Northern and Central California include so many different USDA climate zones that little can be said that is true of gardening throughout the area. Except this: May is a delightful month of moderate temperatures and occasionally glorious days, even in the Bay Area. Flowers are in abundance and growth is lush even in the central valley before the temperatures begin to soar. It’s a great time to be outside in the garden!
Summer annuals offer endless possibilities for garden design using waves of color to draw the eye. Good choices are cosmos, pentas, marigold, nasturtiums, periwinkle and petunias. Try some of the new “Wave” petunias, some of which grow to 18 inches and are covered with vibrant blooms.
May is a good month to plant perennials, especially natives like prairie penstemon, coneflower, some of the many salvias, nicotiana, verbena or sage. Prepare your beds by digging in lots of organic material and some grit, such as expanded shale or coarse sand to help drainage. When you’ve planted your flowers, mulch around them with organic mulch, such as shredded pine bark or pine straw. Even leaves you’ve raked from your yard will make good mulch, especially if you shred them.
Put your spring flowering bulbs to bed by tying up their foliage with twine until it turns yellow, then you can cut it back. Once that happens, overplant the bulb beds with colorful annuals.
Plant warm season vegetables this month when all danger of frost is past. Tomatoes, peppers, melons and squash need as much sun as they can get. In the warmer zones, though, remember that tomatoes won’t set fruit when the temperature gets above 90 degrees. Shade cloth and misting can help, as can thick mulch to protect the roots from heat and conserve water.
Near the coast, cool season crops like lettuce and broccoli can grow all year round. Lettuce, chicory, mustards and other greens can be harvested with scissors in a cut and come again fashion that will keep them productive for months. Use row cover if insects are a problem. Sow directly in the ground or in raised beds every two weeks for a continuous harvest until fall.
If you have planted potatoes, continue to hill them up either with soil or moist mulch to give tubers room to grow. After the plants bloom, look for new potatoes and harvest without digging up the whole plant. Leave some tubers to mature.
In the cool coastal areas, asparagus thrive, especially in raised beds with lots of mulch on top. Dig a trench 6 to 8 inches deep and lay the bare roots on the bottom. Cover with a couple of inches of soil. When the ferny growth pops through, add more soil until the trench is full, then mulch around the plants. The first year, just let the ferns grow. Next year, you can harvest for a few weeks, then let the ferns grow. After that, you will have thick tasty spears for six weeks or more every spring for many years.
The white flowered bush anemone is a great choice for backyard gardening. It’s a native and has won many awards as a must-have plant for California gardens. It is pleasantly fragrant and tough as nails.
Refresh the mulch under roses now and make sure that when you water, you do it from underneath. Keep water off the leaves when the sun is shining to avoid burn. Pick up and get rid of (not in the compost) yellowed leaves that fall. Check for insect damage and treat as necessary.
Prune spring and winter flowering shrubs if they need it and refresh their mulch.
This is a good month to think through whether you want to continue to support a large area of turf grass, with all its water and fertilizer requirements, not to mention mowing. If you want to get rid of a section of turf grass in favor of a landscaped area using native plants, use a glyphosate-based product, such as Roundup or Kleenup. Water the grass for several days to encourage new growth, then, on a windless day after covering plants you want to keep with plastic, spray the turf with the grass killer as per instructions on the label. Wait several days and do it all again. Wait a week or so for the glyphosate to dissipate and begin planting other things. Try ornamental grasses, hardy salvias and ceanothus. Or succulents like aloe, yucca and agave.
For the turf you keep, mow high and often, keeping the grass at 3 to 4 inches long, which will protect its roots, conserve water and keep it healthy. Don’t cut more than a third of its length at any mowing. Get a mulching kit for your mower and leave the clippings on the lawn to return nutrients to the soil.