Late arrivals, Frost and Hardening off plants.

Late Arrivals!

As I'm writing this it's mid April and the first of our swallows have returned, flying in and around our potting shed, it's wonderful to hear their shrill calls once again. 

Most of the plants on the nursery have sprung into growth. Soft green shoots are emerging left, right and centre. But some of our pots appear to be empty, devoid of greenery or any obvious signs of life! We have entire batches of plants without a single green leaf showing. Maybe these so called “perennials” have succumbed to the harsh winter or been eaten by vermin?  Every year it’s the same; it’s tempting to just get a wheelbarrow and start throwing them away!!  

But if we're too hasty we may be adding perfectly good plants to the compost heap. 

Plants are a little like people. Our bodies have their own natural rhythm. Some of us wake early, while a few of us are night owls and prefer a later start to the day. Similarly plants have their own natural rhythm. Even plants from the same Genus break their winter dormancy and start into growth at different times. This can trick us into thinking that plants have failed, when in fact they are still lying dormant.  

As we inspect our plants on the nursery, we can see that THALICTRUM flavum subsp. glaucum, a native of Spain, Portugal and North America where it grows in damp meadowland and by streams, is already growing enthusiastically. However the Thalictrum delavyii types, native to south west China, where in Yunnan, they grow in alpine meadows and scrub, have only just started to grow. Their delicate new shoots and leaves are difficult to see against the deep brown colour of the compost. And in a garden these shoots could easily fall foul of the gardeners hoe. Worse still these late arrivals may have already been dug out in disgust because a hasty gardener had assumed that they'd perished.  

EUTROCHIUM maculatum Atropurpureum Group (Eupatorium) which hails from North America, could be likened to a youthful teenager. It’s normal for these Eutrochium to appear late on the scene but when they arrive they have an energy which others just can’t keep up with. Their growth rate quickly outpacing plants which emerged weeks earlier. 


So what triggers a plant to break its winter dormancy* and burst into growth?   

Winter dormancy, which is by definition ‘a temporary suspension of visible growth of any plant structure containing a meristem’ has an important part to play in the survival of our herbaceous perennials. During autumn as the days shorten and the nights lengthen, stem and leaf growth slows or stops. The shortening day length triggers some plants to develop underground storage organs i.e a crown, tuber, bulb or corm. Finally leaf and stem senescence takes place. As the plant dies down for winter the buds either below ground or within the crown of the plant lie dormant, protected from freezing temperatures and other adverse soil or weather conditions. 

With the onset of spring, increasing temperature (above freezing up to around 15°C) and day length are key factors in breaking winter dormancy (*ecodormancy) , the only other environmental requirement is sufficient rainfall. 



But danger can now lie ahead. A sudden and prolonged early warm spell can trigger some plants into growth prematurely. And as we nurserymen know to our cost, a really hard spring frost can damage soft green shoots, causing them to appear scorched or in the case of Dahlias, blackened. Generally this is not fatal. Plants will send up new shoots. But if they are continually knocked back by the frost they will take a lot longer to get growing. Sometimes it may be early summer before a frost damaged plant fully recovers. You may recall that the spring of 2021 was particularly cruel, with multiple hard frosts! Again it was tempting to give up on some plants which struggled for weeks before finally recovering. To be a gardener you need patience !  

Of course there are things we can do to help protect our plants from frost:  covering with horticultural fleece; mulching around the crowns of plants;  moving pots into a protected environment eg. a cold frame. If you garden in an area which is particularly prone to frost or garden in a frost pocket then selecting tough, hardy plants to grow is of paramount importance if you want your garden to succeed. If you are unsure about Hardiness ratings then you will find the RHS guide in our previous article about Tender Perennials





According to the Department of Energy & Climate Change here in the UK the thermal growing season officially begins “at the start of a period of five successive days where the daily-average temperature is greater than 5.0°C”  and “ends on the day before a period of five successive days when the daily average temperature is less than 5.0°C” Interestingly 

“The earliest start of the thermal growing season was in 2002 when it began on 13 January. The longest growing season in the 240-year series was 330 days, in 2000. The shortest growing season was 181 days in 1782 and 1859. In 2012 the thermal growing season was 282 days, up from 279 days in 2011 and above the 1961-1990 average of 252 days.” And if you want to know more then here’s the link to the data 


Hardening Plants Off

If you have chosen to overwinter some of your young plants in a glasshouse, cold frame or polythene tunnel then at some point in the spring these plants are going to have to be hardened off and moved outdoors prior to planting them out.

Before you move any plants though it is wise to remember the old adage which says "One swallow doesn't make a summer". Just because it's relatively warm outside doesn't mean it will remain warm. Frost can still strike even in the middle of May. 

It takes around three weeks for a plant to fully adapt to the cooler, less humid and frequently a lot windier conditions outside. And it's best to allow plants to adjust in small stages. If you have a cold frame you could start by lifting its roof up throughout the day and then closing it again in the evening. Or move the plants outside and then put them back in overnight. Next leave the cold frame roof up all of the time,  unless there is a risk of hard frost. Finally in the third week move your plants to a moderately sheltered location outside for a further week before planting them out. You may still need to cover or fleece plants if extreme frosts are forecast. But by this point your plants should have built up enough resilience to cope with all but the worst weather conditions.  

Tender perennials are best left inside in a protected environment, until the last of the spring frosts have passed. 

 * There is a great deal we still don’t know about the breaking of winter dormancy in herbaceous perennials. Due to climate change there’s lots of research going on. The study of winter dormancy in fruit trees has found that inherited genes play a part in determining how many weeks or hours of cold must be endured before a tree can break winter dormancy - or period of endodormancy.

Endodormancy inhibits premature sprouting of shoots during winter. The original global origin (latitude) of the plant species may determine how many days or hours of cold must be endured before endormancy ends. Once Endodormancy has been broken a phase of ecodormancy starts. This dormancy can be broken by external environmental factors - a combination of  increased warmth, day length, rain. This research has commercial significance in the production and future breeding of both food and ornamental crops. Currently only limited research has been carried out on herbaceous perennials. 

Many plants need a set cold period prior to growth to enable good flowering and or fruit production.