Northerner gardeners are so eager to start playing in the dirt that we are in danger of forgetting everything we know about gardening in our climate. Even Mother Nature has gotten a little bit ahead of herself; many of the perennials that were poking up their heads several weeks ago when temperatures were in the 80s may have died down when the temperatures dipped below freezing only a few days later. The cold spell was followed shortly by a shower of snow; but make no mistake it is the freezing temperatures that damaged the plant tissue — not the snow. In fact if the snow had come first it would have served as insulation from the killing frost.
We would be wise to remember that we still live and garden in zone 4, and the last average date of frost in Minnesota is May 15. Keeping that in mind, there are some outdoor chores we can do in anticipation of the impending gardening season, so here is a list of do’s and don’ts for the month of May.
DO remove winter mulch gradually as the temperatures warm, keeping it close at hand should the temperatures threaten to dip below freezing overnight. If no mulch is available you can cover emerging perennials with old sheets or towels, anything that will contain the thermal heat.
DON’T rake your lawn before the ground has warmed up, and is dry and firm. If your footsteps leave imprints in the grass, it is too early to rake. You should avoid heavy traffic on the lawn lest the soil becomes compacted. DO rake the lawn lightly after it is dry and firm. This will help the soil warm up faster and the grass begin to grow.
DON’T be hasty in cleaning up your garden beds; many pollinators may be overwintering in the hollow stems and leaf debris left over from last year. Wait until the temperatures are consistently over 50° so these pollinators have a chance to wake up and start foraging for food.
DO RECONSIDER “No Mow May.” No Mow May has been promoted the last few years as a way to provide flowers for food for these early pollinators. The idea was based on a research paper that has since been retracted because the practice was challenging for the lawn. The lawns in our area consist of cool season turf grasses that have two flushes of yearly growth; one in early summer and one in early fall. Not mowing in May could result in grass 12”-18” high. Trying to mow grass that tall presents a number of challenges. Cutting more than 1/3 of the grass blades is considered ‘scalping,’ which is stressful for the lawn. It is even more stressful in high temperatures, like we have experienced in the last two Junes. Also cutting off that much grass at once means the clippings are not mulched and ‘fed back’ to your lawn, but instead are composted with other yard waste. Instead of No Mow May it is now recommended that for the month of May you mow as high as your mower will allow and then only mow to remove 1/3 of the blade. For example, if your mower will allow you to mow at a height of 4” mow once your grass is 6” high. This will allow some flowers to bloom in your lawn to feed the pollinators.
DO CONSIDER applying pre-emergent herbicide for crabgrass. In the Twin Cities the best time to do this has historically been between May 5and May 20. Pre-emergent herbicides work by preventing annual weed seeds from germinating, which crabgrass usually does as soon as the ground warms up. If you do apply a pre-emergent DO NOT overseed your lawn, as the herbicide will also prevent the grass seed from germinating.
DON’T trim oak or elm trees after mid-April. This reduces susceptibility to Oak Wilt and Dutch Elm that is transmitted by insects.
DO trim spring flowering shrubs, lilacs, azaleas and rhododendrons soon after they are done flowering. They begin to set next year’s buds shortly after they flower, so you risk eliminating next year’s blooms if you wait too long to trim.
DO sow cold tolerant seeds and plant cold tolerant annual seedlings. By “cold tolerant” we mean seeds that will germinate and grow in cold ground and seedlings that will survive spring frost. Seeds that can be sown directly into the garden in early spring include bachelor buttons (Centaurea cyanus), sweet peas (Lathyrus odoratus), sweet alyssum (Lobularia maritime), California poppy (Eschscholzia California), sunflower (Helianthus annus) and black-eyed Susan (Rudbeckia hirta). Cold tolerant seedlings can be planted in the garden before the frost-free date of May 15 and include, snapdragon (Antirrhinum majus), ornamental kale (Brassica oleracea), dusty miller (Centaurea cineraria), pinks (Dianthus chinensis), phlox (Phlox drummondii), and pansy/violet/viola (Viola spp.) Most of these are hardy to the mid 20°s F, actually preferring cooler conditions.
DON’T plant warm weather plants or seeds until after Memorial Day. Even if we are lucky enough to avoid frost in May, being planted in ground that is too cold can stunt the plant’s growth for the entire growing season.
DO enjoy the imminent gardening season!
Plant carrots in January and you’ll never have to eat carrots – Author Unknown