Hardy Annuals and the RHS Rating System
As far as hardy annuals go, not all hardy annuals are created equal. I think that’s one of the mistakes I made as a newcomer to flower farmer, and one that I still see being made by others.
I didn’t know for example that there was a major difference in hardiness between calendula, Icelandic poppies and bells of Ireland (do you know off the top of your head which is the hardiest and which is the most tender?). Too many times I see them all being lumped together as just the hardy annuals that can be planted for spring blooms - but without paying attention to their own individual hardiness and needs.
There are a lot of factors that can contribute to hardy annuals’ survival and success, including how freely the soil drains, what type of microclimate they are located in, precipitation, and even variety of the species you’re growing, but by far the most important factor is their hardiness. And although there are some resources out there regarding the hardiness of these annuals, I think the Royal Horticultural Society (hereafter referred to as the RHS) rating system has a better way of describing the overall hardiness of hardy annuals.
(As a side note, of COURSE the Brits have a great rating system for hardiness. Although it’s helpful to point out that most of Great Britain is a very mild climate due to the warm air from the Gulf Stream and the buffering effect of the ocean surrounding them, with the most dire hardiness ating being a balmy USDA Zone 7a and pointedly talking about the poor saps of “continental Europe” that have to suffer the equivalent of USDA Zone 6a or colder.)
You’ll notice that the rating system runs from H2 to H7, and the higher the rating, the more hardy the species is. This is of course the opposite of the USDA hardiness zone system (where the lower the zone the more hardy the species is) but I’ve included the USDA hardiness zone equivalent and the equivalent temperatures so you an apply it to your situation.
It’s EXTREMELY important that you recognize what kind of temperatures you have so that you can evaluate the flowers you will be growing and check for the hardiness. For example, we tried to grow strawflowers for a fall planting one year and did everything correct - only to watch them all die off completely, wasting an entire bed for spring that could have been filled with a more productive crop. If I had known that strawflowers had an H2 rating - meaning that they are only hardy down to 34 F and that only happens in a Zone 10b (where they never get a hard freeze) I wouldn’t have wasted my time, opting instead for an early spring planting instead.
In essence, by knowing your hardiness of your hardy annuals, you’re going to find life as a flower farmer to be MUCH easier.
H2 Rating - 34 to 41° F (1 to 5 C)
USDA EQUIVALENT: Zone 10b
IN A NUTSHELL: Can survive low temperatures, but not a hard frost
SPECIES: Freesia, Strawflower, Didiscus
Overall, these annuals are the most delicate of all the hardy annuals - one might call them half-hardy since they are really only hardy into the few temperatures just above freezing. And it’s important to NOT let them freeze. While they can endure some frost damage to their above-ground foliage, if the ground freezes they are toast.
With the only guaranteed survival being in a Zone 10b winter, we find it best to only plant these out on the last frost date - and even still, we will stand by with frost cloth, ready to cover if the temperatures dip.
H3 Rating - 23 to 34° F (-5 to 1 C)
USDA EQUIVALENT: Zone 10a-9b
IN A NUTSHELL: Can survive with some protection (i.e. hoop house or polytunnel)
SPECIES: Nasturtium, Cerinthe, Love-in-a-mist (Nigella hispanica/papillosa), Rudbeckia hirta (annual varieties), Sweet Peas
Some surprises here - didn’t know that there was a difference in hardiness between the two types of Nigella/Love-in-a-mist (Nigella damascena is much hardier with an H5 rating/USDA 7b-8a survival rate).
Also surprising that Cerinthe is a lot less hardy than I had thought. Now I don’t feel so bad about the fact I haven’t been able to overwinter it. Again, the beauty of the RHS hardiness system to put things into perspective.
