Which are the Hardiest Cut Flowers to Grow

The arrival of meteorological spring has brought with it glorious clear blue skies filled with sunshine.

During the day although temperatures are cold, under the rays of the sun it feels pleasantly warm. But of course these clear nights also bring with them a continuous period of cold and frosty weather.

Night after night, I find myself meticulously covering my young plants and seedlings with frost protection, desperately willing them to survive.

But do I need to be covering them all? Which annuals actually need protection, and which ones will manage just fine on their own.

My Cut Flower Garden in July

You will find so much contradictory information regarding cool flowers and hardy annuals, when they can be planted and how tolerant they are of winter cold.

These contradictions do not mean that one grower is wrong, and one is right, it simply means that they have had different growing experiences within their own plot.

An exposed site in the same growing zone as a more sheltered spot, or dry versus wet growing conditions or even growers that let plants rough it versus growers that offer overnight protection, can totally alter the success of over wintered crops. Not only that but each winter can vary in its severity, meaning that one year all your crops may survive with very little intervention, and others may be a lot of hard work for little reward.

In the UK The Royal Horticultural Society has created a plant hardiness rating table, which is a great aid if you are just starting to grow. I know when I started, I thought that all hardy annuals were equally tolerant of cold, so I treated them all as if they were totally hardy to frost and cold, and the result was a lot of dead frosted plants, that would not recover.

 I also have my own contradictory results even in comparison to the RHS ratings I know I myself have lost Ami Majus, to continued frosts, despite its rating of surviving temperatures as low as minus 20, but of course my site is very exposed, and I had not made any attempt to offer any form of crop protection, but this error was an invaluable lesson for me.

Unfortunately for me a lot of my garden is simply too wet for over wintering annuals, so I am lucky to have an area in my poly tunnel for this. My plan next year is to make better use of my two low tunnels and raised bed, to get some cold tolerant annuals planted outside in the autumn for flowering the following spring.

I can use my previous experiences to help me decide which plants are potentially suitable for overwintering outside, and of course the RHS plant hardiness ratings will also be helpful for this.

If you are not familiar with the RHS hardiness ratings, they provide the absolute minimum winter temperature range, and you can find them listed for all plants with an RHS listing.

There are 9 categories with the least tolerant of cold being H1a needing temperatures of above 15 Celsius, these are tropical plants. Through to category H7 which are very hardy tolerating temperatures of minus 20 Celsius and below.

So I thought I would take a look at some of the varieties I grow and see which category they actually are listed as.

During this research I was quite surprised just how hardy some of these plants could be if given the right spot and favorable conditions.

I am going to skip H1a, H1b, and H1c as all of these need temperatures or 5 Celsius or above, and winters here are never consistently above that, in the US these would represent zones 11 TO 13.

Antirrhinum (Snapdragon) 

We are going to start with the least hardy categories and work our way up to the hardiest, beginning with H2 which is the equivalent to a zone 10b in the US. Tolerant of low temperatures 1 to 5 Celsius, but will not survive being frozen.

Except in frost free inner city areas or coastal extremities requires glasshouse conditions in winter, but can be grown outdoors once risk of frost is over. Or if crop protection is provided. I was really surprised by the plants in this category which include:

·       Amaranthus caudatus (Love lies bleeding)

·       Helichrysum bracteatum (Strawflower)

·       Tagetes erecta (African marigold)

·       Zinnia

·       Antirrhinum (Snapdragon)

·       Ageratum (floss flower)

Latheyrus (Sweet pea)

H3 Half hardy minus 5 to 1 Celsius, hardy in coastal mild areas except in hard winters and at risk from sudden early frosts.

May be hardy elsewhere with wall shelter or good micro climate. Can survive with artificial winter protection. Includes varieties such as:

·       Cosmos bipinnatus

·       Nigella damascene (Love in a mist)

·       Rudbeckia hirta

·       Latheyrus (Sweet pea)

·       Omphalodes linfolia

I would never of considered Nigella to be comparable to Cosmos and am not convinced it belongs here in this category, as I have always overwintered it with no protection, so now I am worried about all the Nigella I have not been protecting.

Antheum graveolens ( Florists Dill)

H4 minus 0 to minus 5 Celsius, Hardy throughout most of the UK apart from inland valleys, at altitude & central northerly locations.

May suffer foliage damage and stem die back in harsh winters in cold gardens. Plants in pots are more vulnerable, includes:

·       Matthiola incana (Column Stock)

·       Phacelia Tanacetifolia

·       Antheum graveolens (Dill)

·       Limonium sinuatum (Statice)

·       Moluccella laevis (Bells of Ireland)

·       Lavatera timestris

Calendula

H5 Hardy through most of the UK even in severe winters.

Minus 15 to minus 10 Celsius, but may not withstand open or exposed sites or central/northerly locations. Many evergreens suffer foliage damage and plants in pots will be at increased risk.

·       Calendula

·       Agrostemma githago (corncockle)

·       Bupleurum

·       Scabiosa atropupurea

Ami Majus

·       H6 now we have reached those super hardy varieties, minus 20 to minus 15 Celsius, Hardy throughout the UK and northern Europe. Many plants in containers will be damaged unless protection is given.

·       Ami Majus

·       Delphinium consolida (larkspur)

·       Centaurea cyanus (cornflower)

·       Godetia grandiflora

·       Phlox Drummondii

Centaurea cyanus (cornflower)

The final category is H7, minus 20 Celsius, hardy in the severest European continental climates including exposed upland locations in the UK. I only found Daucus for this group although I am sure there are lots more.

The important thing to remember is that if you have started any seeds inside, and your thinking about planting them out before your last frost date, you must acclimatise the plants to the outside growing conditions, to avoid transplant shock. And if you think there is any chance that your plants will not tolerate a frost, offer them some form of protection.