Half hardy herbaceous perennials

GeraniumLast spring Judy and I were invited to visit a newly created garden in Warwickshire that had been planted up the previous autumn, and we were shocked to see many apparently hardy plants had been killed by last winter’s frosts. Obviously, when we design a planting plan, we are careful to select plants that are suitably hardy. So what caused these plants to succumb last winter? (It wasn’t one of our designs, by the way.) And if we plant new herbaceous perennials now, will they suffer the same fate this winter?

It’s the formation of ice crystals, which rupture the plant’s cell walls, that causes frost damage. Some plants are not at all hardy, whilst others are hardy to varying extents. A plant’s ability to withstand freezing temperatures derives from its physiology and acclimatization. Hardy plants use certain “tricks” to help withstand frost, including:

  • strengthened cell walls;
  • desiccation: they lower the water content in the cells during the winter (ice in the spaces between cells is much less of a problem for a plant);
  • natural anti-freeze: they manufacture chemicals inside their cells that inhibit freezing.

Importantly, hardy plants generally deploy these survival techniques as a reaction to shortened daylight hours and lower temperatures. This process takes time (which is why locally grown stock is hardier than plants imported from somewhere warmer). New growth is more susceptible as the cell walls are thinner and, by implication, growth means warmth and so less time for acclimatization.

fresh from Italy?

Last winter was characterized by some very severe frosts – down to -15ºC here in south Wiltshire – and the frosts came in late November, which is early for such severe frost. It was the combination of (early) timing, and severity, which was to blame for the damage. The cold weather also lasted for a considerable time – several weeks – which allowed the frost to penetrate deep into the ground, reaching roots that would normally remain frost-free.

And what about soil water content? Well, it is certainly true that wet soil will lose heat much more quickly than dry soil, and so will allow a frost to penetrate deeply.

OK, enough science – is it too late to plant? No, it isn’t. In fact, as long as the plants are sufficiently mature (not just new growth) and have been properly acclimatized, there is no scientific reason why you shouldn’t be able to plant all year round (as long as the ground isn’t frozen solid!). As they start to go into dormancy, plants are less likely to suffer from the “shock” of transplanting, and there’s still plenty of warmth in the ground at the moment for the roots to get established. Another great advantage is that autumn transplants are much less likely to suffer from drought. Of course, it does all still rely on adequate soil preparation, but that’s another story.

Frederick Warne / Observer The Observer's Book of Garden Flowers (Observer's Pocket)
Book (Frederick Warne / Observer)

Pt. 2 - Sheet Mulching around Perennials

2003-06-26 17:59:09 by pro

Sheet mulching around your perennials will require more work. Keep mulch 6 to 12 inches away from woody perennials and several inches from herbaceous. Weeds growing close to their root crowns will need to be hand weeded if possible.
For weeds you cannot kill that way, carefully wipe their leaves with Roundup. Attach sponges to the tips of spring-loaded meat tongs with rubber bands, dip them in Roundup and wipe both sides of the leaf.
Weed the area near the perennial crowns first then sheet mulch keeping the sheet away from them and thin out the mulch as it gets close to the woody stem

Despite soft wood cuttings these are hard wood +

2004-11-11 13:59:30 by AnitaMoPlants

Woody herbaceous plants which take a lot longer to send out roots than many true softwood herbaceous perennials .
The temperature and amount of light is going to have a large impact on how well your propagation goes.
You will do best with bottom heat and good bright light.
If you don't have a heating mat then a cold frame with a gravel bed facing south west would be best.
You can increase and dispell accumulated daytime heat by placing clear jugs of water ( empty milk jugs ) near your cold frame and or place your coldframe near a rock wall.
If you don't have a cold frame , which is pretty inexpensive to make, then you are going to have a rough go of it doing herbaceous cuttings during these cold months.

Have to change the struture

2007-03-27 11:57:13 by bockman

You don't need fertilizer, you need vast amounts of organic material. Very few cultivated herbaceous perennials and woody plants enjoy heavy clay as a growing medium, at best some will tolerate it.
Top dressing with things like good compost, composted cow manure, stable tailings, shredded leaves, thin layers of green grass clippings, pine needles, etc. will start the process of good soil structure as all that material slowly leaches downwards.

Powdery Mildew?

2004-06-30 12:00:01 by pro

Glad you didn't put it there! But, seriously, it's pretty common for Dahlia's to get PM. They won't die from it. Dahlias are herbaceous perennials so they die back naturally every year. Next year you'll have fresh, new, clean foliage (for a while) and you can try to prevent it from spreading before the leaves get yucky.
"Cultural Practices: Shade and moderate temperatures favor most powdery mildews. Locate plants in sunny areas as much as possible, provide good air circulation, and avoid excess fertilizer. A good alternative is to use slow-release fertilizer

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