Q: I'd love to hear a deep discussion on forsythia; specifically on propagating a forsythia hedgerow from scratch. I would like them as a garden border and bird habitat. Would cuttings be best? Or should I purchase plants and bury some of the branches to thicken the stand? In short, what are the best ways to propagate new forsythia?
----Chris in Harrisburg, PA
A: I have some really old forsythia shrubs at my house, and can say with assurance that the plants would make a good 'hedgerow'. In fact, I spend a lot of time preventing my two old giants from moving towards each other and BECOMING a hedge.
Why don't I let them? Let's see; there's the huge old climbing rose and blue holly in between them; the magnificent white pine off to one side, the ancient peonies that will soon pop up out of the ground on the other side, the Gold Dust Plant next to that, the two peach trees planted between the forsythia and the house that absolutely require decent sun and airflow, the azaleas….
If they were left unpruned, my two 'standard' forsythia would be a bigger threat to those plants than Godzilla is to high-tension lines—probably reaching twenty feet high by now. They would get taller, but at that height, the new growth tends to bend down, and every part of the plant that touches the ground 'tip roots' and becomes a new plant. (Cue 'The Sorcerer's Apprentice' music from Fantasia). The rest of my front landscape would have been swallowed up by their walking invasion long ago if I didn't control the spread every year.
(These attributes make standard types perfect for the kind of screening we're discussing here. There are also 'dwarf'-sized better-behaved varieties for folks who want the color and other benefits without the height.)
But that possible Future World of my ginormous forsythia would be wonderful in its own way. The sight of that entire area ablaze twenty feet high in a shade of yellow that's unparalleled in the plant world every Spring would certainly not be hard to take. And it would not be awful for the massive numbers of birds that would happily live in the protection of those giant shrubs; or for the great diversity of native bees and other pollinators that would swarm all over the plants in bloom; or for the amazing beneficial insect known as the Spring Tiphia wasp.
…A really cool insect. A wasp that looks like a winged carpenter ant, it feeds on the extra sugars that forsythia pumps out in the Spring (as do peonies and firethorn), and then lays its eggs inside developing Japanese beetle grubs in the soil nearby. Then, instead of a rose-chewing beetle, another predatory wasp eventually emerges from that spot in the soil. (Just like in the movie 'Alien', but with a grub as the victim instead of John Hurt.)
People are often surprised when they hear that these wasps look like flying ants. But as we often try and explain, many insects don't "look like themselves". There are bees that look like flies, and wasps that look like dragonflies. The 'cuckoo wasp' is a brighter metallic color than the shiniest new hot rod. There's a cuckoo BEE that looks just like a traditional wasp. And there's a very waspy-looking wasp called the five-banded Tiphiid that looks terrifying, but only attacks grubs in the soil, much like the Spring Tiphia (which, we remind you, looks like a flying ant).
Hey! Wait a minute—beneficial insects aren't this week's question! We're supposed to be making a forsythia hedge. So, back to the topic:
Our listener should either buy some new plants or "layer" new plants from the branches on existing shrubs. It's super easy; just peg a live branch down onto bare soil, keep the area moist and then cut the new plant loose when it has its own roots. Then, when you use these plants to create the actual hedge, don't crowd them too close together, as the crowns of forsythia tend to get large pretty quickly.
That's why I would not layer or 'tip root' the lower branches between these starter plants to fill things in as our listener suggests. I think there's a better way: Allow the new plants to grow unmolested for a season; then, right after they finish blooming the following year, prune back just the tops. Those top spikes shoot up really high, and it's good to keep those bad boys under a little control. But don't prune the branches coming off the sides; allow them to grow laterally and they should naturally join together, like a series of sideways curtains.
That way, you should be able to achieve a solid wall of Springtime color within a few years without having the ground so full of roots that the plants can't get any air, water or nutrients. (Speaking of nutrients, don't feed them anything stronger than a yearly mulch of compost. Unfed Forsythia quickly grow into huge plants, and if you gave them a lot of additional incentive they might get up and start walking around like Triffids.)
No matter what, be sure to avoid the biggest mistake people make with these plants, which is late season pruning. Forsythia sends up tall spikes all spring and summer, and people who cut these shoots after June often remove what would have been the best flowering parts. Prune them as much as you want right after the flowers fade and for a month or so after that; but then stop. And remember—they're supposed to look a little rangy, so don't try and turn them into boxwoods.