H4 Rating - 14 to 23° F (-10 to -5 C)
USDA EQUIVALENT: Zone 9a-8b
IN A NUTSHELL: Hardy in an a mild winter winter, may have some die back of the foliage in colder environments but root systems should survive
SPECIES: Scabiosa (Scabiosa atropurpurea), Bells of Ireland, Statice (Limonium sinuatum), Phacelia, Dill, Stock, Ranunculus, Anemone (Anemone coronaria)
We here in our zone 7b are a bit on the drier, milder side as far as 7b’s go, which means that we have an easier time overwintering certain varieties, especially if we have a mild winter. Scabiosa in particular, does a great job overwintering here in the more sheltered parts of our property (it reseeds readily and we find plenty of volunteers) but it isn’t a 100% guarantee that they will survive.
We also have a hit-or-miss success rate with statice. It overwinters just fine sometimes, but sometimes it doesn’t - three years ago we had a giant crop of statice that was doing great until we had a late freak freeze that killed them off in March. Definitely one of those crops that needs a cover for us here.
And surprisingly enough ranunculus and anemones are much more hardy than I think most people give them credit for. Of course you’re going to need shelter for them, and ideally you would want to grow them in a tunnel or hoop house - but it’s good to know that they’re relatively hardy and with a bit of protection they’ll be just fine.
H5 Rating - 5 to 14° F (-15 to -10 C)
USDA EQUIVALENT: Zone 8a-7b
IN A NUTSHELL: Hardy down to the single digits
SPECIES: Clary Sage (Salvia viridis), Calendula, Love-in-a-Mist (Nigella damascena), Bupleurum, Toadflax (Linaria), Corn Cockle (Agrostemma), Foxglove (Digitalis), Shirley/Opium/Corn Poppy (Papaver rhoeas, Papaver somniferum), Annual Baby’s Breath (Gypsophila elegans), Chinese Forget-me-nots (Cynoglossum),Viola and Pansies
Here’s where we start seeing more of the usual hardy annuals with bupleurum, nigella, calendula, agrostemma, digitalis, and corn poppies showing up. It’s also where we start seeing temperatures down into the single digits, which is more of where we see winter temperatures in the continental U.S.
A lot of the overwintering also depends on how well-draining your soil is. We can grow clary sage very well here because our winters are on the dry side, where if we were getting precipitation the same way as we would in the Midwest or Northeast or Northwest it might not be the same rate of survival.
Note that even these cold-hardy annuals can still succumb to extreme temperatures, which leads us to the next group…
H6 Rating - -4 to 5° F (-20 to -15 C)
USDA EQUIVALENT: Zone 7a-6b
IN A NUTSHELL: Hardy down to single digits
SPECIES: Bachelor buttons, Queen Anne’s Lace (Daucus), False Queen Anne’s Lace (Ammi majus), Green Mist (Ammi visnaga), Phlox (annual), Larkspur, Brown Eyed Susan (Rudbeckia triloba), Black Eyed Susan (Rudbeckia fulgida), Cress, Clarkia (Godetia)
The H6 group are another large group of the hardy annuals that are pretty tough, surviving down to zero and below. And again, although you will do better by providing them some shelter from the wind and cold via hoop house/tunnels/floating row cover and a microclimate that provides some shelter, they are pretty hardy overall.
Note that the major fillers of spring - Daucus, Ammi Majus, Ammi visnaga are all of around the same hardiness that (despite preconceived notions) will all probably be just fine as small seedlings. The only other umbellifer filler that is missing from this group is Orlaya - which actually is even hardier with an H7 rating (see the next section).
Also surprising is the fact that Phlox drummondii - aka annual phlox with the well known varieties of ‘Caramel Cherry’ ‘Creme Brulee’ ‘Phlox of Sheep’ and ‘Blushing Bride’ is hardy down to the single digits - who knew?
H7 Rating - -4<° F (-20< C)
USDA EQUIVALENT: Zone 5b or less
IN A NUTSHELL: Bulletproof - will reliably survive the coldest temperatures, even subzero temps
SPECIES: Icelandic Poppies, Orlaya grandiflora, Snapdragons, Dianthus
Alright, you have the hardiest of the hardy annuals here as far as cold goes. Icelandic poppies don’t surprise me at all given that they live in some of the harshest, worst environments in the world and have survived pretty much anything you can throw at them, but I am surprised by the orlaya to be honest.
