Daikon Soil Preparation: Preventing Forked and Stunted Roots

Everything that forks a daikon is physical

A daikon root grows downward as a single tip. While that tip is intact the root is one straight white column. Damage it or stop it, and growth transfers to side roots which then thicken, and you lift a root that splits into two or three legs, or one that is short and blunt with a beard of hairy roots.

There are only four things that do this, and all four are decided before the seed goes in: a stone or clod in the path, a lump of unrotted manure or compost, a compacted layer the tip cannot push through, and root damage from transplanting or careless thinning. None of them can be corrected once the crop is growing.

It is worth digging up the site of any forked root you pull and finding the cause. The obstruction is almost always still there, at exactly the depth where the root divided.

Dig deeper than the root will go

A full sized daikon runs twelve to sixteen inches (30 to 40 cm) down and two to three inches (5 to 8 cm) across at the shoulder. Aim to have soil the tip can move through to at least fifteen inches (38 cm), which in practice means digging a full spade depth and then loosening the layer below that with a fork.

Fine crumbly soil on top of an untouched pan is the classic trap. The plant grows beautifully for six weeks, then the root reaches the pan, stops, and thickens into a stubby cone. From above the leaves look perfect right up until you pull it.

1. Turn a full spit Push the spade in vertically to its full depth and turn the soil over across the whole bed. Do not worry about clods at this stage; the aim is to break any compacted layer.
2. Loosen the layer below Drive a fork into the base of the trench and lever it back and forth without lifting the soil. This gives the tip somewhere to go in the last few weeks of bulking.
3. Take out every stone Work through the turned soil and remove anything bigger than a fingernail, plus old roots, stems and bits of plastic. Treat one stone as one forked root.
4. Break the clods to a crumb Reduce the top eight inches (20 cm) to an even crumb with the back of a rake. On clay, choose a day when it is slightly on the dry side and the clods shatter instead of smearing.
5. Let the bed settle Make the bed and leave it for a week or more so rain settles it. Sowing into freshly fluffed soil means it sinks unevenly underneath the seedlings and the roots grow crooked.

Where the bed will be sown in autumn, do the deep digging in high summer and leave it rough. Where it will be sown in spring, dig it rough in the coldest weeks and let frost do the clod breaking for you.

Manure and compost must be finished breaking down

Fresh manure is the worst single input for daikon. It continues to break down in the soil, releasing ammonia and heat around the root tip, and a tip that meets a lump of straw or half rotted material stops and divides. The old advice about never manuring for roots is not superstition, it is a description of exactly this.

Use only well rotted material, get it in at least a month before sowing, and mix it evenly through the digging depth instead of leaving a layer. If your only compost is coarse or clearly unfinished, put it on the crop before the daikon and sow into the residue a season later.

Lime and compost applied on the same day react with each other and waste nitrogen. Separate them by at least two weeks, lime first.

Keep the feeding even, not strong

Concentration is a bigger problem than quantity. A band of fertilizer under the seed row is something the tip will certainly meet, and the result is a scorched tip and a split root. Scatter the base dressing over the whole bed surface and rake it through the top few inches so there is no rich pocket anywhere.

Too much nitrogen early gives huge leaves and a slim root, and it makes the plant softer and more attractive to aphids. Daikon does most of its bulking in the second half of its life, so keep the base dressing modest and top dress after thinning instead.

Shallow ground, heavy clay and containers

If you cannot get fifteen inches of workable depth, build up rather than dig down. A raised bed eight to ten inches (20 to 25 cm) high on top of loosened ground gives the root the depth it needs and drains better, which matters because daikon rots in waterlogged soil but splits if it dries hard and is then soaked.

In containers, depth is the only thing that counts. A tub at least sixteen inches (40 cm) deep filled with sieved bought compost will grow a full length root; anything shallower will grow a short blunt one no matter how wide the pot is. Where depth is genuinely limited, switch to a short or round rooted type rather than accepting a forked long one every year.