B. Rosie Lerner
Extension Consumer Horticulturist
Rhubarb Flowers Blooming or Bolting?
We humans can be so difficult to please. If plants flower when we want
them to, we call it blooming. But if plants flowers when we don't want
them to, we call it bolting. Flowering is an undesirable trait when growing
rhubarb; therefore, bolting describes the event.
Gardeners frequently ask why their rhubarb is bolting. Well, if you think
of it from the plant's perspective, it is just a part of the plant's natural
life cycle. Flowering is part of the reproductive phase that leads to
the production of fruit and seed.
But from the gardener's perspective, the production of flowers, fruit
and seed wastes the plant's resources, which could be better spent on
producing edible stalks or storing carbohydrates to use for the coming
season. And if allowed to mature seed, the resulting seedling offspring
are often less desirable than the mother plant, which we paid good money
to buy as a named cultivar. In fact, seedling offspring are often more
likely to bolt than some of the more modern hybrid cultivars and can be
vigorous enough or just numerous enough to take over the planting.
It does appear that some rhubarb plants are more prone to flowering than
others. Old-fashioned varieties, such as Victoria and MacDonald, are heavy
seed stalk producers. Canada Red and Valentine are less likely to bolt.
Plant maturity is also a factor, with more mature plants being more likely
to bolt than youngsters. Dividing the crowns every 4-5 years should help
rejuvenate the planting. Applying moderate amounts of fertilizer, either
a balanced fertilizer, such as 12-12-12, or well-composted manure each
spring should also discourage bolting.
Weather no doubt has a role to play as well. Rhubarb is a cool season
perennial that can remain productive for 8-15 years, if given proper care.
Plant stress, such as temperatures above 90 F, prolonged drought during
hot weather, poor nutrition, etc., may also promote bolting.
The bottom line is that rhubarb may bolt for a variety -- and likely a
combination -- of several factors. Many gardeners may not know what cultivar
they have, and there's not much we can do about the weather. So, if your
rhubarb should happen to bolt, remove the flowering stalks, just as soon
as they are visible, to which the plant will likely respond by sending
up another. If you keep at it, soon the plant will return to the desired
priority for foliage production.
Another question that sometimes comes up is whether the flowering makes
the leaf stalks poisonous. The answer is no, the leaf stalks remain edible,
regardless of whether flower stalks are present. However, the leafy blade
portion is always poisonous due to a high level of oxalic acid.
There are a number of species of rhubarb relatives, which are grown as
ornamentals, especially in Europe. So if you happen to find rhubarb flowers
attractive, you can let them bloom, but cut the stalks as soon as they
fade, to prevent seed from maturing.