For something as delicate as orlaya, you would think that they would be a bit more fragile when it comes to the cold - but interestingly enough they are hardy enough to where they can take some of the worst weather overall and come out just fine. Color me surprised, I had no idea they were such a tough little plant.
This is an interesting group of flowers, and I can’t help but notice that you would have an easy combination of materials if these were grown together - Icelandic poppies as the focal, orlaya as the filler, snapdragons as a vertical element and dianthus as a focal/secondary flower as well. You could very easily run a flower farm off just those four alone!
Some Details on hardiness ratings
Add a hardiness zone if you have a poly tunnel or hoop house (that provides protection from wind and adds an insulative bubble of air that further buffers temperatures).
Add a hardiness zone if you cover your crops with floating row cover (i.e. Agribon). And yes, the hoop house and floating row cover stack to provide further hardiness and protection (giving you in essence 2 zones more hardiness)
For floating row cover, this protection only works if you keep the fabric up and off the plants. If the plants touch the fabric, that means they’re going to be exposed to the outside temperatures (similar to if you are in your house but you have a hand touching a glass window that hand is going to be much colder than the rest of your body in the house). You will require hoops or some sort of support that will keep the row cover up and above the plants, forming an insulative air bubble that will give you additional temperature buffering.
Note also that unheated poly tunnels/hoop houses and plastic low tunnels can provide a very nice and warm environment but this can encourage your hardy annuals to get lazy and start producing lovely tender succulent and soft growth. This is fine if your temperatures stay relatively mild - but if you get a polar vortex and temperatures drop considerably, your happy and healthy seedlings can die quickly if you don’t offer them additional protection. Place floating row covers and blankets on your plants while the temperatures are low, even offering supplemental heat as needed to get them through the cold temperatures.
Floating row cover offers protection down to 26 F for AG-30, and 28 F for AG-19, so it’s not going to prevent your plants from freezing BUT it offers them a bit more protection from the drying effects of wind and the damage to foliage that can occur through dessication via freezing (instead trapping the humidity inside the floating row cover’s interior, allowing for more protection to the plants) so it may help your seedlings not only survive but grow more readily when the weather gets better.
We use floating row cover only - we don’t have the need for a large polytunnel/hoop house, and low tunnels are far too high maintenance (since you have to be constantly opening and closing them) and find that row covers are actually far easier. They are self-ventilating (no running out to open the tunnel if its sunny) and they also allow for the plants to stay relatively cold-hardy. This means that if we do have a sudden dip in temperatures (which seems to be more and more consistently occurring in the past five years) the plants have some defenses against the cold temperatures. And when needed, we can throw an additional layer or two of floating row cover to add additional protection against the cold (just as in fashion and winter in the Midwest, layering is EVERYTHING).
A heavy layer of snow (which is very insulating and protects plants) that lasts throughout winter is actually means the hardiness rating is going to be more consistent interestingly enough. A cold and dry winter is going to be less consistent (since you’re dealing with increased cold due to a lack of insulation in the form of snow around the plants and you will subsequently see more cold damage to the foliage and structure of the plant) That’s where it can get strange - a Zone 3a winter with heavy snowfall can be much more successful overwintering hardy annuals than a Zone 7a with little snowfall.
The exception to the snow rule are a few odd ducks out of the hardy annuals that actually prefer it on the dry side. Clary sage is one of those, as is annual Rudbeckia hirta - we are very successful at overwintering the fancy rudbeckia like ‘Sahara’ and ‘Cherokee Sunset’. Both of these absolutely HATE wet soil and will rot easily if the soil gets waterlogged, and with as little precipitation as we receive here that is not a problem.
The hardiness rating is more likely to apply to small seedlings - we’re talking 1-2 true leaves - than it will larger more developed seedlings. The reason for this is that smaller seedlings are more plastic and adaptable when it comes to cold and freezes - larger, more tender and succulent leaf growth and roots means more surface area for damage from frost and cold, whereas small leaves and fine roots are more likely to survive freezes and cold.
