Warm is such a benign word. It's friendly, perhaps even cozy, comfortable, unintimidating. That is not the climate in which we grow :D Warm is 80 degrees on a sunny May day. Warm is fourth-of-July picnic with a breezy 87F. That is not the climate in which we grow.
Here, it is HOT. Mid-march when our ranunculus is flushing, we get 90 degrees. July is dangerous with 120F heat indexes. We are warned to stay inside! If you wait for 'warm' day to sow your zinnia seeds in summer, you'd have to wait the equivalent of 2 whole months (55 days) for temperatures to fall below 100F. Basically, autumn (or hotumn as we like to call it :p) you can sow them when it's only 95F and there's a shot at germination. But a freak freeze at halloween will kill them and then we'll have 80's through Christmas. Dramatic? Those are real temperatures on real dates, occurring in real time, with frequency, over the last 6 years of farming in Dallas, Texas. This outdated but helpful heat zone map shows that Texas gets 120-150 days every year with temps over 80F on average. In contrast, Oregon (same hardiness zone) gets just 14-30 days of temps over 80F on average. Big difference.
But WAIT! What about the 3 inch coating of ice we get every February or -2F wind chill for a whole week or 17 inches of snow (2010)? That's decidedly NOT hot. Yep, you're gonna have to farm through that.
If you live here, you already know all this. You know that tornadoes hit in May or Christmas, and straight line 80mph winds accompany spring; you know that the clay earth gets so dry and cracks so deep by August that you could stick your entire hand in the crevices.
Yet we farm flowers. And farm successfully. Here's how.
First, ignore hardiness zone. It's useless. Well, mostly. Hardiness zones in the U.S. tells you the average low temperature in winter. That's just one season out of four. And it's an average to boot. Oregon and Dallas are both zone 8, yet in May (Spring), Dallas temperatures are typically 20 degrees higher than Oregon. That makes a huge difference in the flowering cycle of hardy annuals. In real terms, it means that by May, it will be too hot for your hardy annuals to flower well. The plant has to be ready to flower in April for a quality cut harvest (April is also when we reach long-day lengths, a significant blooming trigger for many cold hardy annuals). Making this group of plants ready to bloom by April brings us to our second point:
Second, fall plant. This is perhaps the most important part. Hardy annuals need cool for roughly 2-3 months to establish roots and grow vegetatively. The only chance of cool we have is November-January and even that is not a given. Then, when we skip cool and get COLD in February. The plant needs to be established to handle 10F for a whole week. Again, the hardiness zone map of the U.S. is not particularly helpful. I recommend using the RHS hardiness scale. The Royal Horticultural Society lists actual temperature ranges for each type of flower plant. You'll have to translate from Celsius to Fahrenheit but you'll have a more accurate picture of the cold your plant can handle (see point four below). It can handle it if established. Planting 4-6 weeks before your average last frost date (March 15th here) is to plant in February. That's prime ice season. Go ahead and cross that out in your farming books. Nobody should be doing that here. Period. :)
Additionally, fall planting is necessary because of vernalization, which is prolonged exposure to cold that induces some plants to flower in the spring. Without the cold exposure, harvests are not of sufficient quality. Sometimes, long daylengths can substitute for cold exposure, but in our hot climate, long days arrive only with very hot temperatures, which signal the plant to shut down, not bloom. Fall planting is necessary for us so that some cut flowers receive enough cold to produce high quality blooms in the spring. I found this to be true specifically for delphinium, larkspur, foxglove, peonies, and even lisianthus.
Third, order plugs, it's a great climate hack. In late September we still had 100F degrees outside. So if I have a seeded tray of any cold hardy plant that needs to grow at 50-60 degrees for 8 weeks to fall plant, where am I going to put that tray? I have a walk-in cooler and I use it solely for seed growing under lights at 60 degrees in September but this means that I'm not harvesting and using that cooler for cut flower storage. It's a trade off. You could build two coolers, or if you're just starting out and don't have a cooler at all, plugs are an excellent hack. Even I order some plugs simply because my cooler isn't big enough (from Farmer Bailey, yay!) Plugs are grown in cool temperatures in a place NOT 100F degrees and arrive ready to be transplanted in that magical two week window of actual autumn that we get (the magical window is a fast moving target, anywhere from mid-october to mid-november. Good luck. haha!) This is even more useful for direct seeded crops. Seeds like larkspur require constant moisture for good germination and this can be difficult in a hot season climate where the soil dries out quickly. Plugs for the win.
