As days grow shorter and nights grow longer in mid-August, plants have cooler nights to recover from hot daytime temperatures. You’ll see a flush of new leaf and twig growth as well as increased flower bud production during the next few weeks. Heat and drought stress in late June and all of July cause plants to become semi-dormant in order to survive our extreme summer conditions. Growth that slowed or stopped last month will resume (slowly) this month.
There’s no good reason to fertilize a semi-dormant plant, but as plants regain vigor this month they will need nutrients. Here’s a quick guide to late summer/early fall fertilization for our central San Joaquin Valley climate.
Roses – Roses bloom reliably six to eight weeks after fertilization. Our roses are at their best of the year in September/October when we have few insect or fungal disease problems. Deadhead any old flowers and feed each rose in mid-August with a half cup of a low-nitrogen, higher phosphorus fertilizer, say a 4-6-5 formulation. You don’t necessarily need to buy a rose-only fertilizer. The formulations for some tomato and vegetable fertilizers are very similar and will have the same effect.
Roses grow best in soil with neutral or slightly acid soil (< 7) and our soil and water can be highly alkaline with a high Ph level (> 7). Apply a half cup of sulfur granules to the root zone of each rose bush now if you didn’t add sulfur at the first spring feeding.
Lawns – Bermuda grass and other warm-season grasses will have a growth spurt in late summer as their roots take up and store nutrients. Feed them with one of the newer lower nitrogen/ higher iron lawn foods.
Cool-season grasses including fescue are dormant during the hottest months. Give them a light feeding with the lower nitrogen/higher iron lawn food at half the recommended rate in late August or early September.
Shade plants – Azaleas, camellias and hydrangeas are fed monthly during the summer months. Apply a last feeding now of a cup per plant of a shade plant fertilizer with additional sulfur (at least 3 percent) or add a half-cup of sulfur granules to the same fertilizer you used on your roses and tomatoes.
Summer vegetables and annuals – Feed at half the recommended rate with a low-nitrogen, high-phosphorus fertilizer to encourage flower and fruit production. Deadhead any spent flowers and remove old fruit. Pinch back growing tips on vine-type or indeterminate tomato varieties.
Observations – Because newer fertilizing guidelines recommend applying high-phosphorus bone meal and low-nitrogen compost or humus instead of commercial fertilizer, I have done just that in my garden this year with mixed results.
Tomatoes, peppers, squash and roses treated with bone meal and organic compost did set flowers well but did not set a normal amount of leaves. The plants are normal height but spindly and a little sunburned. I’ll be fertilizing this week with a low nitrogen commercial food to see if leaf production can be improved.
Send Elinor Teague plant questions at firstname.lastname@example.org.