Gadenin日本語Get the appWhy an old crown needs splitting
A rhubarb crown expands outward year on year until the center becomes a woody, congested mass. The buds crowd each other, the stalks come up thinner and more numerous, and the plant starts throwing flower spikes as if it were under stress, because effectively it is. Dividing resets it, and the divisions are your free new plants.
Plan on lifting and splitting each crown every five or six years. You do not have to wait for it to decline; a healthy crown divides more easily and the pieces establish faster than those from a plant already struggling. If you only want to keep the same number of plants, replant one good division and give the rest away.
The signals that a crown is due are consistent: many thin stalks instead of a few thick ones, a woody or hollow center with no growth in the middle, and flower spikes appearing two seasons running despite good feeding and watering.
| Signal | What it means | Action |
|---|---|---|
| Stalks thinner every year | Congested crown or exhausted soil | Divide and replant in fresh, enriched ground |
| Dead woody center | The middle of the crown has aged out | Divide, discard the center, keep the outer buds |
| Flower spikes two years running | The plant is under stress | Divide in the next dormant season |
| Plant simply very large | Healthy and outgrowing its space | Divide for new plants, no urgency |
Do it while the crown is dormant
The window for dividing is the dormant season: from after the leaves have died back in autumn until just before the buds break in early spring. A dormant crown has no leaves to support, so the disturbance costs it almost nothing, and the roots knit back into the soil before growth starts.
Late autumn suits mild areas, where the divisions settle in over winter and start away strongly. In cold areas where the ground freezes hard, wait until late winter or very early spring so a newly cut division is not sitting in frozen wet soil for months.
Do not divide a plant in full leaf in summer. It can be done in an emergency but the plant sulks for a whole season, and there is no advantage over waiting a few months.
Splitting the crown
The job is straightforward but physical. A mature crown can weigh a great deal and the roots run deeper than expected, so allow more time and more digging than the size of the plant above ground suggests.
- Lift the whole crown Dig a trench right round the plant, a foot (30 cm) out from the buds and a full spade deep, then lever it out from several sides at once. Expect to break some roots. Knock the loose soil off so you can see where the buds are.
- Find the buds before you cut Look over the top of the crown for the fat pink or white buds. Every piece you make must carry at least one of them, and pieces with two or three establish faster. A chunk of root with no bud will never grow.
- Cut with a spade or an old knife Drive a sharp spade straight down between the buds and split the crown into pieces about the size of a fist or larger, each with a bud on top and a decent length of root below. Clean, decisive cuts heal better than tearing.
- Throw the old center away The woody, budless middle of an old crown will not regrow. Discard it along with any piece that is soft, hollow, brown or foul smelling, which indicates rot.
- Let the cuts dry briefly, then plant Leave the divisions in a cool, airy, frost free place for a few hours so the cut surfaces dry over. Then plant each one as a new crown, with the bud just at or barely below the soil surface, three feet (90 cm) apart in well manured ground.
If you cannot plant immediately, heel the divisions into a spare corner of soil or pot them into large containers of gritty potting mix. Do not let bare roots dry out or sit in a bucket of water.
Looking after the divisions
Water a spring planted division whenever the soil dries in its first season, and mulch around it while keeping the material off the bud. Autumn planted divisions usually need no watering at all until the following summer.
Treat a division exactly as you would a newly bought crown: take no stalks in its first full season, and only a very light pick in the second. It is a large piece of root with established reserves, which makes it tempting to pick early, but the crown that is left alone for a year outperforms the one that was picked for the rest of its life.
Cut out any flower spike that appears on a new division immediately. A freshly divided plant is under stress and is more likely than usual to try to flower.
Why not grow it from seed, and what to write down
Rhubarb can be raised from seed, and it is the only route if you cannot get hold of a crown, but it takes two to three years to reach a pickable size and the seedlings vary enormously. Some come up thin, some stringy, some prone to bolting. Division gives you an exact copy of a plant you already know is good, at cropping size much sooner.
Record the date you divided, how many divisions you made, and where each one went. Note the parent plant too. In three years, when one division is cropping heavily and another is not, the position and the parentage are the only two things that can explain the difference, and neither is something you will remember unaided.
Also note the year of division against each crown so you know when the next split is due. Five or six years is long enough to forget entirely.