Fourth, strategize your winter protection. In hot climates, plants often grow large and bushy and are unable to handle sudden cold weather. One trick here is to plant your transplants several weeks before we drop down to 10 hours of daylength. Even if warm weather encourages your plant to grow, the shorter day lengths will signal it to go slower. We want established, but small plants, heading into our freak weather events. A smaller plant is also easier to cover. Because our winter ice storms usually give us 2 to 3 inches of ice-coating, a normal low tunnel easily pancakes under that weight. (Oh! let's not forget our notorious straight-line-winds that simply fling off the entire low tunnel to begin with). My hack is to use left-over bulb crates upside down as structural support under the frost cloth. There's no pancaking and the sides are easier for me to secure. Or it's useful for creating a double layer of protection in extreme cold. Winter irrigation is also important in hot climates. Often we have a cold system arrive with no accompanying moisture. This means dropping from 80F to 17F with cold harsh winds blowing all the moisture right out of the soil. Plants often die from desiccation more than the cold itself. So water well before a cold front. Have a plan in place for irrigation if an ice-storm arrives and you're using plastic, not frost cloth, to cover your plants. The plastic can freeze to the weights on the outside of the tunnel but when the sun comes out it'll quickly bake your plants on the inside. Venting is crucial and a fast thaw is damaging to plants. If it's been a very cold night but a sunny day is coming, I'll uncover my plants in the morning before the sun arrives, even if temperatures are still a bit below freezing. The sun can heat up those tunnels faster than you can imagine (put a temperature sensor in one of them for enlightenment), having them vented early slows the thaw.
Fifth, know your soil. We farm in blackland prairie clay. With our deluge-drought cycles, it's very important that we mound up our rows for drainage in the deluge cycle. A few hours away, farmer friends have sand and they don't need to mound up. We mound a good 2-3 inches high. In HOT climates, it's even more important to know your soil because irrigation is a huge factor in crop quality. If you have sand you'll need a massive irrigation plan in a hot climate to keep your plants hydrated. In our clay soil, we don't have to worry about irrigation in the deluge cycles, but because the soil is SOO warm, we cannot use landscape fabric, especially in the wetter spring months. Hot + moist soil is a dangerous recipe. We have steamed more root systems to death than we can count (one sad bupleurum crop comes to mind). Deal with the weeds another way and ditch the landscape fabric.
Sixth, shade cloth. YEAR ROUND if you have a high tunnel. Seriously. If you don't want your ranunculus to start shutting down during a sunny 80F Christmas week (in which your tunnel temp will be 95F fully vented), you need shade cloth. It happens every year. If you don't have a tunnel, the shade cloth will be necessary during the summer months. Actual air temperatures of 109F for weeks (before factoring in the heat index) means your plants will struggle. Even stalwart zinnias. You could declare summer your off-season like we do and rest, or you could be a badass, get shade cloth, and keep farming. Your choice.
Seventh, invest in natives and perennials. This is an excellent strategy no matter your climate. Native plants have ecological resilience. I grow my lisianthus in the field. It's native to our blackland prairies and is used to the crazy temperature and moisture swings. This makes it pretty easy for me to grow. The same is true with perennials. They handle temperature swings much better, with less irrigation and winter protection needed than many sophisticated cut flower annuals. It's a smart strategy for successful flower farming. I have several other blog posts on great perennials and natives for our region.
Eight, budget. There's only so much you can do to mitigate the weather. Be super smart about the plants you grow. Consult culture sheets (so easy to find online) on the varieties you grow. Then, consider the historical weather data for your area to find the dates when your climate will likely give you the conditions your plant needs to thrive. If the cut flower you want to grow demands conditions that are not naturally occurring in your climate, realize that it will cost you money to create those conditions. A clear budget and clear climate data should inform your crop decisions. On some level, my climate does not give me ideal growing conditions every single moment. There's always a moment (or several) during a crop's life cycle when I am creating an ideal condition because it does not exist naturally. Factor in the cost of those moments. They will occur. Count on it.
Lastly, I know my own growing climate. I don't know yours. It's probably different in some crucial way. So keep your own data records and use science (yay, science!!) to grow well. Join the ASCFG which is a wealth of science at your fingertips. If I can do it, so can you! :)