Go into any nursery and try to find lavender and Russian sage; they will be in the perennial section. Check these plants online or in books, and most sources will tell you they are perennials. Lavender and Russian sage are subshrubs, a fancy name for a small shrub. Once you understand they are NOT perennials, they become easy to grow.
What is a Subshrub?
Subshrubs are short shrubs. They have a woody base that is similar to a tree – think single woody trunk reaching the ground, where roots develop. The term is used for a number of plants, like lavender, that tend to have a woody bottom with a more herbaceous-like top. Unlike most shrubs, the herbaceous top does not harden off before winter and true dormancy is never reached. Because of this, the top part is susceptible to frost damage.
Unlike perennials, subhrubs have a single stem and root system, making them difficult to divide. Attempts at division usually result in death of all sections of the plant. I’ll discuss propagation methods below.
Examples of Subshrubs
Here is a list of some of the more common subshrubs.
Deerwood (Lotus scoparius)
English Lavender (Lavandula angustifolia)
Heather (Calluna and Erica)
Lavender cottons (Santolina chamaecyparissus)
Oregano (Origanum vulgare, Origanum rotundifolium)
Rock Rose (Helianthemum alpestre)
Rosemary (Rosmarinus officinalis)
Russian Sage (Perovskia atriplicifolia)
Sage (Salvia officinalis, Salvia elegans)
Sunset Hyssop (Agastache rupestris)
Thyme (Thymus vulgaris)
Wormwood (Artemisia species)
Subshrubs in Cold Climates
In cold climates, the top herbaceous part can be killed off in winter. In my zone 5 garden the top is almost always killed to some extent. The amount of damage depends very much on how cold it gets and how much snow we have. With enough snow cover, the parts under snow can survive quite well.
This presents a problem for the gardener. In spring, you will have no idea how much of the stems have been damaged, and new buds will only form on undamaged stems. Buds also tend to develop more easily on younger wood. For these reasons you should not prune back the shrubs until you see the new growth.
Subshrubs in Warm Climates
In warm climates subshrubs tend to grow all year long and can be pruned any time. It is best to prune them after flowering.
High temperatures and too much humidity can make it difficult to grow some types. Check with local sources to see which varieties grow best in your climate.
Cutting Back Early Bloomers
Some subshrubs bloom earlier in the year; lavender for example. These can be cut back right after flowering by deadheading and shaping them a bit. Always leave several green leaves on each stem so they can continue to grow and develop buds.
Late bloomers should not be cut back.
It is important to prune your subshrub at least once a year to keep it compact and prevent it from getting too leggy.
Because gardeners in cold climates do not know which buds are killed it is best to wait until spring so that you can see the new buds growing. As soon as you see them, you can prune back to a point so that at least some green buds remain.
If you cut below the live buds, some subshrubs will form newer buds lower down, but many will just die.
Lavender tends to only make buds higher up on newer wood. Russian Sage seems to always have buds at ground level.
One of the main reasons people have trouble growing lavender is improper pruning.
Since subshrubs get woody at the bottom and new buds develop mostly higher up on the plant, they get taller each year and after a few years they start looking very leggy with most of the green showing above dead-looking stems. To look their best they need to be re-newed every 4-5 years.
One option is to remove them and buy new plants – but where is the fun in that?
A second option is to propagate them buy taking cuttings. Early summer cuttings usually work best. This is a standard process that can be used for all kinds of perennials and woody plants.
A third option is to propagate them by hilling. In spring, cover the old plant with soil so that the new buds are showing just above the soil (picture on the left). Leave them like this for the summer. By the following spring, each stem will have made new roots in the hilled-up soil. The whole plant can now be dug up, divided, and each rooted stem can be planted to produce a new plant. This method does take time, but is very easy and successful.
These plants can also be grown from seed, but seed will not come true. Each seedling will be different from the parents, and in most cases they will be inferior to the parents. It is best to use one of the other methods if you have named clones.