Planting a Shikuwasa Tree: Warmth, Shelter and Choosing Pot or Ground

Start from the hardiness, because it narrows everything else

Shikuwasa is a small subtropical sour citrus, sometimes sold as flat lemon, grown for a heavy crop of thin-skinned fruit little bigger than a walnut. Its weak point as a garden tree is cold. It is noticeably less hardy than the sour citrus most people compare it with, and where yuzu shrugs off a hard freeze and sudachi tolerates a short one, shikuwasa is unhappy at temperatures that only just dip below freezing and is damaged outright by a freeze that lasts more than a night or two.

That single fact settles the planting question before you pick up a spade. If your winters are essentially frost free, or frost arrives as an occasional light overnight event that burns off by mid-morning, you can plant in the ground and treat it as a permanent tree. If you get repeated frosts, or any period of days below freezing, plant it in a pot you can move and do not talk yourself into anything else. A tree in the ground in a cold-winter garden is not a bold experiment; it is a tree you will lose in its second or third winter.

The compensation is that the plant is unusually well suited to container life. It naturally makes a small, dense, twiggy tree rather than a sprawling one, it flowers and fruits freely at a size you can still move on a sack truck, and it crops heavily enough that a potted specimen gives you a real harvest rather than a novelty handful. Growing it in a pot is not a lesser version of growing it properly. For most people it is the correct way.

The site: sun first, then shelter from wind

Give it the sunniest position you have, and no less than six hours of direct sun through the growing season. Sun does two jobs on this tree. It drives the crop, and it builds the aromatic oil in the rind that is a large part of what you are growing. A shikuwasa in half shade will still make fruit, but the fruit smells thin and the juice is flatter, and no amount of feeding corrects it.

A wall or solid fence facing the midday sun is worth going out of your way for. Stored heat in masonry lifts night minimums by several degrees, which on a plant this tender is the difference between a tree that sails through a marginal night and a tree that drops half its leaves. The same wall gives you something to fix row cover to when you need it.

Wind is the second consideration and it is more important here than on most citrus, because the fruit is thin skinned. A branch rubbing a cluster of fruit against a neighboring twig in a breeze will mark every fruit in that cluster, and marked rind on this fruit is not cosmetic damage but lost crop. A hedge, a fence or another shrub upwind pays for itself in the first year of cropping.

Do not plant it in a frost pocket, meaning the bottom of a slope or a hollow where cold air collects and sits. Those positions run several degrees colder than ground twenty feet away on the same night, and on a tree with this little cold tolerance that difference decides the outcome.

Planting into the ground

Plant in spring once the soil has warmed thoroughly and all risk of frost is behind you. Never plant in autumn. A spring planting gets a whole season to establish roots before it meets its first cold, and on a plant of this tenderness that season is the entire safety margin you have.

1. Dig wide rather than deep Make the hole three times the width of the rootball and exactly as deep as the rootball is tall. Break up the sides with a fork so the roots can leave the pit instead of circling around inside it.
2. Backfill with the soil you removed Add no more than a fifth coarse grit or bark if the ground is heavy. A hole filled with rich compost holds water like a bucket and keeps the roots inside the planting pit for years instead of spreading out.
3. Set the rootball slightly proud Finish with the top of the rootball one to two inches (3-5 cm) above the surrounding soil, because it will settle. A citrus planted an inch too deep sulks for years and is far more prone to collar rot.
4. Keep the graft union well clear of soil The swollen union must sit at least four inches (10 cm) above the soil and above the eventual mulch line. Once buried, the top roots for itself and whatever cold tolerance the rootstock offered is gone.
5. Water in, then mulch with a bare collar Firm gently, water with about four gallons (15 liters), then spread two to three inches (5-8 cm) of mulch across the root zone while pulling it back to leave a hand's width of bare soil around the trunk.
6. Stake only if the site is exposed A low stake and a soft tie for the first two years is enough. Tie loosely so the trunk can flex; a rigidly staked tree never thickens properly at the base.

Planting into a pot

Resist the temptation to start big. A young tree dropped into an oversized container sits in a volume of wet compost its roots cannot dry out, and root rot follows within a season or two. Move up two inches (5 cm) of pot diameter at a time and the problem never arises.

Use a free-draining mix rather than ordinary potting mix alone. Roughly two parts loam-based compost to one part coarse grit or bark gives you both the drainage the roots need over winter and the weight the tree needs to stay upright. Weight genuinely matters on this plant: a small dense tree carrying a heavy crop in a lightweight peat mix goes over in the first autumn gale.

Choose a pot with real drainage holes and stand it on feet so water runs away freely. Never leave it sitting in a full saucer, and never use a decorative outer pot with no outlet. Before you buy the pot, work out where the tree will spend winter and whether you can physically get it there, because that is the constraint that will bite you.

Keep the graft union above the compost surface in a container too. Top-dressing every spring slowly raises the level, and after several years the union is buried without anyone having noticed the day it happened.

The first season, and what to write down

For the first year, the job is roots rather than fruit. Water thoroughly and never let the rootball dry out completely, and feed lightly from the first sign of growth until the end of summer. A newly planted tree that flowers heavily in its first spring is better relieved of most of that fruit, because a crop taken from an unestablished root system costs you the growth that would have supported next year's.

Record the date you planted, whether the tree is grafted or on its own roots, the pot diameter or the exact position in the ground, and the height at planting. Photograph the base of the trunk so that in three years, when the bark has thickened and suckers start to appear, you can still identify the graft union with confidence.

Then add one line every winter: the lowest temperature the tree actually experienced, whether you fleeced it or moved it under cover, and how much leaf drop or dieback showed the following spring. Three winters of those notes give you a hardiness figure for your own garden, which is worth far more than any published number.