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Forsythia survives in part shade and flowers properly only in full sun. Six hours of direct light is the working minimum and eight is better. In shade the shrub still makes plenty of long green wands, but the buds set thinly along them, and you get a scattering of yellow instead of the solid wall of color that made you want the plant in the first place. This is the most common reason a healthy-looking forsythia disappoints, and no amount of feeding fixes it.
The light that matters is the light the shrub gets in the summer before the flowering, not the light in early spring. Flower buds are formed on the current season's new growth during summer and then sit through the winter waiting. A site that is sunny while the surrounding trees are still bare, but shaded once they leaf out, is a shaded site as far as forsythia is concerned. Judge the position at the height of the growing season.
Beyond light, forsythia is genuinely undemanding. It takes clay, sand, acid or limy ground, coastal wind and city air. It will not accept soil that stays saturated for days at a time, and it grows poorly under the drip line of a large conifer, where the ground is both dry and dark. Everywhere else, the plant is not the limiting factor.
Be honest about the mature size
A full-size forsythia reaches eight to ten feet (2.4 to 3 m) tall and just as wide, and the arching outer branches lean out further still. The compact kinds settle at three to five feet (0.9 to 1.5 m). People plant them three feet from a path or two feet from a fence because the nursery plant is a knee-high bundle of sticks, and then spend every year afterwards cutting the shrub back to keep the path open, which as the pruning guide explains is the fastest way to lose the flowers.
The shape is the whole point of the plant. Forsythia flowers along fountaining stems that sweep outward and downward, and the display only reads properly when those stems can hang free. Boxed in between a wall and a path, the same shrub becomes a clipped green lump that flowers on its top few inches. Give it a position where the outline can be left alone.
This makes forsythia an excellent shrub for a boundary, a bank, the back of a mixed border or a rough corner of the garden, and a poor one for a narrow front-garden strip. If the only space you have is narrow, choose a compact type and accept that it will still want four feet of width.
| Position | Spacing to allow | How it works out |
|---|---|---|
| Single specimen, full-size | 8 to 10 ft (2.4 to 3 m) clear all round | Stems arch freely, flowers to the ground |
| Informal screen or boundary | 5 to 6 ft (1.5 to 1.8 m) between plants | Closes into a continuous band in three or four years |
| Compact type in a border | 4 to 5 ft (1.2 to 1.5 m) clear | Stays in scale without annual shearing |
| Against a wall or fence | 3 ft (0.9 m) off the structure minimum | Air moves behind, back stems stay alive |
| Beside a path | Half the mature width, plus 2 ft (0.6 m) | Path stays open without cutting flowering wood |
Setting the plant
The best planting windows are autumn, once the leaves drop and the soil is still warm, and late winter to early spring before the buds break. Autumn is the stronger of the two, because the roots move while the top is dormant and the plant is established before its first summer. Container plants can go in during the growing season if you commit to watering, but a hot, dry spell straight after planting will cost you a season of growth.
Dig a hole two to three times the width of the root ball and no deeper than the root ball is tall. Width is what matters, because forsythia roots run shallow and wide. Depth is where people go wrong: soil settles after planting, and a shrub set two inches low ends up with its crown buried, which rots the base and encourages the plant to throw suckers from below ground in odd places.
Backfill with the soil you dug out rather than a rich compost mix. A pocket of luxury soil inside ordinary ground teaches roots to circle in the pocket instead of running out into the surrounding soil, and it pushes soft, leafy growth on a shrub that has no shortage of vigor to begin with. Firm the soil in stages, water it in to collapse air pockets, and finish level.
- Loosen the sides of the hole In clay especially, run a fork down the walls of the hole to break the smeared surface a spade leaves. A glazed hole in clay behaves like a plant pot and holds water around the roots.
- Free the root ball If roots are circling the pot, cut down through them in three or four places and pull the outer inch loose. Circling roots stay circling and choke the shrub years later.
- Set the depth with a straight edge Lay a cane across the hole and set the top of the root ball level with it. Level or a shade proud, never below.
- Water heavily, then mulch Give five to ten gallons (20 to 40 liters) slowly so it soaks in rather than running off. Mulch two to three inches (5 to 8 cm) deep across the root area, pulled back a hand's width from the stems.
- Shorten nothing except damage Resist tidying a newly planted shrub into shape. Cut out only broken or dead wood. Every stem you remove is flower buds you will not see, and the plant needs its leaf area to root out.
The first year after planting
Water deeply once a week through the first growing season whenever a week passes without an inch (25 mm) of rain, and keep going into autumn if the season is dry. One long soak beats three light sprinklings, because it wets the full depth of the root ball and pulls the roots downward. After the first year an established forsythia rarely needs watering at all except in a real drought.
Do not feed at planting and do not feed through the first summer beyond the mulch. Forsythia grows fast on ordinary soil and nitrogen simply lengthens the stems at the expense of the flower buds. If the shrub puts on two feet of new growth in its first summer, which is normal, it is being fed quite enough by the ground it is in.
Expect a modest first spring. A shrub planted in autumn usually flowers the following spring on the wood it arrived with, but a plant that was cut back at the nursery, or planted in spring, may give almost nothing until its second year. That is the plant catching up, not a fault.
Forsythia layers itself wherever an arching stem rests on bare soil, so any low branch touching the ground will quietly root and widen the shrub outward. Decide at planting whether you want that. If not, keep a mulched, stem-free gap around the base so the tips have nothing to root into.
What to write down on planting day
Record the planting date, the exact position, and the mature width you have allowed for. That last figure is the one you will argue with yourself about in three years, when the shrub reaches the path and you are deciding between moving the path and cutting the plant. Having the number written down turns a vague guess into a decision you already made.
Note the aspect and roughly how many hours of summer sun the site gets once the surrounding trees are in leaf. If flowering later disappoints, that note tells you immediately whether you are looking at a light problem or a pruning problem, and those two have completely different fixes. Add the first spring you see a full display, which becomes the baseline every later year is measured against.