The rule of thumb has always been to plant in spring and fall when the weather is cooler. But a rising temperature doesn't mean you can't continue to fill the empty spaces in your garden with plants. Sure, there are some things you shouldn't plant in summer, such as a bare-root, newly dug or newly divided plant.
Summer isn't as ideal of a time to plant as spring or early fall, but with a few precautions, most plants will do fine. Peak sunlight and searing heat are added stressors on young plants with small roots, not to mention, summer soil tends to be drier.
The general answer is around 90 degrees F, with some exceptions to the rule. This means that when temps rise above 90 and remain there for a lengthy spell: Leaves wilt. Water evaporates into the atmosphere more quickly in high heat, draining a plant's reserves.
If your summer is flying by and you haven't gotten to everything on your to do list, it is not too late to plant seeds. July is actually an optimal time as the ground is warm and the daytime and nightime temperatures are warm so your seeds will germinate faster and your plants will grow very well.
July is not too late to start a garden, and you can still get a bountiful harvest in just a few weeks or months. However, you should prepare the garden quickly if you are starting in July and sow seeds or plant young transplants as early as possible.
There are natural advantages to sowing seed in summer, too. The warmth of the sun and longer days provide perfect growing conditions, plus you'll benefit from mess-free sowing outdoors.
The general rule of thumb is that most plants freeze when temperatures remain at 28°F for five hours. Of course, there are exceptions to this rule. Seedlings, with their tender new leaves, often give up the ghost when temperatures dip to 32-33°F. Tropical plants have differing low-temperature thresholds.
If extreme heat continues for weeks at a time, plants can actually die from a depletion of their food reserves. High temperatures can also cause severe water loss (desiccation) when transpiration (the process by which leaves release water vapor to the atmosphere) exceeds moisture absorption by the roots.
Too Hot to Germinate
However, soil can be too warm for seeds to germinate. For instance, lettuce won't germinate if the soil is above 85 degrees. For spinach, 75 is too hot to germinate.
The time needed to recover from stress is proportional to the severity of the damage. Plant heat stress recovery may take months for crops that have been ignored for too long during a heatwave; in extreme cases, it may be impossible to bring them back to life.
During extremely hot weather (daytime temperatures above 90F and nighttime temperatures above 70F), try to water daily or every other day. In a 10x10-foot garden, this would mean giving your plants 8 to 9 gallons of water each day.
In the desert southwest, including parts of Texas and California, you should wait until the average daytime temperature drops under 90F before you plant. The issue isn't only the air temperature, but the temperature of the ground.
Yep. For the most part if you water well you can easily and successfully plant trees, shrubs, and perennials in summer. Water deeply every other day for the first few weeks. After this time reduce to about 2-3 times per week depending on your climate, soil, etc.
Most of these are fall and winter vegetables (root crops, salad and cooking greens, broccoli, cabbage, etc), but you can still plant a few fast-growing summer crops like green beans, summer squash, and cucumbers – depending on your climate.
The books are right – planting in the summer when it is hot and dry is not the best time to move new plants into or around the garden. During a particularly bad heat wave it can be hard enough to keep the soil hydrated, let alone provide ample moisture for plants that are establishing new roots.
Most plants grow best in temperatures ranging from 59°–86°F. When temperatures above 90°F are sustained for long periods, plant growth is slowed, and some plants begin to show signs of stress.
Make some shade
You can also use old bedsheets, rigged-up cheesecloth or propped-up cardboard to give your plants a break from the scorching rays, especially in the afternoon when temps reach their peak. Make sure your shade covering allows air to circulate freely around the plants.
Most houseplants are native to tropical areas and will not tolerate freezing temperatures. Many indoor plants will see damage when temperatures drop to the mid to lower 40s°F and nearly all will die if exposed to below freezing temperatures.
Though some species are more cold tolerant than others, most will perish or enter diapause once temperatures fall below 32°F, which answers the question, “how cold is too cold for mosquitoes?”
Regularly misting the plant or placing it on a pebble tray with water can help meet its humidity requirements. Temperature: It prefers warmer temperatures, ideally between 65-75°F (18-24°C).
In truth, each season is different and garden planting dates have been even harder to peg down than the traditional wisdom would lead one to believe! I want to emphasize that it's not too late to plant a great garden, even in June and July.