley, primitive seeming people compared to 'modern' rural New England....)

There are several different locations where the time portal suddenly opens. The first is near the 'star watching rock' in the garden/fields around Polly's grandparents home. Another time it is next to the swimming pool. When she goes with Zachary it is also in the rural area near the 'star watching rock'. Here is the description of the passage through the portal, when Polly went with Zachary:

Under their feet the ground seemed to tremble. There was a faint rumble, as of distant thunder. The air about them quivered with concealed lightning. .... The trunks of the trees thickened, the branches reached upwards. Ahead of them, sunlight glinted off water.

**Additional Information: I'm sure there were a group of them - all children/teenagers. Mix of genders.

Close but not exact. From modern times, Polly is a teen I believe. Zachary is a young adult. Bishop Colubra on the other hand, is not young at all. The main characters in the past appear to be young, though I don't believe their ages are given.

Setting might well have been England, but might have been elsewhere.

Rural New England?

The only other detail I remember is that there was a series of books about this.**

Partial match. This is the last book in the "Time Quintet" series, but the books are independent and this is the only one about this time portal. Though, in "A Swiftly Tilting Planet", Charles Wallace, then a teen (who later will be Polly's uncle) does do a form of time travel, in the same location, including spending time with the People of the Wind. He does his time travel while riding on a unicorn though, not through anything resembling a wormhole.

The characters actually come together from several different books by Madeleine L'Engle. Polly first appears in "The Arm of the Starfish". Zachary in "Meet the Austins". The Murrys (Polly's grandparents) along with their children (and son-in-law to be, Polly's father) and Dr. Colubra (the bishop's sister) appear in "A Wrinkle in Time" and/or its sequels.

> Read 15/20 years ago Possible. First published in 1989.

A description of the book from a website in the author's name:

A flash of lightning, quivering ground, and, instead of her grandparents’ farm, Polly sees mist and jagged mountains — and coming toward her, a group of young men carrying spears. Why has a time gate opened and dropped Polly into a world that existed 3,000 years ago? Will she be able to get back to the present before the time gate closes — and leaves her to face a group of people who believe in human sacrifice?

Some additional points to see if they jog your memory: Polly and friends are caught in the middle of conflict between the tribe they have met ("The People of the Wind") and the tribe across the lake. Zachary has a heart condition, and he is used as a pawn because of his fear of death and wish for a cure. At one point Polly is a captive of the tribe across the lake, but she manages to escape (with the help of a dog) by swimming across the lake.

(Note: Polly is the daughter of Meg Murry and Calvin O'Keefe, from "A Wrinkle in Time" and its sequels)

This could possibly be Philip Pullman's His Dark Materials trilogy.

I read these books as a kid...some 15/20 years ago

The first book (Northern Lights) was published in 1995, and the last (The Amber Spyglass) in 2000.

the stories/books had a dark atmosphere

There's kidnapping, child experimentation, and murder in the first book, and plenty of other dark themes and events in the others.

the main characters were young

One of the main characters is around eleven at the beginning of the first book. The other main character, introduced in the second book, is a similar age.

they found some kind of wormhole in a garden that would take them to another place.

There are several windows between worlds, though they act like simple doorways rather than wormholes. Some are found - the first one in some bushes; others are opened intentionally, including at least one in a garden.

I'm sure there were a group of them - all children/teenagers. Mix of genders.

Not an exact match here. The main characters are a boy and a girl, which covers mixed genders. There are other children involved at points in the story, but none of them would be considered main characters.

Setting might well have been England, but might have been elsewhere.

There's both what would be considered "our" England and an alternate universe England in the books.