Lowland ecotype Cyperus rotundus L. has been reported to infest irrigated lowland ricefields in the Philippines. Its effects on the growth and yield of rice under flooded conditions are unknown. Two experimental runs were conducted in a screenhouse to determine the growth of lowland ecotype C. rotundus with transplanted and wet direct-seeded rice at sowing densities of 0, 22, 44, 66, and 88 initial germinating tubers m–2, as well as its effects on the growth and yield of rice. Except for height, growth variables of lowland ecotype C. rotundus were increased with the increase in its germinating tuber densities. Compared with 22 germinating tubers m–2, its number of off-shoots and tubers plus shoot and root biomass at 44–88 germinating tubers m–2 increased by up to three times. Growth variables of rice were reduced by the weed. Grain yield of transplanted rice was reduced by 14–38%, whereas that of wet direct-seeded rice was reduced by 11–28%. The estimated yield losses of transplanted rice per weed plant were 0.8 and 0.1% as germinating tuber density and number of off-shoots approached zero (i) and 75.7 and 66.1% as the variables approached infinity (a). Estimated yield losses of wet direct-seeded rice per weed plant were 0.6 and 0.1% as the two variables approached i and 63.8 and 100% as variables approached a. Results suggest that lowland ecotype C. rotundus can grow well and reduce the growth and yield of rice under flooded conditions. Although the findings were obtained under screenhouse conditions, the results imply the need of developing a holistic weed control strategy against the weed.

C. rotundus is known to reduce grain yields of transplanted (TPR) and wet direct-seeded rice (WDSR) under flooded conditions. Information regarding its critical period of control is not yet known. Two identical experiments were performed to determine the critical periods of controlling C. rotundus in TPR and WDSR. Following the procedure of two crop-weed competition periods, the weed was grown with TPR and WDSR in a randomized complete block design arrangement with three replications. Under the critical time of weed removal, C. rotundus had the lowest shoot biomass when grown with TPR and WDSR from 0–10 DAT/DAS and the highest from 0–100 DAT/ 0–120 DAS. When grown from 0–20 up to 60 DAT/DAS, shoot biomass increased significantly thereafter. TPR grain yield was not affected by the weed from 0–10 up to 20 DAT but reduced thereafter by 10.7–40% from 0–30 up to 100 DAT. WDSR grain yield was also unaffected from 0–10 up to 20 DAS; however, it decreased thereafter by 15–34.1% from 0–30 up to 120 DAS. Under the critical weed-free period, the shoot biomass of the weed was highest from 10–100 DAT/DAS for both TPR and WDSR; it decreased significantly from 20–100/120 and up to 60–100/120 DAT/DAS. TPR grain yield decreased to 6.1% when the weed was present from 10–100 and up to 60–100 DAT; WDSR decreased by 16.9 and 4.1% from 10–120 and 20–120 DAS, respectively. There was no yield reduction in WDSR from 0–10 and up to 60–120 DAS. The classical method and statistical model suggest that the critical period for controlling C. rotundus is from 14–29 DAT in TPR and 18–26 DAS in WDSR. Appropriate and effective weed control techniques should be employed during these periods to avoid having more than 5% grain yield reduction.