How to Divide Rhubarb

Rhubarb: it's the gift that keeps on giving.  Around here, almost everyone knows someone who has a rhubarb plant in their backyard (even if they don't harvest a thing from it), and it's safe to bet that everyone's rhubarb plant originally came from someone else's rhubarb plant.  It is by far the easiest way to propagate rhubarb (rhubarb seed is pretty unpredictable) and the reddest varieties are only available by means of plant division.  But dividing rhubarb is not only a means of propagation, it's also an important process in caring for your rhubarb patch.

There are a number of reasons to divide your rhubarb regularly:

The Basics: 
It is best to divide rhubarb plants every 5 or 6 years.  It won't necessarily hurt the plant to go a little longer between divisions, but when you start to notice that the buds popping up from the crown are getting smaller and more crowded (and as a result, your rhubarb stalks will be smaller and thinner), it's definitely time to take some action.  Take a look at how small and crowded the buds are in the middle of the plant, compared to the outer edges of the plant:
Rhubarb can be divided in either the fall or the spring.  If you are dividing rhubarb in the fall, it will be more cumbersome to deal with the the larger plant, and transplants will require a lot more care to ensure that they get re-established before the first frost, but it will have the smallest impact on your rhubarb harvest, as the plants will have more time to recover before the following rhubarb season.  On the other hand, dividing rhubarb in the spring is a bit easier.  When new growth is just starting out, it is easier to make more precise divisions and the stress on the new plants will be considerably less, but it will somewhat limit how much you will be able to harvest from the plant (only for that year).  My personal preference is for spring divisions.

Ideally, divisions should be staggered over two or three seasons, so there will always be some rhubarb growing and producing normally.  In my case (our rhubarb came with the house), the plant condition and the location pretty much left me with no other choice but to do it all at once this year, but when the time comes for future divisions, I will divide no more than half of the patch in any one year, so I'll still be able to harvest regularly from the other plants.

Dividing Rhubarb:
Start by digging around the plant and loosen up the soil and roots (technically a rhizome).  As in any perennial division, the goal is to keep as much of the root system attached to the plant as possible, so start digging a good 12 inches from the plant.  You should be able to remove a good amount of soil from around the root mass and get a better look at what you'll be working with.
Rhubarb has a pretty impressive root system, especially if it hasn't been divided in a number of years.  Try to gently lift up from underneath the plant with a shovel or fork.  You will likely need to work around the plant, doing this on all sides(maybe even a couple of times) until the plant is free to be lifted out of its location.  Some natural divisions in the plant might become visible (or happen on their own) during this process.
The next step is to actually divide the rhubarb plant.  I prefer to use a sharp knife to make sure that my cuts are as precise and controlled as possible, but you could certainly use a spade or shovel, if you prefer.  Carefully cut the rhubarb plant between the buds (even though the roots might look hard, they are actually quite easy to cut through with a sharp knife).  Be extremely careful not to damage the delicate buds.  Each new plant should have at least 3 or 4 buds and at least several inches of  rhizome (they say as little as two or three inches will be successful, but I like to err on the side of caution and give each new plant more along the lines of  5 inches or more (and as you can see, with a plant that had not been divided in at least 8 years, there was plenty of root for each new plant).
This is a good time to examine each new division for signs of rot or disease.  If rot is present in small areas, it can usually be carved out quite easily, but if large sections of the crown or root is rotten, those parts should be discarded.  Once you are satisfied with the condition of the plants, set them in the shade or cover them with a wet towel to prevent the roots from drying out while you prepare the soil for transplanting.

Transplanting Rhubarb:
Rhubarb has a pretty basic list of needs: full sun, rich soil, regular water, and good drainage.  To prepare the soil, I worked in a lot of compost (rhubarb loves compost).  Now I keep the soil there pretty rich in organic matter to start with (I always let the old foliage go right back into the ground every year), but because I will only have the opportunity to work in organic matter this deeply every 5 or 6 years, I made sure that I really worked that compost in.

Ideally, rhubarb plants should be spaced out about 3-4 feet apart (keep in mind that mature rhubarb stalks are about a foot long, and often the leaves are nearly as long as the stalk).
Prepare a hole that is large enough to contain the entire root mass.  Add a couple of generous scoops of compost (have I mentioned that rhubarb loves compost?) and a lot of water.  Gently place the rhubarb division into the hole and start to fill in dirt and compost underneath and around the root mass.  The crown should sit just 1 or 2 inches below the surface of the soil, with the buds at the same ground level they were at prior to the division.
Once the plant is firmly (put not compacted) in place, give it another good drink of water.  The plant will need a lot of water in the first weeks to compensate for the disruption.  After a week or so of daily watering  gradually cut back, extending the time between watering until the plant is reestablished.  Pay close attention  on hot days and during dry spells, when plants might be more likely to experience some distress and need more frequent watering.

If you don't have room for all your new plants, be sure to keep the roots from drying out until they can be transplanted into the garden of a family member, friend, or neighbor (this year my brother Paul and my good friend Natalie were the lucky recipients of my extra rhubarb!).  If the plants will fit into a pot with some potting medium, that is ideal, but if that is not possible, place the roots in a shallow tub and keep the roots covered with a wet towel.  A few days out of the ground shouldn't hurt the plants, but you will want to get them transplanted as soon as possible.
In the season immediately following the division, you might notice that the rhubarb will not produce quite as vigorously as it usually does.  It is safe to harvest some of the rhubarb stalks the same year as a transplant, but do so sparingly as to not cause too much stress on the plants.  The rule of thumb with rhubarb is stop harvesting all together after the 4th of July, which gives the plant plenty of time to store up energy for the next season.  Also, if the rhubarb sends up a flower head, be sure to cut it off as close to the ground as possible - you don't want a new plant wasting any energy on flowering this year (and as a rule in general, saving rhubarb seeds isn't recommended as they do not produce true to type consistently).

By the time fall rolls around, the rhubarb should be happily reestablished and ready to face the winter, and by next season you'll have so much healthy, happy, and productive rhubarb, you'll need to expand your repertoire just to keep up!