One of the surviving-summer strategies I’m using heavily in my zone 3 gardens is choosing heat-tolerant plants. Heat tolerance is an important thing to select for, but it’s not as obvious as it first seems. For instance, I don’t mean drought tolerant. Most vegetables need regular water to grow any bits worth eating, no matter how hot they like it. Drought tolerance is easy to predict from the look of a plant. Silver or grey colouring reflects heat and light. Hairy or furry stems and leaves trap plant sweat, air moisture and dew. Narrow, tiny leaves or false leaves such as on acacias and sheoaks restrict water loss. Succulence stores water. Shrunken or absent flower petals save water. Tough or sclerophyllous leaves hold their cells in shape and functioning correctly as they lose moisture. All these are characteristics easy to spot at a glance. Heat tolerance? Not so much. The information about that comes from the stories of plants. Where they originated, where they are grown as crops, which cultures think they’re important, the seasons for planting written in garden books. You can’t read it straight off the stem as easily.
I started thinking seriously about heat tolerance last year, when I planted rosella for the first time. I like planting unusual things, I had a free packet of seeds, the instructions said “grow in a similar fashion to tomatoes”. So I did. They thrived. It came to secondsummer and I wanted to put shadecloth over the vegie bed to help nurse my crops through the heatwaves. I was battling fatigue a lot and didn’t manage to get any proper covering set up. I did however have a piece of shadecloth I’d originally bought to make a wicking bed with. It was the same length and width as the garden bed, so I used that. Of course, by the time you’ve tented a piece of fabric over a few support poles it no longer covers the whole bed, and the rosella and tomatoes had grown a bit tall to fit under the cover anyway so I left them out in the full sun. To my surprise, no matter how hot it got they thrived. Their leaves never folded up to protect themself from the heat, they only showed signs of wilt if I missed a watering and not always even then, they held onto all their fruit and flowers in the heatwaves without dropping them. Being in direct hot sun for hours at a time was just fine. The melons under the shadecloth did OK too, they’re a warm-soil crop, but any bits outside the shade took permanent heat damage in the heatwaves. The silverbeet under the cloth stayed alive through the heatwaves but showed all the usual signs of heat stress – wilting stems, heatburn on leaves, increased bitterness. It managed to stay productive and not bolt to seed though so it was still within tolerance if only barely.
Heat tolerance is something we only really look at in passing in our vegie gardens, and I think we could do better. In Melbourne it wasn’t something I needed to select for. I just planted peas and spinach to grow through winter and beans and silverbeet to grow through summer, because that’s what you did. Peas like it colder, beans warmer. Tomatoes get planted out around Melbourne Cup day, potatoes before the end of October. It’s all about soil temperature. And in Melbourne and the cooler temperate parts of Australia, you’ve got a growing climate that suits all the food plants we’re most familiar with so there’s no reason to alter anything except the day you mark on your calendar to get the seeds in the ground. However, Perth. Lovely warm Mediterranean climate. Best growing time is winter. This is even more true as you head north to Geraldton, once home of a significant tomato export industry. But, I want to eat vegetables year round, not just from April to October. Heat tolerance is the key.
There’s a reasonable amount of information out there. Much of it is offerings of heat-tolerant varieties of the vegies we already know, and that has a place in extending the season of our favourites. And there are the hot-climate vegetables we’ve widely adopted as summer crops, such as squashes, chillies, tomatoes and corn. But there’s also room to look more closely at altogether different vegetables, ones less established in our British-derived culture. For instance, the things you might find on sale in a village market in, say, Southern Italy, like fennel or eggplant. Or in Mexico, like avocado, tomatillo or pepino dulce. Maybe okra from the southern USA, or the Pacific’s sweet potato, or the mustard greens of South East Asia. Reading about the grains and starches grown in hotter places with less reliable rainfall introduced me to sorghum and teff. I also learnt that in the tropics they use green banana sometimes as a substitute for potatoes, which can’t grow without winter cold.
I haven’t yet found direct information on what temperatures different plants tolerate. I do know some from experience – for instance, peas < beans < cowpeas. Or, spinach < silverbeet < New Zealand Spinach (Warrigal greens) < sweet potato leaf and rosella leaf. And again, this is not about how much water they need – they all need water. It’s about whether they can thrive and hold onto their edible bits as the temperature climbs. Extra waterings can only get a suffering plant so far up the thermometer. After that, a change of thinking is needed.