Planting Gentians: Getting the Soil, the Aspect and the Depth Right the First Time

Decide the soil before you dig anything

The most important part of planting a gentian happens before the spade comes out, and it is the decision about lime. The spring-flowering alpine kinds generally want lime and are at home in neutral to alkaline ground. The autumn-flowering Asian kinds need acid, peaty, humus-rich soil with no free lime anywhere in the root zone. Get this backwards and the plant will not fail immediately; it will grow on for a couple of seasons, flower thinly, fade and then be gone, and you will never know why.

This means the two groups need separate ground, and mixing them in one bed for the sake of a display is a mistake that will cost you half the plants. If you want both, build them separately: a limestone-chipping scree or a trough with alkaline mix for the alpines, and an acid bed or a large container of acidic potting mix for the autumn-flowering kinds. Keep two bags of grit, one limestone and one lime-free, and label them.

Test the ground before committing, particularly against a house wall or beside a path, where buried mortar rubble makes soil far more alkaline than the rest of the garden. That single fact explains a large proportion of the autumn-flowering gentians that fail in otherwise good gardens, and it is the reason a container of controlled compost is often the safer answer for those plants.

A cool aspect, not a dark one

Gentians want cool roots and enough light to open their flowers, and those two requirements pull in different directions unless you site the plant carefully. A north-facing bed, the cool side of a low wall, or a position with morning sun and shade through the hottest part of the afternoon, gives you both. The ground stays cool and moist at root level while the plant still gets the direct light it needs to open its trumpets.

What ruins them is a hot, open, south-facing spot with thin soil, where the shallow root run dries and bakes and the foliage scorches. What disappoints them is the other extreme, deep shade under an evergreen or against a solid fence that never sees the sun, where the plant makes leaves and few flowers and the buds it does make barely open. Light shade is right; heavy shade is not.

Elevation helps in every respect. A raised bed, a scree slope, a trough or a gap in the top of a low retaining wall lifts the crown above winter wet, drains sharply by construction, and puts the flowers nearer eye level where the low mat-forming kinds are much better seen. If your garden soil is heavy clay, a trough or raised bed is not a compromise for gentians; it is the better option outright, and it lets you control the lime question completely.

Building the root zone and putting the plant in

The target is ground that holds moisture at depth all season yet never stands wet, so build it over a wide area rather than in a narrow hole. Gentian roots run out further than the visible plant, and a deep pit dug in unimproved clay just fills with water and drowns the plant it was meant to help. Wear ordinary gloves and take your time over the preparation, because this is the part you cannot revisit later without disturbing the roots.

1. Test the drainage first Dig a hole a foot (30 cm) deep and fill it with water. Draining within a couple of hours is fine. Water still standing the next morning means you must build the bed up or plant in a raised bed instead.
2. Improve a wide area, not a deep pit Work a third by volume of coarse grit plus generous leaf mold through the top eight inches (20 cm) of a patch two feet (60 cm) across. Match the grit to the type of gentian.
3. Plant in spring, as growth begins Spring planting gives a full season of root growth before the first cold spell. Autumn works in mild areas with sharply drained ground, but a new plant sitting wet and cold through winter often rots.
4. Soak the plant in its pot before planting Stand the pot in water for half an hour. A dry root ball sheds soil as you handle it and then repels water for weeks after it goes in the ground.
5. Set the crown exactly at the finished surface The point where the shoots leave the roots must sit level with the soil, never below it. A buried gentian crown rots at the collar during the first wet spell.
6. Backfill, firm gently and water in Work the gritty mix in around the roots with your fingers and firm with the flat of your hand rather than treading. Water in slowly to close air gaps around the roots.
7. Top-dress with grit right up to the collar Spread an inch (2 to 3 cm) of the correct grit over the whole root area, taking it up to the neck of the plant. This keeps the collar dry, the roots cool and the soil off the foliage.

Containers, troughs and the case for growing them separately

A container is often the best home for a gentian rather than a fallback, because it gives you complete control over the one thing that decides whether the plant lives: the soil reaction. Mix a loam-based compost with about a third coarse grit and a generous amount of leaf mold, using acidic potting mix and lime-free grit for the autumn-flowering kinds. Choose a deep pot or a stone trough rather than a shallow bowl, since the roots go down before they go out, and make sure the drainage holes are genuinely open.

Stand the container where it gets light but not the full heat of an afternoon in high summer, and remember that a pot heats up and dries out much faster than open ground. Check the moisture below the grit twice a week in warm weather. Use rainwater throughout for the acid-loving kinds, because a season of hard tap water in the confined volume of a pot does far more damage than the same water applied to open ground.

Never leave a pot standing in a saucer of water, particularly through the cold months. Raise it on pot feet or a couple of tiles so water runs straight out of the base, and move it to the shelter of a wall where winter rain is relentless. Repot only when the plant is genuinely outgrowing its container, in spring, keeping the root ball intact and disturbing it as little as you possibly can.

Keep the two groups in separate containers even if they will stand side by side. One shared trough means one soil reaction, and whichever camp it suits, the other half of the planting will slowly fade out of it.

The first two seasons, and what to record

A newly planted gentian spends its first season building roots and often flowers sparsely or not at all. That is normal. Keep the root run evenly moist through the first two summers, checking two inches (5 cm) down beneath the grit rather than watering to a routine, keep the top dressing topped up, and weed by hand only. Do not feed heavily in the hope of speeding things up, because rich feeding at this stage produces soft growth and no flowers.

Above all, do not move it. A gentian lifted and replanted in its first or second year loses most of the roots it has built and starts again from nothing, and repeated moving is how these plants get a reputation for being difficult. Decide the position once, mark the edge of the root run with something visible, and then leave the ground alone.

Write down, on a sketch of the bed, exactly where each plant went, the date it was planted, which camp you judged it to belong to, and therefore which grit and which water it gets. Record the result of the drainage test and note whether water pooled anywhere after the first heavy rain. Add the date growth first appeared each spring and the date of the first open flower. Those few entries will tell you, two seasons from now, whether the position was right, and they are the difference between diagnosing a problem and guessing at it.