Should Herbs Be Replaced Every Few Years?
Answered by: Conrad Richter
Question from: Cynthia van Hazinga
Posted on: April 22, 2004

I am writing a column "Ask The Experts" for the magazine Gardening and Deck Design, published by Woman’s Day magazine.

One of the questions that’s come to us is this:

"I’ve heard that herb plants should be pulled up and replaced every few years for maximum flavor. Is there any truth to this rumor?"

Personally, I never heard such, thought soil and climate might have more to do with flavor? Or is it timely picking? Many herbs we can grow are perennial anyway.

I would truly appreciate your expert opinion!

The short answer is yes: it is true replacing herbs every few years can be beneficial. The longer answer is yes and no: replacing herbs works for many herbs, but not necessarily all.

Actually, the greater issue is the health of the plant. Perennial herbs such as sage, thyme, lavender, mint and oregano spread by sending up more stems. Eventually the stems become so dense at the centre of the plant that they become weak and more prone to contracting diseases. Slower and weaker growth retards the development of oil glands which are responsible for flavour and aroma of many popular herbs. Diseases can interfere with the development of the oil glands and add their own unpleasant notes to the flavour also.

When plants are overcrowded they can be divided. Using a spade or shovel the clump can be dug up and split into two or more pieces each of which can be planted to get new plants. This process of dividing herbs does wonders for perking up tired old herbs. Dividing is an opportunity to add compost or well-rooted manure to boost the soil fertility too. Divided herbs will survive winter cold better than old, crowded herbs.

Because people usually don’t have the extra room needed to plant divisions, and because perennial herbs often become infested with weeds, the strategy of replacing perennial herbs every 3-5 years is a simple way to keep the herb garden vigourous, clean and flavourful.

But not every herb benefits from division. Slower growing woody herbs such as rosemary, bay and myrtle can be kept healthy and vigourous for years with just normal pruning, repotting and feeding. Some herbs such as chives are susceptible to overcrowding, but the effect is not much of a problem even after years of growth.

There is a little understood but very real phenomenon of physiological aging in plants. With some medicinal plants especially we have noticed that active constituents in leaves can drop in older plants. Years ago we did a study on feverfew, a medicinal herb for migraine, and we found that second year plants always had far less parthenolide than first year plants. So, although feverfew is a perennial, farmers are better off replanting it every year. I don’t think that feverfew is unique in this regard; I think many culinary herbs age physiologically also, but the aging effect is not as dramatic as it is in feverfew.

What to do if you are unsure if a plant should be replaced or not? If each year the plant is sending up more and more stems at the centre of the plant or patch and the stems seem to be getting crowded and weaker, then you probably should replace the plant.

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