Fielding Questions: Is rhubarb safe to eat after a frost, can I still prune a hydrangea this spring and pinching petunias

A: This rumor circulates whenever we have our common spring frosts. Rhubarb leaves contain oxalic acid, which can be a toxic substance if consumed in large enough quantities, which is why we don’t eat the leaves of rhubarb. When rhubarb plants are exposed to frost, some of the oxalic acid can travel from the leaves down into the stalks, but there are obvious clues when this happens.

So how do you know if oxalic acid has traveled downward, making the rhubarb stalks unsafe? Many research universities have studied this, and agree on the following: If stalks become limp after frost, don’t harvest them. Instead, pull and discard. If leaves show frost damage, such as blackened edges and water-soaked tissue, pull and discard the stalks, even if the stalks themselves are firm. If stalks and leaves appear normal after a frost, the rhubarb is safe to eat.

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If stalks or leaves are damaged, and you’ve pulled the injured stalks, all future regrowth is safe to eat. If in doubt or you’re hesitant, just don’t use it.

Rhubarb begins to grow in late April and early May, and because our region’s final frost can be mid-May or later, rhubarb plants have been getting frosted since pioneer days. Unless the stalks or leaves show injury as described, the rhubarb is fine.

Q: Should I prune off the last year’s brown flowers from my Quick Fire hydrangea bushes? I hesitate because of a very few white flowers on the old brown clusters. Is it too late this spring to do so? — Les Hazlett, Fargo.

A: Now is a fine time to prune off last year’s flower clusters on hydrangea, which are quite decorative when left intact over winter. The Quick Fire hydrangea cultivar belongs to the wonderfully adapted Hydrangea paniculata species, with pyramidal, cone-shaped flowers. Remove each dried flower cluster below the point of attachment, down to where you see a new green bud swelling along the twig.

Q: I read in a gardening magazine that it’s best to pinch off all the blossoms and buds of annual flowers, like petunias and marigolds, when you plant them outside. Is this true? It kills me to cut off the nice flowers. — Linda S., Alexandria, Minn.

A: The answer is yes, and no. Removing the flowers of bedding plants by pinching or cutting is meant to coax a young plant to put its energy into forming a larger plant structure with more branching that will give a larger floral display in the long run. You sacrifice a few small, early blossoms at planting time, but are rewarded with a better floral show all summer.

My mother taught me this many years ago, as one of the old-time secrets of better flower gardens. My wife, Mary, and I always cut back the flowers on the four-packs and six-packs of flowers that we plant, pinching off the blossoms, buds and sometimes the plant’s central growing point to encourage branching. This is effective especially on petunias, marigolds, salvia and others.

However, if you buy annual plants in large, individual pots, such as the common 4-inch diameter size, the plants are usually quite well-developed, well-branched and were probably already pinched back early in their growth by the greenhouse grower. Removing their blossoms has less impact, although you can if you like. These larger-potted annuals are great for instant color in outdoor containers.

If you have a gardening or lawn care question, email Don Kinzler at [email protected] All questions will be answered, and those with broad appeal may be published, so please include your name, city and state for appropriate advice.


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