How to grow impatiens
Impatiens are jungle species. They're understorey plants with very soft stems and intolerant of direct sunlight. Impatiens species include two fundamental forms, the large, bushy form and similar but much smaller dwarf species.
Impatiens can be difficult plants to grow, and they don't adapt well to some environments. You need to be well aware of the problems before planting them, because they can cause serious issues in the garden.
1. Impatiens become highly invasive in favorable habitats. They literally take over large areas of the garden, and worse, remove a lot of nutrients from the soil while doing so.The large form of Impatiens blocks light from other plants, and may provide habitat for vermin.
2. Impatiens require a lot of water, and are extremely sensitive to sunlight. They shrivel visibly, and quickly, in strong sunlight, and look as if they're dying, but recover when the sunlight goes off them. They really do look terrible, though, so it's a good idea not to plant them in areas which get a lot of sun, particularly in summer.
3. They're best planted under canopy, preferably under large broadleaf trees. Do not plant them under "territorial" plants like pines or eucalypts. They thrive in wet areas, and can be used effectively to mask "swamp zones" in natural settings, provided you're wary of their invasive habits. Think of invasive Impatiens as "horizontal kudzu", and you'll understand the risks.
4. They're not suitable for cold environments, and require sheltered micro climates even in temperate zones.
5. They're prone to fungal rot, and can therefore contaminate compost. Although they do make excellent green matter for compost heaps, providing lots of useful nutrients, this is one of their bad habits.
6. Impatiens are very easy to uproot and move around. They dislike sandy or impacted soil, and prefer soft loams. Sandy soils drain too quickly for them, and they will usually fail in these environments.
Impatiens don't really need fertilizer. Their growth habit allows them to get a lot of energy from foliage, and if you fertilize them, they'll go on the rampage in any suitable environment. Dwarf species are easier to manage, but may also clump in the wrong places in garden settings. Be ruthless with any overgrowth.
*****Pruning back Impatiens makes them bushier, but being such soft plants, they may also show signs of rot or insect attack. Keep an eye on them after cutting them back to avoid spread of fungi or pests.
In fairness, in the right environment, Impatiens can be spectacularly effective, creating lush green understorey. They really look like a miniature, flowering forest, and if you've got a cat or dog, you can expect them to exploit the possibilities of a nice shady place.
Impatiens are very reliable in terms of flowering, area cover and garden aspect for dense foliage plants. If you think of them as "tall ground cover" you'll get an idea of their potential in dealing with difficult spots.
If they cause problems, take them out, because these situations only deteriorate. They belong in well defined, case-specific environments, not garden beds.