Our seeding season is coming to a close with the last greens that can be sown this late in the year and still provide a harvest over the winter. I usually recommend sowing winter hardy lettuce, corn salad and arugula through the first week of September, but given how cool this season has been I am planting them this week. These are all small, quick plants than can be sown anywhere and everywhere there is a space in the garden now. I mostly seed them between the cut stumps of corn (I leave the corn roots in the soil until spring to add organic matter) and under plants that will be finished this fall such as beans, tomatoes, peppers and winter squash. To sow now under plants that are still growing, pull back the mulch and broadcast the seeds on the surface, then rake lightly over the soil to cover the seeds without disturbing roots of the existing plants. When the old plants are finished, cut them at the soil line rather than pulling them so as not to disturb the roots of the new crop.
Final check for thinning and weeding: With only a few good growing weeks left and days now so short, it is important to make sure that plant growth isn’t slowed by over-crowding or weed competition. I find it especially easy to forget carrots and other crops (winter radish, Chinese cabbage, etc.) growing out of sight under insect netting or floating row covers. Even though my carrots were thinned last month, I know I need to lift the netting this week and do a last check for weeds and crowded roots. As long as you take the netting off during the day and replace it by late afternoon, you won’t risk attack by carrot rust flies or cabbage maggot flies while the bed is uncovered (adult flies are active at dusk). And a reminder: Insect covers have to stay in place until the end of October.
Late summer disease problems:
It looks like late blight is showing up on tomatoes here and there and with the damper weather this summer that is no surprise. Early blight (Alternaria) is another fungus disease that would thrive in this cooler summer. If you have been lulled by the last few years of dry summers into growing tomatoes in the garden without a rain cover you might want to rig up a shelter to keep plants dry as long as possible into the fall. Once late blight strikes, plants go down quickly and totally (early blight results in a longer, slower decline and you might still get a reasonable harvest from affected plants). The late blight organism is a water mold, which needs water on the leaf to germinate. If you can keep leaves from getting wet from rain, irrigation or condensation, you can avoid infections. Plants in greenhouses and high tunnels ventilated well enough to prevent condensation from forming and wetting the leaves can do very well. Also plants in pots against the house where they are sheltered from rain by the eaves. [In case you saw this on the internet, it is a myth that removing the leaves of tomato plants prevents late blight infections!] Plants infected with early or late blight should be destroyed (burn, bury, dispose) and not composted; then don’t grow tomatoes or related plants (potatoes, eggplant, peppers) in that soil for as long as possible (at least 4 years).
Powdery mildews are rapidly spreading now too as they always do at this end of the year when the air is more humid. Unlike late blight, free water on leaves actually prevents powdery mildew spores from germinating, but high humidity around dry leaves favours spore germination. Several different species of fungi are called “powdery mildew”, because the white spores on the leaves look like white powder. You can’t control the disease by removing affected leaves as the spores are everywhere—and removing leaves just further stunts the plant. For winter squash that already has set fruit that is now maturing, you don’t usually need to do anything about powdery mildew. For summer squash that you want to keep producing all fall, feeding liquid fertilizer for the next weeks will stimulate the growth of new leaves, which are much less susceptible to powdery mildew than old leaves. You can also slow the spread of the fungus by rinsing the leaves with water in the middle of the day, 3or 4 times a week, to prevent spores from germinating. Don’t discard leaves or plants of kale or Swiss chard with powdery mildew even if you don’t want to harvest right now: when it starts to rain regularly this fall the new leaves won’t be infected and overwintered plants will produce lots of good leaves in the spring without any sign of mildew. You can compost plant material that has had powdery mildew.